README.md

@@ -175,4 +175,240 @@ curl --cookie cookies.txt http://localhost:8080/cart/add/TEST-SKU-123
 
 - Always regenerate protobuf Go code after modifying any `.proto` files (messages/cart_actor/control_plane)
 - The generated `messages.pb.go` file should not be edited manually
-- Make sure your PATH includes the protoc-gen-go binary location (usually `$GOPATH/bin`)
+- Make sure your PATH includes the protoc-gen-go binary location (usually `$GOPATH/bin`)
+
+---
+
+## Architecture Overview
+
+The system is a distributed, sharded (by cart id) actor model implementation:
+
+- Each cart is a grain (an in‑memory struct `*CartGrain`) that owns and mutates its own state.
+- A **local grain pool** holds grains owned by the node.
+- A **synced (cluster) pool** (`SyncedPool`) coordinates multiple nodes and exposes local or remote grains through a uniform interface (`GrainPool`).
+- All inter‑node communication is gRPC:
+  - Cart mutation & state RPCs (CartActor service).
+  - Control plane RPCs (ControlPlane service) for membership, ownership negotiation, liveness, and graceful shutdown.
+
+### Key Processes
+
+1. Client HTTP request (or gRPC client) arrives with a cart identifier (cookie or path).
+2. The pool resolves ownership:
+   - If local grain exists → use it.
+   - If a remote host is known owner → a remote grain proxy (`RemoteGrainGRPC`) is used; it performs gRPC calls to the owning node.
+   - If ownership is unknown → node attempts to claim ownership (quorum negotiation) and spawns a local grain.
+3. Mutation is executed via the **mutation registry** (registry wraps domain logic + optional totals recomputation).
+4. Updated state returned to caller; ownership preserved unless relinquished later (not yet implemented to shed load).
+
+---
+
+## Grain & Mutation Model
+
+- `CartGrain` holds items, deliveries, pricing aggregates, and checkout/order metadata.
+- All mutations are registered via `RegisterMutation[T]` with signature:
+  ```
+  func(*CartGrain, *T) error
+  ```
+- `WithTotals()` flag triggers automatic recalculation of totals after successful handlers.
+- The old giant `switch` in `CartGrain.Apply` has been replaced by registry dispatch; unregistered mutations fail fast.
+- Adding a mutation:
+  1. Define proto message.
+  2. Generate code.
+  3. Register handler (optionally WithTotals).
+  4. Add gRPC RPC + request wrapper if the mutation must be remotely invokable.
+  5. (Optional) Add HTTP endpoint mapping to the mutation.
+
+---
+
+## Local Grain Pool
+
+- Manages an in‑memory map `map[CartId]*CartGrain`.
+- Lazy spawn: first mutation or explicit access triggers `spawn(id)`.
+- TTL / purge loop periodically removes expired grains unless they changed recently (basic memory pressure management).
+- Capacity limit (`PoolSize`); oldest expired grain evicted first when full.
+
+---
+
+## Synced (Cluster) Pool
+
+`SyncedPool` wraps a local pool and tracks:
+
+- `remoteHosts`: known peer nodes (gRPC connections).
+- `remoteIndex`: mapping of cart id → remote grain proxy (`RemoteGrainGRPC`) for carts owned elsewhere.
+
+Responsibilities:
+
+1. Discovery integration (via a `Discovery` interface) adds/removes hosts.
+2. Periodic ping health checks (ControlPlane.Ping).
+3. Ownership negotiation:
+   - On first contention / unknown owner, node calls `ConfirmOwner` on peers to achieve quorum before making a local grain authoritative.
+4. Remote spawning:
+   - When a remote host reports its cart ids (`GetCartIds`), the pool creates remote proxies for fast routing.
+
+---
+
+## Remote Grain Proxies
+
+A `RemoteGrainGRPC` implements the `Grain` interface but delegates:
+
+- `Apply` → Specific CartActor per‑mutation RPC (e.g., `AddItem`, `RemoveItem`) constructed from the mutation type. (Legacy envelope removed.)
+- `GetCurrentState` → `CartActor.GetState`.
+
+Return path:
+
+1. gRPC reply (CartMutationReply / StateReply) → proto `CartState`.
+2. `ToCartState` / mapping reconstructs a local `CartGrain` snapshot for callers expecting grain semantics.
+
+---
+
+## Control Plane (Inter‑Node Coordination)
+
+Defined in `proto/control_plane.proto`:
+
+| RPC | Purpose |
+|-----|---------|
+| `Ping` | Liveness; increments missed ping counter if failing. |
+| `Negotiate` | Merges membership views; used after discovery events. |
+| `GetCartIds` | Enumerate locally owned carts for remote index seeding. |
+| `ConfirmOwner` | Quorum acknowledgment for ownership claim. |
+| `Closing` | Graceful shutdown notice; peers remove host & associated remote grains. |
+
+### Ownership / Quorum Rules
+
+- If total participating hosts < 3 → all must accept.
+- Otherwise majority acceptance (`ok >= total/2`).
+- On failure → local tentative grain is removed (rollback to avoid split‑brain).
+
+---
+
+## Request / Mutation Flow Examples
+
+### Local Mutation
+1. HTTP handler parses request → determines cart id.
+2. `SyncedPool.Apply`:
+   - Finds local grain (or spawns new after quorum).
+   - Executes registry mutation.
+3. Totals updated if flagged.
+4. HTTP response returns updated JSON (via `ToCartState`).
+
+### Remote Mutation
+1. `SyncedPool.Apply` sees cart mapped to a remote host.
+2. Routes to `RemoteGrainGRPC.Apply`.
+3. Remote node executes mutation locally and returns updated state over gRPC.
+4. Proxy materializes snapshot locally (not authoritative, read‑only view).
+
+### Checkout (Side‑Effecting, Non-Pure)
+- HTTP `/checkout` uses current grain snapshot to build payload (pure function).
+- Calls Klarna externally (not a mutation).
+- Applies `InitializeCheckout` mutation to persist reference + status.
+- Returns Klarna order JSON to client.
+
+---
+
+## Scaling & Deployment
+
+- **Horizontal scaling**: Add more nodes; discovery layer (Kubernetes / service registry) feeds hosts to `SyncedPool`.
+- **Sharding**: Implicit by cart id hash. Ownership is first-claim with quorum acceptance.
+- **Hot spots**: A single popular cart remains on one node; for heavy multi-client concurrency, future work could add read replicas or partitioning (not implemented).
+- **Capacity tuning**: Increase `PoolSize` & memory limits; adjust TTL for stale cart eviction.
+
+### Adding Nodes
+1. Node starts gRPC server (CartActor + ControlPlane).
+2. After brief delay, begins discovery watch; on event:
+   - New host → dial + negotiate → seed remote cart ids.
+3. Pings maintain health; failed hosts removed (proxies invalidated).
+
+---
+
+## Failure Handling
+
+| Scenario | Behavior |
+|----------|----------|
+| Remote host unreachable | Pings increment `MissedPings`; after threshold host removed. |
+| Ownership negotiation fails | Tentative local grain discarded. |
+| gRPC call error on remote mutation | Error bubbled to caller; no local fallback. |
+| Missing mutation registration | Fast failure with explicit error message. |
+| Partial checkout (Klarna fails) | No local state mutation for checkout; client sees error; cart remains unchanged. |
+
+---
+
+## Mutation Registry Summary
+
+- Central, type-safe registry prevents silent omission.
+- Each handler:
+  - Validates input.
+  - Mutates `*CartGrain`.
+  - Returns error for rejection.
+- Automatic totals recomputation reduces boilerplate and consistency risk.
+- Coverage test (add separately) can enforce all proto mutations are registered.
+
+---
+
+## gRPC Interfaces
+
+- **CartActor**: Per-mutation unary RPCs + `GetState`. (Checkout logic intentionally excluded; handled at HTTP layer.)
+- **ControlPlane**: Cluster coordination (Ping, Negotiate, ConfirmOwner, etc.).
+
+**Ports** (default / implied):
+- CartActor & ControlPlane share the same gRPC server/listener (single port, e.g. `:1337`).
+- Legacy frame/TCP code has been removed.
+
+---
+
+## Security & Future Enhancements
+
+| Area | Potential Improvement |
+|------|------------------------|
+| Transport Security | Add TLS / mTLS to gRPC servers & clients. |
+| Auth / RBAC | Intercept CartActor RPCs with auth metadata. |
+| Backpressure | Rate-limit remote mutation calls per host. |
+| Observability | Add per-mutation Prometheus metrics & tracing spans. |
+| Ownership | Add lease timeouts / fencing tokens for stricter guarantees. |
+| Batch Ops | Introduce batch mutation RPC or streaming updates (WatchState). |
+| Persistence | Reintroduce event log or snapshot persistence layer if durability required. |
+
+---
+
+## Adding a New Node (Operational Checklist)
+
+1. Deploy binary/container with same proto + registry.
+2. Expose gRPC port.
+3. Ensure discovery lists the new host.
+4. Node dials peers, negotiates membership.
+5. Remote cart proxies seeded.
+6. Traffic routed automatically based on ownership.
+
+---
+
+## Adding a New Mutation (Checklist Recap)
+
+1. Define proto message (+ request wrapper & RPC if remote invocation needed).
+2. Regenerate protobuf code.
+3. Implement & register handler (`RegisterMutation`).
+4. Add client (HTTP/gRPC) endpoint.
+5. Write unit + integration tests.
+6. (Optional) Add to coverage test list and docs.
+
+---
+
+## High-Level Data Flow Diagram (Text)
+
+```
+Client -> HTTP Handler -> SyncedPool -> (local?) -> Registry -> Grain State
+                       \-> (remote?) -> RemoteGrainGRPC -> gRPC -> Remote CartActor -> Registry -> Grain
+ControlPlane: Discovery Events <-> Negotiation/Ping/ConfirmOwner <-> SyncedPool state
+```
+
+---
+
+## Troubleshooting
+
+| Symptom | Likely Cause | Action |
+|---------|--------------|--------|
+| New cart every request | Secure cookie over plain HTTP or not sending cookie jar | Disable Secure locally or use HTTPS & proper curl `-b` |
+| Unsupported mutation error | Missing registry handler | Add `RegisterMutation` for that proto |
+| Ownership flapping | Quorum failing due to intermittent peers | Investigate `ConfirmOwner` errors / network |
+| Remote mutation latency | Network / serialization overhead | Consider batching or colocating hot carts |
+| Checkout returns 500 | Klarna call failed | Inspect logs; no grain state mutated |
+
+---

TODO.md

@@ -0,0 +1,245 @@
+# TODO / Roadmap
+
+A living roadmap for improving the cart actor system. Focus areas:
+1. Reliability & correctness
+2. Simplicity of mutation & ownership flows
+3. Developer experience (DX)
+4. Operability (observability, tracing, metrics)
+5. Performance & scalability
+6. Security & multi-tenant readiness
+
+---
+
+## 1. Immediate Next Steps (High-Leverage)
+
+| Priority | Task | Goal | Effort | Owner | Notes |
+|----------|------|------|--------|-------|-------|
+| P0 | Add mutation registry coverage test | Ensure no unregistered mutations silently fail | S |  | Failing fast in CI |
+| P0 | Add decodeJSON helper + 400 mapping for EOF | Reduce noisy 500 logs | S |  | Improves client API clarity |
+| P0 | Regenerate protos & prune unused messages (CreateCheckoutOrder, Checkout RPC remnants) | Eliminate dead types | S |  | Avoid confusion |
+| P0 | Add integration test: multi-node ownership negotiation | Validate quorum logic | M |  | Spin up 2–3 nodes ephemeral |
+| P1 | Export Prometheus metrics for per-mutation counts & latency | Operability | M |  | Wrap registry handlers |
+| P1 | Add graceful shutdown ordering (Closing → wait for acks → stop gRPC) | Reduce in-flight mutation failures | S |  | Add context cancellation |
+| P1 | Add coverage for InitializeCheckout / OrderCreated flows | Checkout reliability | S |  | Simulate Klarna stub |
+| P2 | Add optional batching client (apply multiple mutations locally then persist) | Performance | M |  | Only if needed |
+
+---
+
+## 2. Simplification Opportunities
+
+### A. RemoteGrain Proxy Mapping
+Current: manual switch building each RPC call.
+Simplify by:
+- Generating a thin client adapter from proto RPC descriptors (codegen).
+- Or using a registry similar to mutation registry but for “outbound call constructors”.
+Benefit: adding a new mutation = add proto + register server handler + register outbound invoker (no switch edits).
+
+### B. Ownership Negotiation
+Current: ad hoc quorum rule in `SyncedPool`.
+Simplify:
+- Introduce explicit `OwnershipLease{holder, expiresAt, version}`.
+- Use monotonic version increment—reject stale ConfirmOwner replies.
+- Optional: add randomized backoff to reduce thundering herd on contested cart ids.
+
+### C. CartId Handling
+Current: ephemeral 16-byte array with trimmed string semantics.
+Simplify:
+- Use ULID / UUIDv7 (time-ordered, collision-resistant) for easier external correlation.
+- Provide helper `NewCartIdString()` and keep internal fixed-size if still desired.
+
+### D. Mutation Signatures
+Current: registry assumes `func(*CartGrain, *T) error`.
+Extension option: allow pure transforms returning a delta struct (for audit/logging):
+```
+type MutationResult struct {
+  Changed bool
+  Events  []interface{}
+}
+```
+Only implement if auditing/event-sourcing reintroduced.
+
+---
+
+## 3. Developer Experience Improvements
+
+| Task | Rationale | Approach |
+|------|-----------|----------|
+| Makefile targets: `make run-single`, `make run-multi N=3` | Faster local cluster spin-up | Docker compose or background “mini cluster” scripts |
+| Template for new mutation (generator) | Reduce boilerplate | `go:generate` scanning proto for new RPCs |
+| Lint config (golangci-lint) | Catch subtle issues early | Add `.golangci.yml` |
+| Pre-commit hook for proto regeneration check | Avoid stale generated code | Script compares git diff after `make protogen` |
+| Example client (Go + curl snippets auto-generated) | Onboarding | Codegen a markdown from proto comments |
+
+---
+
+## 4. Observability / Metrics / Tracing
+
+| Area | Metric / Trace | Notes |
+|------|----------------|-------|
+| Mutation registry | `cart_mutations_total{type,success}`; duration histogram | Wrap handler |
+| Ownership negotiation | `cart_ownership_attempts_total{result}` | result=accepted,rejected,timeout |
+| Remote latency | `cart_remote_mutation_seconds{method}` | Use client interceptors |
+| Pings | `cart_remote_missed_pings_total{host}` | Already count, expose |
+| Checkout flow | `checkout_attempts_total`, `checkout_failures_total` | Differentiate Klarna vs internal errors |
+| Tracing | Span: HTTP handler → SyncedPool.Apply → (Remote?) gRPC → mutation handler | Add OpenTelemetry instrumentation |
+
+---
+
+## 5. Performance & Scalability
+
+| Concern | Idea | Trade-Off |
+|---------|------|-----------|
+| High mutation rate on single cart | Introduce optional mutation queue (serialize explicitly) | Slight latency increase per op |
+| Remote call overhead | Add client-side gRPC pooling & per-host circuit breaker | Complexity vs resilience |
+| TTL purge efficiency | Use min-heap or timing wheel instead of slice scan | More code, better big-N performance |
+| Batch network latency | Add `BatchMutate` RPC (list of mutations applied atomically) | Lost single-op simplicity |
+
+---
+
+## 6. Reliability Features
+
+| Feature | Description | Priority |
+|---------|-------------|----------|
+| Lease fencing token | Include `ownership_version` in all remote mutate requests | M |
+| Retry policy | Limited retry for transient network errors (idempotent mutations only) | L |
+| Dead host reconciliation | On host removal, proactively attempt re-acquire of its carts | M |
+| Drain mode | Node marks itself “draining” → refuses new ownership claims | M |
+
+---
+
+## 7. Security & Hardening
+
+| Area | Next Step | Detail |
+|------|-----------|--------|
+| Transport | mTLS on gRPC | Use SPIFFE IDs or simple CA |
+| AuthN/AuthZ | Interceptor enforcing service token | Inject metadata header |
+| Input validation | Strengthen JSON decode responses | Disallow unknown fields globally |
+| Rate limiting | Per-IP / per-cart throttling | Guard hotspot abuse |
+| Multi-tenancy | Tenant id dimension in cart id or metadata | Partition metrics & ownership |
+
+---
+
+## 8. Testing Strategy Enhancements
+
+| Gap | Improvement |
+|-----|------------|
+| No multi-node integration test in CI | Spin ephemeral in-process servers on randomized ports |
+| Mutation regression | Table-driven tests auto-discover handlers via registry |
+| Ownership race | Stress test: concurrent Apply on same new cart id from N goroutines |
+| Checkout external dependency | Klarna mock server (HTTptest) + deterministic responses |
+| Fuzzing | Fuzz `BuildCheckoutOrderPayload` & mutation handlers for panics |
+
+---
+
+## 9. Cleanup / Tech Debt
+
+| Item | Action |
+|------|--------|
+| Remove deprecated proto remnants (CreateCheckoutOrder, Checkout RPC) | Delete & regenerate |
+| Consolidate duplicate tax computations | Single helper with tax config |
+| Delivery price hard-coded (4900) | Config or pricing strategy interface |
+| Mixed naming (camel vs snake JSON historically) | Provide stable external API doc; accept old forms if needed |
+| Manual remote mutation switch (if still present) | Replace with generated outbound registry |
+| Mixed error responses (string bodies) | Standardize JSON: `{ "error": "...", "code": 400 }` |
+
+---
+
+## 10. Potential Future Features
+
+| Feature | Value | Complexity |
+|---------|-------|------------|
+| Streaming `WatchState` RPC | Real-time cart updates for clients | Medium |
+| Event sourcing / audit log | Replay, analytics, debugging | High |
+| Promotion / coupon engine plugin | Business extensibility | Medium |
+| Partial cart reservation / inventory lock | Stock accuracy under concurrency | High |
+| Multi-currency pricing | Globalization | Medium |
+| GraphQL facade | Client flexibility | Medium |
+
+---
+
+## 11. Suggested Prioritized Backlog (Condensed)
+
+1. Coverage test + decode error mapping (P0)
+2. Proto regeneration & cleanup (P0)
+3. Metrics wrapper for registry (P1)
+4. Multi-node ownership integration test (P1)
+5. Delivery pricing abstraction (P2)
+6. Lease version in remote RPCs (P2)
+7. BatchMutate evaluation (P3)
+8. TLS / auth hardening (P3) if going multi-tenant/public
+9. Event sourcing (Evaluate after stability) (P4)
+
+---
+
+## 12. Simplifying the Developer Workflow
+
+| Pain | Simplifier |
+|------|------------|
+| Manual mutation boilerplate | Code generator for registry stubs |
+| Forgetting totals | Enforce WithTotals lint: fail if mutation touches items/deliveries without flag |
+| Hard to inspect remote ownership | `/internal/ownership` debug endpoint (JSON of local + remoteIndex) |
+| Hard to see mutation timings | Add `?debug=latency` header to return per-mutation durations |
+| Cookie dev confusion (Secure flag) | Env var: `DEV_INSECURE_COOKIES=1` |
+
+---
+
+## 13. Example: Mutation Codegen Sketch (Future)
+
+Input: cart_actor.proto  
+Output: `mutation_auto.go`
+- Detect messages used in RPC wrappers (e.g., `AddItemRequest` → payload field).
+- Generate `RegisterMutation` template if handler not found.
+- Mark with `// TODO implement logic`.
+
+---
+
+## 14. Risk / Impact Matrix (Abbreviated)
+
+| Change | Risk | Mitigation |
+|--------|------|-----------|
+| Replace remote switch with registry | Possible missing registration → runtime error | Coverage test gating CI |
+| Lease introduction | Split-brain if version mishandled | Increment + assert monotonic; test race |
+| BatchMutate | Large atomic operations starving others | Size limits & fair scheduling |
+| Event sourcing | Storage + replay complexity | Start with append-only log + compaction job |
+
+---
+
+## 15. Contributing Workflow (Proposed)
+
+1. Add / modify proto → run `make protogen`
+2. Implement mutation logic → add `RegisterMutation` invocation
+3. Add/Update tests (unit + integration)
+4. Run `make verify` (lint, test, coverage, proto diff)
+5. Open PR (template auto-checklist referencing this TODO)
+6. Merge requires green CI + coverage threshold
+
+---
+
+## 16. Open Questions
+
+| Question | Notes |
+|----------|-------|
+| Do we need sticky sessions for HTTP layer scaling? | Currently cart id routing suffices |
+| Should deliveries prune invalid line references on SetCartRequest? | Inconsistency risk; add optional cleanup |
+| Is checkout idempotency strict enough? | Multiple create vs update semantics |
+| Add version field to CartState for optimistic concurrency? | Could enable external CAS writes |
+
+---
+
+## 17. Tracking
+
+Mark any completed tasks with `[x]`:
+
+- [ ] Coverage test
+- [ ] Decode helper + 400 mapping
+- [ ] Proto cleanup
+- [ ] Registry metrics instrumentation
+- [ ] Ownership multi-node test
+- [ ] Lease versioning
+- [ ] Delivery pricing abstraction
+- [ ] TLS/mTLS internal
+- [ ] BatchMutate design doc
+
+---
+
+_Last updated: roadmap draft – refine after first metrics & scaling test run._

amqp-order-handler.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	amqp "github.com/rabbitmq/amqp091-go"
+)
+
+type AmqpOrderHandler struct {
+	Url        string
+	Connection *amqp.Connection
+	Channel    *amqp.Channel
+}
+
+func (h *AmqpOrderHandler) Connect() error {
+	conn, err := amqp.Dial(h.Url)
+	if err != nil {
+		return fmt.Errorf("failed to connect to RabbitMQ: %w", err)
+	}
+	h.Connection = conn
+
+	ch, err := conn.Channel()
+	if err != nil {
+		return fmt.Errorf("failed to open a channel: %w", err)
+	}
+	h.Channel = ch
+
+	return nil
+}
+
+func (h *AmqpOrderHandler) Close() error {
+	if h.Channel != nil {
+		h.Channel.Close()
+	}
+	if h.Connection != nil {
+		return h.Connection.Close()
+	}
+	return nil
+}
+
+func (h *AmqpOrderHandler) OrderCompleted(body []byte) error {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	err := h.Channel.PublishWithContext(ctx,
+		"orders", // exchange
+		"new",    // routing key
+		false,    // mandatory
+		false,    // immediate
+		amqp.Publishing{
+			ContentType: "application/json",
+			Body:        body,
+		})
+	if err != nil {
+		return fmt.Errorf("failed to publish a message: %w", err)
+	}
+
+	return nil
+}

cart-grain.go

@@ -1,11 +1,8 @@
 package main
 
 import (
-	"bytes"
 	"encoding/json"
 	"fmt"
-	"log"
-	"slices"
 	"sync"
 
 	messages "git.tornberg.me/go-cart-actor/proto"
@@ -265,235 +262,14 @@ func GetTaxAmount(total int64, tax int) int64 {
 func (c *CartGrain) Apply(content interface{}, isReplay bool) (*CartGrain, error) {
 	grainMutations.Inc()
 
-	switch msg := content.(type) {
-
-	case *messages.SetCartRequest:
-		c.mu.Lock()
-		c.Items = make([]*CartItem, 0, len(msg.Items))
-		c.mu.Unlock()
-		for _, it := range msg.Items {
-			c.AddItem(it.Sku, int(it.Quantity), it.Country, it.StoreId)
+	updated, err := ApplyRegistered(c, content)
+	if err != nil {
+		if err == ErrMutationNotRegistered {
+			return nil, fmt.Errorf("unsupported mutation type %T (not registered)", content)
 		}
-
-	case *messages.AddRequest:
-		if existing, found := c.FindItemWithSku(msg.Sku); found {
-			existing.Quantity += int(msg.Quantity)
-			c.UpdateTotals()
-		} else {
-			return c.AddItem(msg.Sku, int(msg.Quantity), msg.Country, msg.StoreId)
-		}
-
-	case *messages.AddItem:
-		if msg.Quantity < 1 {
-			return nil, fmt.Errorf("invalid quantity")
-		}
-		if existing, found := c.FindItemWithSku(msg.Sku); found {
-			existing.Quantity += int(msg.Quantity)
-			c.UpdateTotals()
-		} else {
-			c.mu.Lock()
-			c.lastItemId++
-			tax := 2500
-			if msg.Tax > 0 {
-				tax = int(msg.Tax)
-			}
-			taxAmount := GetTaxAmount(msg.Price, tax)
-			c.Items = append(c.Items, &CartItem{
-				Id:          c.lastItemId,
-				ItemId:      int(msg.ItemId),
-				Quantity:    int(msg.Quantity),
-				Sku:         msg.Sku,
-				Name:        msg.Name,
-				Price:       msg.Price,
-				TotalPrice:  msg.Price * int64(msg.Quantity),
-				TotalTax:    int64(taxAmount * int64(msg.Quantity)),
-				Image:       msg.Image,
-				Stock:       StockStatus(msg.Stock),
-				Disclaimer:  msg.Disclaimer,
-				Brand:       msg.Brand,
-				Category:    msg.Category,
-				Category2:   msg.Category2,
-				Category3:   msg.Category3,
-				Category4:   msg.Category4,
-				Category5:   msg.Category5,
-				OrgPrice:    msg.OrgPrice,
-				ArticleType: msg.ArticleType,
-				Outlet:      msg.Outlet,
-				SellerId:    msg.SellerId,
-				SellerName:  msg.SellerName,
-				Tax:         int(taxAmount),
-				TaxRate:     tax,
-				StoreId:     msg.StoreId,
-			})
-			c.UpdateTotals()
-			c.mu.Unlock()
-		}
-
-	case *messages.ChangeQuantity:
-		for i, item := range c.Items {
-			if item.Id == int(msg.Id) {
-				if msg.Quantity <= 0 {
-					c.Items = append(c.Items[:i], c.Items[i+1:]...)
-				} else {
-					item.Quantity = int(msg.Quantity)
-				}
-				break
-			}
-		}
-		c.UpdateTotals()
-
-	case *messages.RemoveItem:
-		newItems := make([]*CartItem, 0, len(c.Items))
-		for _, it := range c.Items {
-			if it.Id != int(msg.Id) {
-				newItems = append(newItems, it)
-			}
-		}
-		c.Items = newItems
-		c.UpdateTotals()
-
-	case *messages.SetDelivery:
-		c.lastDeliveryId++
-		items := make([]int, 0)
-		withDelivery := c.ItemsWithDelivery()
-		if len(msg.Items) == 0 {
-			items = append(items, c.ItemsWithoutDelivery()...)
-		} else {
-			for _, id := range msg.Items {
-				for _, it := range c.Items {
-					if it.Id == int(id) {
-						if slices.Contains(withDelivery, it.Id) {
-							return nil, fmt.Errorf("item already has delivery")
-						}
-						items = append(items, int(it.Id))
-						break
-					}
-				}
-			}
-		}
-		if len(items) > 0 {
-			c.Deliveries = append(c.Deliveries, &CartDelivery{
-				Id:          c.lastDeliveryId,
-				Provider:    msg.Provider,
-				PickupPoint: msg.PickupPoint,
-				Price:       4900,
-				Items:       items,
-			})
-			c.UpdateTotals()
-		}
-
-	case *messages.RemoveDelivery:
-		dels := make([]*CartDelivery, 0, len(c.Deliveries))
-		for _, d := range c.Deliveries {
-			if d.Id == int(msg.Id) {
-				c.TotalPrice -= d.Price
-			} else {
-				dels = append(dels, d)
-			}
-		}
-		c.Deliveries = dels
-		c.UpdateTotals()
-
-	case *messages.SetPickupPoint:
-		for _, d := range c.Deliveries {
-			if d.Id == int(msg.DeliveryId) {
-				d.PickupPoint = &messages.PickupPoint{
-					Id:      msg.Id,
-					Address: msg.Address,
-					City:    msg.City,
-					Zip:     msg.Zip,
-					Country: msg.Country,
-					Name:    msg.Name,
-				}
-				break
-			}
-		}
-
-	case *messages.CreateCheckoutOrder:
-		orderLines := make([]*Line, 0, len(c.Items))
-		c.PaymentInProgress = true
-		c.Processing = true
-		for _, item := range c.Items {
-			orderLines = append(orderLines, &Line{
-				Type:           "physical",
-				Reference:      item.Sku,
-				Name:           item.Name,
-				Quantity:       item.Quantity,
-				UnitPrice:      int(item.Price),
-				TaxRate:        2500,
-				QuantityUnit:   "st",
-				TotalAmount:    int(item.TotalPrice),
-				TotalTaxAmount: int(item.TotalTax),
-				ImageURL:       fmt.Sprintf("https://www.elgiganten.se%s", item.Image),
-			})
-		}
-		for _, line := range c.Deliveries {
-			if line.Price > 0 {
-				orderLines = append(orderLines, &Line{
-					Type:           "shipping_fee",
-					Reference:      line.Provider,
-					Name:           "Delivery",
-					Quantity:       1,
-					UnitPrice:      int(line.Price),
-					TaxRate:        2500,
-					QuantityUnit:   "st",
-					TotalAmount:    int(line.Price),
-					TotalTaxAmount: int(GetTaxAmount(line.Price, 2500)),
-				})
-			}
-		}
-		order := CheckoutOrder{
-			PurchaseCountry:    "SE",
-			PurchaseCurrency:   "SEK",
-			Locale:             "sv-se",
-			OrderAmount:        int(c.TotalPrice),
-			OrderTaxAmount:     int(c.TotalTax),
-			OrderLines:         orderLines,
-			MerchantReference1: c.Id.String(),
-			MerchantURLS: &CheckoutMerchantURLS{
-				Terms:        msg.Terms,
-				Checkout:     msg.Checkout,
-				Confirmation: msg.Confirmation,
-				Validation:   msg.Validation,
-				Push:         msg.Push,
-			},
-		}
-		orderPayload, err := json.Marshal(order)
-		if err != nil {
-			return nil, err
-		}
-		var klarnaOrder *CheckoutOrder
-		var klarnaError error
-		if c.OrderReference != "" {
-			log.Printf("Updating order id %s", c.OrderReference)
-			klarnaOrder, klarnaError = KlarnaInstance.UpdateOrder(c.OrderReference, bytes.NewReader(orderPayload))
-		} else {
-			klarnaOrder, klarnaError = KlarnaInstance.CreateOrder(bytes.NewReader(orderPayload))
-		}
-		if klarnaError != nil {
-			log.Printf("error from klarna: %v", klarnaError)
-			return nil, klarnaError
-		}
-		if c.OrderReference == "" {
-			c.OrderReference = klarnaOrder.ID
-			c.PaymentStatus = klarnaOrder.Status
-		}
-		// This originally returned a FrameWithPayload; now returns the grain state.
-		// The caller (gRPC handler) is responsible for wrapping this.
-		return c, nil
-
-	case *messages.OrderCreated:
-		c.OrderReference = msg.OrderId
-		c.PaymentStatus = msg.Status
-		c.PaymentInProgress = false
-
-	default:
-		return nil, fmt.Errorf("unsupported mutation type %T", content)
+		return nil, err
 	}
-
-	// (Optional) Append to new storage mechanism here if still required.
-
-	return c, nil
+	return updated, nil
 }
 
 func (c *CartGrain) UpdateTotals() {

checkout_builder.go

@@ -0,0 +1,119 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// CheckoutMeta carries the external / URL metadata required to build a
+// Klarna CheckoutOrder from a CartGrain snapshot. It deliberately excludes
+// any Klarna-specific response fields (HTML snippet, client token, etc.).
+type CheckoutMeta struct {
+	Terms        string
+	Checkout     string
+	Confirmation string
+	Validation   string
+	Push         string
+	Country      string
+	Currency     string // optional override (defaults to "SEK" if empty)
+	Locale       string // optional override (defaults to "sv-se" if empty)
+}
+
+// BuildCheckoutOrderPayload converts the current cart grain + meta information
+// into a CheckoutOrder domain struct and returns its JSON-serialized payload
+// (to send to Klarna) alongside the structured CheckoutOrder object.
+//
+// This function is PURE: it does not perform any network I/O or mutate the
+// grain. The caller is responsible for:
+//
+//  1. Choosing whether to create or update the Klarna order.
+//  2. Invoking KlarnaClient.CreateOrder / UpdateOrder with the returned payload.
+//  3. Applying an InitializeCheckout mutation (or equivalent) with the
+//     resulting Klarna order id + status.
+//
+// If you later need to support different tax rates per line, you can extend
+// CartItem / Delivery to expose that data and propagate it here.
+func BuildCheckoutOrderPayload(grain *CartGrain, meta *CheckoutMeta) ([]byte, *CheckoutOrder, error) {
+	if grain == nil {
+		return nil, nil, fmt.Errorf("nil grain")
+	}
+	if meta == nil {
+		return nil, nil, fmt.Errorf("nil checkout meta")
+	}
+
+	currency := meta.Currency
+	if currency == "" {
+		currency = "SEK"
+	}
+	locale := meta.Locale
+	if locale == "" {
+		locale = "sv-se"
+	}
+	country := meta.Country
+	if country == "" {
+		country = "SE" // sensible default; adjust if multi-country support changes
+	}
+
+	lines := make([]*Line, 0, len(grain.Items)+len(grain.Deliveries))
+
+	// Item lines
+	for _, it := range grain.Items {
+		if it == nil {
+			continue
+		}
+		lines = append(lines, &Line{
+			Type:           "physical",
+			Reference:      it.Sku,
+			Name:           it.Name,
+			Quantity:       it.Quantity,
+			UnitPrice:      int(it.Price),
+			TaxRate:        2500, // TODO: derive if variable tax rates are introduced
+			QuantityUnit:   "st",
+			TotalAmount:    int(it.TotalPrice),
+			TotalTaxAmount: int(it.TotalTax),
+			ImageURL:       fmt.Sprintf("https://www.elgiganten.se%s", it.Image),
+		})
+	}
+
+	// Delivery lines
+	for _, d := range grain.Deliveries {
+		if d == nil || d.Price <= 0 {
+			continue
+		}
+		lines = append(lines, &Line{
+			Type:           "shipping_fee",
+			Reference:      d.Provider,
+			Name:           "Delivery",
+			Quantity:       1,
+			UnitPrice:      int(d.Price),
+			TaxRate:        2500,
+			QuantityUnit:   "st",
+			TotalAmount:    int(d.Price),
+			TotalTaxAmount: int(GetTaxAmount(d.Price, 2500)),
+		})
+	}
+
+	order := &CheckoutOrder{
+		PurchaseCountry:    country,
+		PurchaseCurrency:   currency,
+		Locale:             locale,
+		OrderAmount:        int(grain.TotalPrice),
+		OrderTaxAmount:     int(grain.TotalTax),
+		OrderLines:         lines,
+		MerchantReference1: grain.Id.String(),
+		MerchantURLS: &CheckoutMerchantURLS{
+			Terms:        meta.Terms,
+			Checkout:     meta.Checkout,
+			Confirmation: meta.Confirmation,
+			Validation:   meta.Validation,
+			Push:         meta.Push,
+		},
+	}
+
+	payload, err := json.Marshal(order)
+	if err != nil {
+		return nil, nil, fmt.Errorf("marshal checkout order: %w", err)
+	}
+
+	return payload, order, nil
+}

cookies.txt

@@ -0,0 +1,5 @@
+# Netscape HTTP Cookie File
+# https://curl.se/docs/http-cookies.html
+# This file was generated by libcurl! Edit at your own risk.
+
+#HttpOnly_localhost	FALSE	/	FALSE	1761304670	cartid	4393545184291837

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/google/uuid v1.6.0
 	github.com/matst80/slask-finder v0.0.0-20251009175145-ce05aff5a548
 	github.com/prometheus/client_golang v1.23.2
+	github.com/rabbitmq/amqp091-go v1.10.0
 	google.golang.org/grpc v1.76.0
 	google.golang.org/protobuf v1.36.10
 	k8s.io/api v0.34.1

go.sum

@@ -70,10 +70,14 @@ github.com/json-iterator/go v1.1.12 h1:PV8peI4a0ysnczrg+LtxykD8LfKY9ML6u2jnxaEnr
 github.com/json-iterator/go v1.1.12/go.mod h1:e30LSqwooZae/UwlEbR2852Gd8hjQvJoHmT4TnhNGBo=
 github.com/kisielk/errcheck v1.5.0/go.mod h1:pFxgyoBC7bSaBwPgfKdkLd5X25qrDl4LWUI2bnpBCr8=
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
+github.com/klauspost/compress v1.18.0 h1:c/Cqfb0r+Yi+JtIEq73FWXVkRonBlf0CRNYc8Zttxdo=
+github.com/klauspost/compress v1.18.0/go.mod h1:2Pp+KzxcywXVXMr50+X0Q/Lsb43OQHYWRCY2AiWywWQ=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
 github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/kylelemons/godebug v1.1.0 h1:RPNrshWIDI6G2gRW9EHilWtl7Z6Sb1BR0xunSBf0SNc=
+github.com/kylelemons/godebug v1.1.0/go.mod h1:9/0rRGxNHcop5bhtWyNeEfOS8JIWk580+fNqagV/RAw=
 github.com/matst80/slask-finder v0.0.0-20251009175145-ce05aff5a548 h1:lkxVz5lNPlU78El49cx1c3Rxo/bp7Gbzp2BjSRFgg1U=
 github.com/matst80/slask-finder v0.0.0-20251009175145-ce05aff5a548/go.mod h1:k4lo5gFYb3AqrgftlraCnv95xvjB/w8udRqJQJ12mAE=
 github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=
@@ -100,6 +104,8 @@ github.com/prometheus/common v0.67.1 h1:OTSON1P4DNxzTg4hmKCc37o4ZAZDv0cfXLkOt0oE
 github.com/prometheus/common v0.67.1/go.mod h1:RpmT9v35q2Y+lsieQsdOh5sXZ6ajUGC8NjZAmr8vb0Q=
 github.com/prometheus/procfs v0.17.0 h1:FuLQ+05u4ZI+SS/w9+BWEM2TXiHKsUQ9TADiRH7DuK0=
 github.com/prometheus/procfs v0.17.0/go.mod h1:oPQLaDAMRbA+u8H5Pbfq+dl3VDAvHxMUOVhe0wYB2zw=
+github.com/rabbitmq/amqp091-go v1.10.0 h1:STpn5XsHlHGcecLmMFCtg7mqq0RnD+zFr4uzukfVhBw=
+github.com/rabbitmq/amqp091-go v1.10.0/go.mod h1:Hy4jKW5kQART1u+JkDTF9YYOQUHXqMuhrgxOEeS7G4o=
 github.com/rogpeppe/go-internal v1.13.1 h1:KvO1DLK/DRN07sQ1LQKScxyZJuNnedQ5/wKSR38lUII=
 github.com/rogpeppe/go-internal v1.13.1/go.mod h1:uMEvuHeurkdAXX61udpOXGD/AzZDWNMNyH2VO9fmH0o=
 github.com/spf13/pflag v1.0.6 h1:jFzHGLGAlb3ruxLB8MhbI6A8+AQX/2eW4qeyNZXNp2o=

grain-pool.go

@@ -26,7 +26,7 @@ var (
 )
 
 type GrainPool interface {
-	Process(id CartId, mutations ...interface{}) (*CartGrain, error)
+	Apply(id CartId, mutation interface{}) (*CartGrain, error)
 	Get(id CartId) (*CartGrain, error)
 }
 
@@ -142,18 +142,12 @@ func (p *GrainLocalPool) GetGrain(id CartId) (*CartGrain, error) {
 	return grain, err
 }
 
-func (p *GrainLocalPool) Process(id CartId, mutations ...interface{}) (*CartGrain, error) {
+func (p *GrainLocalPool) Apply(id CartId, mutation interface{}) (*CartGrain, error) {
 	grain, err := p.GetGrain(id)
-	var result *CartGrain
-	if err == nil && grain != nil {
-		for _, m := range mutations {
-			result, err = grain.Apply(m, false)
-			if err != nil {
-				break
-			}
-		}
+	if err != nil || grain == nil {
+		return nil, err
 	}
-	return result, err
+	return grain.Apply(mutation, false)
 }
 
 func (p *GrainLocalPool) Get(id CartId) (*CartGrain, error) {

grpc_integration_test.go

@@ -12,7 +12,7 @@ import (
 
 // TestCartActorMutationAndState validates end-to-end gRPC mutation + state retrieval
 // against a locally started gRPC server (single-node scenario).
-// This test uses the oneof MutationEnvelope directly to avoid hitting external product
+// This test uses the new per-mutation AddItem RPC (breaking v2 API) to avoid external product fetch logic
 // fetching logic (FetchItem) which would require network I/O.
 func TestCartActorMutationAndState(t *testing.T) {
 	// Setup local grain pool + synced pool (no discovery, single host)
@@ -60,32 +60,29 @@ func TestCartActorMutationAndState(t *testing.T) {
 		Country:  "se",
 	}
 
-	// Build oneof envelope directly (no legacy handler/enum)
-	envelope := &messages.MutationEnvelope{
+	// Issue AddItem RPC directly (breaking v2 API)
+	addResp, err := cartClient.AddItem(context.Background(), &messages.AddItemRequest{
 		CartId:          cartID,
 		ClientTimestamp: time.Now().Unix(),
-		Mutation: &messages.MutationEnvelope_AddItem{
-			AddItem: addItem,
-		},
-	}
-
-	// Issue Mutate RPC
-	mutResp, err := cartClient.Mutate(context.Background(), envelope)
+		Payload:         addItem,
+	})
 	if err != nil {
-		t.Fatalf("Mutate RPC error: %v", err)
+		t.Fatalf("AddItem RPC error: %v", err)
 	}
-	if mutResp.StatusCode != 200 {
-		t.Fatalf("Mutate returned non-200 status: %d, error: %s", mutResp.StatusCode, mutResp.GetError())
+	if addResp.StatusCode != 200 {
+		t.Fatalf("AddItem returned non-200 status: %d, error: %s", addResp.StatusCode, addResp.GetError())
 	}
 
-	// Validate the response state
-	state := mutResp.GetState()
+	// Validate the response state (from AddItem)
+	state := addResp.GetState()
 	if state == nil {
-		t.Fatalf("Mutate response state is nil")
+		t.Fatalf("AddItem response state is nil")
 	}
 
+	// (Removed obsolete Mutate response handling)
+
 	if len(state.Items) != 1 {
-		t.Fatalf("Expected 1 item after mutation, got %d", len(state.Items))
+		t.Fatalf("Expected 1 item after AddItem, got %d", len(state.Items))
 	}
 	if state.Items[0].Sku != "test-sku" {
 		t.Fatalf("Unexpected item SKU: %s", state.Items[0].Sku)

grpc_server.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log"
 	"net"
+	"time"
 
 	messages "git.tornberg.me/go-cart-actor/proto"
 	"google.golang.org/grpc"
@@ -17,7 +18,7 @@ type cartActorGRPCServer struct {
 	messages.UnimplementedCartActorServer
 	messages.UnimplementedControlPlaneServer
 
-	pool       GrainPool  // For cart state mutations and queries
+	pool       GrainPool   // For cart state mutations and queries
 	syncedPool *SyncedPool // For cluster membership and control
 }
 
@@ -29,62 +30,125 @@ func NewCartActorGRPCServer(pool GrainPool, syncedPool *SyncedPool) *cartActorGR
 	}
 }
 
-// Mutate applies a mutation from an envelope to the corresponding cart grain.
-func (s *cartActorGRPCServer) Mutate(ctx context.Context, envelope *messages.MutationEnvelope) (*messages.MutationReply, error) {
-	if envelope.GetCartId() == "" {
-		return &messages.MutationReply{
-			StatusCode: 400,
-			Result:     &messages.MutationReply_Error{Error: "cart_id is required"},
-		}, nil
-	}
-	cartID := ToCartId(envelope.GetCartId())
-
-	var mutation interface{}
-	switch m := envelope.Mutation.(type) {
-	case *messages.MutationEnvelope_AddRequest:
-		mutation = m.AddRequest
-	case *messages.MutationEnvelope_AddItem:
-		mutation = m.AddItem
-	case *messages.MutationEnvelope_RemoveItem:
-		mutation = m.RemoveItem
-	case *messages.MutationEnvelope_RemoveDelivery:
-		mutation = m.RemoveDelivery
-	case *messages.MutationEnvelope_ChangeQuantity:
-		mutation = m.ChangeQuantity
-	case *messages.MutationEnvelope_SetDelivery:
-		mutation = m.SetDelivery
-	case *messages.MutationEnvelope_SetPickupPoint:
-		mutation = m.SetPickupPoint
-	case *messages.MutationEnvelope_CreateCheckoutOrder:
-		mutation = m.CreateCheckoutOrder
-	case *messages.MutationEnvelope_SetCartItems:
-		mutation = m.SetCartItems
-	case *messages.MutationEnvelope_OrderCompleted:
-		mutation = m.OrderCompleted
-	default:
-		return &messages.MutationReply{
-			StatusCode: 400,
-			Result:     &messages.MutationReply_Error{Error: fmt.Sprintf("unsupported mutation type: %T", m)},
-		}, nil
-	}
-
-	// Delegate the mutation to the grain pool.
-	// The pool is responsible for routing it to the correct grain (local or remote).
-	grain, err := s.pool.Process(cartID, mutation)
+// applyMutation routes a single cart mutation to the target grain (used by per-mutation RPC handlers).
+func (s *cartActorGRPCServer) applyMutation(cartID string, mutation interface{}) *messages.CartMutationReply {
+	grain, err := s.pool.Apply(ToCartId(cartID), mutation)
 	if err != nil {
-		return &messages.MutationReply{
-			StatusCode: 500,
-			Result:     &messages.MutationReply_Error{Error: err.Error()},
+		return &messages.CartMutationReply{
+			StatusCode:      500,
+			Result:          &messages.CartMutationReply_Error{Error: err.Error()},
+			ServerTimestamp: time.Now().Unix(),
+		}
+	}
+	cartState := ToCartState(grain)
+	return &messages.CartMutationReply{
+		StatusCode:      200,
+		Result:          &messages.CartMutationReply_State{State: cartState},
+		ServerTimestamp: time.Now().Unix(),
+	}
+}
+
+func (s *cartActorGRPCServer) AddRequest(ctx context.Context, req *messages.AddRequestRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
 		}, nil
 	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
+}
 
-	// Map the internal grain state to the protobuf representation.
-	cartState := ToCartState(grain)
+func (s *cartActorGRPCServer) AddItem(ctx context.Context, req *messages.AddItemRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
+		}, nil
+	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
+}
 
-	return &messages.MutationReply{
-		StatusCode: 200,
-		Result:     &messages.MutationReply_State{State: cartState},
-	}, nil
+func (s *cartActorGRPCServer) RemoveItem(ctx context.Context, req *messages.RemoveItemRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
+		}, nil
+	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
+}
+
+func (s *cartActorGRPCServer) RemoveDelivery(ctx context.Context, req *messages.RemoveDeliveryRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
+		}, nil
+	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
+}
+
+func (s *cartActorGRPCServer) ChangeQuantity(ctx context.Context, req *messages.ChangeQuantityRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
+		}, nil
+	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
+}
+
+func (s *cartActorGRPCServer) SetDelivery(ctx context.Context, req *messages.SetDeliveryRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
+		}, nil
+	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
+}
+
+func (s *cartActorGRPCServer) SetPickupPoint(ctx context.Context, req *messages.SetPickupPointRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
+		}, nil
+	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
+}
+
+/*
+Checkout RPC removed. Checkout is handled at the HTTP layer (PoolServer.HandleCheckout).
+*/
+
+func (s *cartActorGRPCServer) SetCartItems(ctx context.Context, req *messages.SetCartItemsRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
+		}, nil
+	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
+}
+
+func (s *cartActorGRPCServer) OrderCompleted(ctx context.Context, req *messages.OrderCompletedRequest) (*messages.CartMutationReply, error) {
+	if req.GetCartId() == "" {
+		return &messages.CartMutationReply{
+			StatusCode:      400,
+			Result:          &messages.CartMutationReply_Error{Error: "cart_id is required"},
+			ServerTimestamp: time.Now().Unix(),
+		}, nil
+	}
+	return s.applyMutation(req.GetCartId(), req.GetPayload()), nil
 }
 
 // GetState retrieves the current state of a cart grain.
@@ -136,4 +200,4 @@ func StartGRPCServer(addr string, pool GrainPool, syncedPool *SyncedPool) (*grpc
 	}()
 
 	return grpcServer, nil
-}
+}

main.go

@@ -1,15 +1,21 @@
 package main
 
 import (
+	"encoding/json"
 	"fmt"
 	"log"
+	"net/http"
+	"net/http/pprof"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"time"
 
+	messages "git.tornberg.me/go-cart-actor/proto"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promauto"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"k8s.io/client-go/kubernetes"
 	"k8s.io/client-go/rest"
 )
@@ -76,13 +82,60 @@ func (a *App) Save() error {
 	return a.storage.saveState()
 }
 
-
+func (a *App) HandleSave(w http.ResponseWriter, r *http.Request) {
+	err := a.Save()
+	if err != nil {
+		w.WriteHeader(http.StatusInternalServerError)
+		w.Write([]byte(err.Error()))
+	} else {
+		w.WriteHeader(http.StatusCreated)
+	}
+}
 
 var podIp = os.Getenv("POD_IP")
 var name = os.Getenv("POD_NAME")
 var amqpUrl = os.Getenv("AMQP_URL")
 var KlarnaInstance = NewKlarnaClient(KlarnaPlaygroundUrl, os.Getenv("KLARNA_API_USERNAME"), os.Getenv("KLARNA_API_PASSWORD"))
 
+var tpl = `<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>s10r testing - checkout</title>
+  </head>
+
+  <body>
+    %s
+  </body>
+</html>
+`
+
+func getCountryFromHost(host string) string {
+	if strings.Contains(strings.ToLower(host), "-no") {
+		return "no"
+	}
+	return "se"
+}
+
+func getCheckoutOrder(host string, cartId CartId) *messages.CreateCheckoutOrder {
+	baseUrl := fmt.Sprintf("https://%s", host)
+	cartBaseUrl := os.Getenv("CART_BASE_URL")
+	if cartBaseUrl == "" {
+		cartBaseUrl = "https://cart.tornberg.me"
+	}
+	country := getCountryFromHost(host)
+
+	return &messages.CreateCheckoutOrder{
+		Terms:        fmt.Sprintf("%s/terms", baseUrl),
+		Checkout:     fmt.Sprintf("%s/checkout?order_id={checkout.order.id}", baseUrl),
+		Confirmation: fmt.Sprintf("%s/confirmation/{checkout.order.id}", baseUrl),
+		Validation:   fmt.Sprintf("%s/validation", cartBaseUrl),
+		Push:         fmt.Sprintf("%s/push?order_id={checkout.order.id}", cartBaseUrl),
+		Country:      country,
+	}
+}
+
 func GetDiscovery() Discovery {
 	if podIp == "" {
 		return nil
@@ -100,8 +153,6 @@ func GetDiscovery() Discovery {
 	return NewK8sDiscovery(client)
 }
 
-
-
 func main() {
 
 	storage, err := NewDiskStorage(fmt.Sprintf("data/%s_state.gob", name))
@@ -134,7 +185,185 @@ func main() {
 			}
 		}
 	}()
+	orderHandler := &AmqpOrderHandler{
+		Url: amqpUrl,
+	}
 
+	syncedServer := NewPoolServer(syncedPool, fmt.Sprintf("%s, %s", name, podIp))
+	mux := http.NewServeMux()
+	mux.Handle("/cart/", http.StripPrefix("/cart", syncedServer.Serve()))
+	// only for local
+	// mux.HandleFunc("GET /add/remote/{host}", func(w http.ResponseWriter, r *http.Request) {
+	// 	syncedPool.AddRemote(r.PathValue("host"))
+	// })
+	// mux.HandleFunc("GET /save", app.HandleSave)
+	//mux.HandleFunc("/", app.RewritePath)
+	mux.HandleFunc("/debug/pprof/", pprof.Index)
+	mux.HandleFunc("/debug/pprof/cmdline", pprof.Cmdline)
+	mux.HandleFunc("/debug/pprof/profile", pprof.Profile)
+	mux.HandleFunc("/debug/pprof/symbol", pprof.Symbol)
+	mux.HandleFunc("/debug/pprof/trace", pprof.Trace)
+	mux.Handle("/metrics", promhttp.Handler())
+	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
+		// Grain pool health: simple capacity check (mirrors previous GrainHandler.IsHealthy)
+		app.pool.mu.RLock()
+		grainCount := len(app.pool.grains)
+		capacity := app.pool.PoolSize
+		app.pool.mu.RUnlock()
+		if grainCount >= capacity {
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte("grain pool at capacity"))
+			return
+		}
+		if !syncedPool.IsHealthy() {
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte("control plane not healthy"))
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("ok"))
+	})
+	mux.HandleFunc("/readyz", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("ok"))
+	})
+	mux.HandleFunc("/livez", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("ok"))
+	})
+
+	mux.HandleFunc("/checkout", func(w http.ResponseWriter, r *http.Request) {
+		orderId := r.URL.Query().Get("order_id")
+		order := &CheckoutOrder{}
+
+		if orderId == "" {
+			cookie, err := r.Cookie("cartid")
+			if err != nil {
+				w.WriteHeader(http.StatusBadRequest)
+				w.Write([]byte(err.Error()))
+				return
+			}
+			if cookie.Value == "" {
+				w.WriteHeader(http.StatusBadRequest)
+				w.Write([]byte("no cart id to checkout is empty"))
+				return
+			}
+			cartId := ToCartId(cookie.Value)
+			_, err = syncedServer.pool.Apply(cartId, getCheckoutOrder(r.Host, cartId))
+			if err != nil {
+				w.WriteHeader(http.StatusInternalServerError)
+				w.Write([]byte(err.Error()))
+			}
+			// v2: Apply now returns *CartGrain; order creation handled inside grain (no payload to unmarshal)
+		} else {
+			prevOrder, err := KlarnaInstance.GetOrder(orderId)
+			if err != nil {
+				w.WriteHeader(http.StatusBadRequest)
+				w.Write([]byte(err.Error()))
+				return
+			}
+			order = prevOrder
+		}
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		w.Header().Set("Permissions-Policy", "payment=(self \"https://js.stripe.com\" \"https://m.stripe.network\" \"https://js.playground.kustom.co\")")
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(fmt.Sprintf(tpl, order.HTMLSnippet)))
+	})
+	mux.HandleFunc("/confirmation/{order_id}", func(w http.ResponseWriter, r *http.Request) {
+
+		orderId := r.PathValue("order_id")
+		order, err := KlarnaInstance.GetOrder(orderId)
+
+		if err != nil {
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte(err.Error()))
+			return
+		}
+
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		if order.Status == "checkout_complete" {
+			http.SetCookie(w, &http.Cookie{
+				Name:     "cartid",
+				Value:    "",
+				Path:     "/",
+				Secure:   true,
+				HttpOnly: true,
+				Expires:  time.Unix(0, 0),
+				SameSite: http.SameSiteLaxMode,
+			})
+		}
+
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(fmt.Sprintf(tpl, order.HTMLSnippet)))
+	})
+	mux.HandleFunc("/validate", func(w http.ResponseWriter, r *http.Request) {
+		log.Printf("Klarna order validation, method: %s", r.Method)
+		if r.Method != "POST" {
+			w.WriteHeader(http.StatusMethodNotAllowed)
+			return
+		}
+		order := &CheckoutOrder{}
+		err := json.NewDecoder(r.Body).Decode(order)
+		if err != nil {
+			w.WriteHeader(http.StatusBadRequest)
+		}
+		log.Printf("Klarna order validation: %s", order.ID)
+		//err = confirmOrder(order, orderHandler)
+		//if err != nil {
+		//	log.Printf("Error validating order: %v\n", err)
+		//	w.WriteHeader(http.StatusInternalServerError)
+		//	return
+		//}
+		//
+		//err = triggerOrderCompleted(err, syncedServer, order)
+		//if err != nil {
+		//	log.Printf("Error processing cart message: %v\n", err)
+		//	w.WriteHeader(http.StatusInternalServerError)
+		//	return
+		//}
+		w.WriteHeader(http.StatusOK)
+	})
+	mux.HandleFunc("/push", func(w http.ResponseWriter, r *http.Request) {
+
+		if r.Method != http.MethodPost {
+			w.WriteHeader(http.StatusMethodNotAllowed)
+			return
+		}
+		orderId := r.URL.Query().Get("order_id")
+		log.Printf("Order confirmation push: %s", orderId)
+
+		order, err := KlarnaInstance.GetOrder(orderId)
+
+		if err != nil {
+			log.Printf("Error creating request: %v\n", err)
+			w.WriteHeader(http.StatusInternalServerError)
+			return
+		}
+
+		err = confirmOrder(order, orderHandler)
+		if err != nil {
+			log.Printf("Error confirming order: %v\n", err)
+			w.WriteHeader(http.StatusInternalServerError)
+			return
+		}
+
+		err = triggerOrderCompleted(err, syncedServer, order)
+		if err != nil {
+			log.Printf("Error processing cart message: %v\n", err)
+			w.WriteHeader(http.StatusInternalServerError)
+			return
+		}
+		err = KlarnaInstance.AcknowledgeOrder(orderId)
+		if err != nil {
+			log.Printf("Error acknowledging order: %v\n", err)
+		}
+
+		w.WriteHeader(http.StatusOK)
+	})
+	mux.HandleFunc("/version", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("1.0.0"))
+	})
 
 	sigs := make(chan os.Signal, 1)
 	done := make(chan bool, 1)
@@ -148,7 +377,34 @@ func main() {
 		done <- true
 	}()
 
-	log.Print("Server started at port 1337")
+	log.Print("Server started at port 8083")
+	go http.ListenAndServe(":8083", mux)
 	<-done
 
 }
+
+func triggerOrderCompleted(err error, syncedServer *PoolServer, order *CheckoutOrder) error {
+	_, err = syncedServer.pool.Apply(ToCartId(order.MerchantReference1), &messages.OrderCreated{
+		OrderId: order.ID,
+		Status:  order.Status,
+	})
+	return err
+}
+
+func confirmOrder(order *CheckoutOrder, orderHandler *AmqpOrderHandler) error {
+	orderToSend, err := json.Marshal(order)
+	if err != nil {
+		return err
+	}
+	err = orderHandler.Connect()
+	if err != nil {
+		return err
+	}
+	defer orderHandler.Close()
+	err = orderHandler.OrderCompleted(orderToSend)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}

mutation_add_item.go

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_add_item.go
+//
+// Registers the AddItem cart mutation in the generic mutation registry.
+// This replaces the legacy switch-based logic previously found in CartGrain.Apply.
+//
+// Behavior:
+//   * Validates quantity > 0
+//   * If an item with same SKU exists -> increases quantity
+//   * Else creates a new CartItem with computed tax amounts
+//   * Totals recalculated automatically via WithTotals()
+//
+// NOTE: Any future field additions in messages.AddItem that affect pricing / tax
+// must keep this handler in sync.
+
+func init() {
+	RegisterMutation[messages.AddItem](
+		"AddItem",
+		func(g *CartGrain, m *messages.AddItem) error {
+			if m == nil {
+				return fmt.Errorf("AddItem: nil payload")
+			}
+			if m.Quantity < 1 {
+				return fmt.Errorf("AddItem: invalid quantity %d", m.Quantity)
+			}
+
+			// Fast path: merge with existing item having same SKU
+			if existing, found := g.FindItemWithSku(m.Sku); found {
+				existing.Quantity += int(m.Quantity)
+				return nil
+			}
+
+			g.mu.Lock()
+			defer g.mu.Unlock()
+
+			g.lastItemId++
+			taxRate := 2500
+			if m.Tax > 0 {
+				taxRate = int(m.Tax)
+			}
+			taxAmountPerUnit := GetTaxAmount(m.Price, taxRate)
+
+			g.Items = append(g.Items, &CartItem{
+				Id:          g.lastItemId,
+				ItemId:      int(m.ItemId),
+				Quantity:    int(m.Quantity),
+				Sku:         m.Sku,
+				Name:        m.Name,
+				Price:       m.Price,
+				TotalPrice:  m.Price * int64(m.Quantity),
+				TotalTax:    int64(taxAmountPerUnit * int64(m.Quantity)),
+				Image:       m.Image,
+				Stock:       StockStatus(m.Stock),
+				Disclaimer:  m.Disclaimer,
+				Brand:       m.Brand,
+				Category:    m.Category,
+				Category2:   m.Category2,
+				Category3:   m.Category3,
+				Category4:   m.Category4,
+				Category5:   m.Category5,
+				OrgPrice:    m.OrgPrice,
+				ArticleType: m.ArticleType,
+				Outlet:      m.Outlet,
+				SellerId:    m.SellerId,
+				SellerName:  m.SellerName,
+				Tax:         int(taxAmountPerUnit),
+				TaxRate:     taxRate,
+				StoreId:     m.StoreId,
+			})
+
+			return nil
+		},
+		WithTotals(), // Recalculate totals after successful mutation
+	)
+}

mutation_add_request.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_add_request.go
+//
+// Registers the AddRequest mutation. This mutation is a higher-level intent
+// (add by SKU + quantity) which may translate into either:
+//   - Increasing quantity of an existing line (same SKU), OR
+//   - Creating a new item by performing a product lookup (via getItemData inside CartGrain.AddItem)
+//
+// Behavior:
+//   - Validates non-empty SKU and quantity > 0
+//   - If an item with the SKU already exists: increments its quantity
+//   - Else delegates to CartGrain.AddItem (which itself produces an AddItem mutation)
+//   - Totals recalculated automatically (WithTotals)
+//
+// NOTE:
+//   - This handler purposely avoids duplicating the detailed AddItem logic;
+//     it reuses CartGrain.AddItem which then flows through the AddItem mutation
+//     registry handler.
+//   - Double total recalculation can occur (AddItem has WithTotals too), but
+//     is acceptable for clarity. Optimize later if needed.
+//
+// Potential future improvements:
+//   - Stock validation before increasing quantity
+//   - Reservation logic or concurrency guards around stock updates
+//   - Coupon / pricing rules applied conditionally during add-by-sku
+func init() {
+	RegisterMutation[messages.AddRequest](
+		"AddRequest",
+		func(g *CartGrain, m *messages.AddRequest) error {
+			if m == nil {
+				return fmt.Errorf("AddRequest: nil payload")
+			}
+			if m.Sku == "" {
+				return fmt.Errorf("AddRequest: sku is empty")
+			}
+			if m.Quantity < 1 {
+				return fmt.Errorf("AddRequest: invalid quantity %d", m.Quantity)
+			}
+
+			// Existing line: accumulate quantity only.
+			if existing, found := g.FindItemWithSku(m.Sku); found {
+				existing.Quantity += int(m.Quantity)
+				return nil
+			}
+
+			// New line: delegate to higher-level AddItem flow (product lookup).
+			// We intentionally ignore the returned *CartGrain; registry will
+			// do totals again after this handler returns (harmless).
+			_, err := g.AddItem(m.Sku, int(m.Quantity), m.Country, m.StoreId)
+			return err
+		},
+		WithTotals(),
+	)
+}

mutation_change_quantity.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_change_quantity.go
+//
+// Registers the ChangeQuantity mutation.
+//
+// Behavior:
+//   - Locates an item by its cart-local line item Id (not source item_id).
+//   - If requested quantity <= 0 the line is removed.
+//   - Otherwise the line's Quantity field is updated.
+//   - Totals are recalculated (WithTotals).
+//
+// Error handling:
+//   - Returns an error if the item Id is not found.
+//   - Returns an error if payload is nil (defensive).
+//
+// Concurrency:
+//   - Uses the grain's RW-safe mutation pattern: we mutate in place under
+//     the grain's implicit expectation that higher layers control access.
+//     (If strict locking is required around every mutation, wrap logic in
+//     an explicit g.mu.Lock()/Unlock(), but current model mirrors prior code.)
+func init() {
+	RegisterMutation[messages.ChangeQuantity](
+		"ChangeQuantity",
+		func(g *CartGrain, m *messages.ChangeQuantity) error {
+			if m == nil {
+				return fmt.Errorf("ChangeQuantity: nil payload")
+			}
+
+			foundIndex := -1
+			for i, it := range g.Items {
+				if it.Id == int(m.Id) {
+					foundIndex = i
+					break
+				}
+			}
+			if foundIndex == -1 {
+				return fmt.Errorf("ChangeQuantity: item id %d not found", m.Id)
+			}
+
+			if m.Quantity <= 0 {
+				// Remove the item
+				g.Items = append(g.Items[:foundIndex], g.Items[foundIndex+1:]...)
+				return nil
+			}
+
+			g.Items[foundIndex].Quantity = int(m.Quantity)
+			return nil
+		},
+		WithTotals(),
+	)
+}

mutation_initialize_checkout.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_initialize_checkout.go
+//
+// Registers the InitializeCheckout mutation.
+// This mutation is invoked AFTER an external Klarna checkout session
+// has been successfully created or updated. It persists the Klarna
+// order reference / status and marks the cart as having a payment in progress.
+//
+// Behavior:
+//   - Sets OrderReference to the Klarna order ID (overwriting if already set).
+//   - Sets PaymentStatus to the current Klarna status.
+//   - Sets / updates PaymentInProgress flag.
+//   - Does NOT alter pricing or line items (so no totals recalculation).
+//
+// Validation:
+//   - Returns an error if payload is nil.
+//   - Returns an error if orderId is empty (integrity guard).
+//
+// Concurrency:
+//   - Relies on upstream mutation serialization for a single grain. If
+//     parallel checkout attempts are possible, add higher-level guards
+//     (e.g. reject if PaymentInProgress already true unless reusing
+//     the same OrderReference).
+func init() {
+	RegisterMutation[messages.InitializeCheckout](
+		"InitializeCheckout",
+		func(g *CartGrain, m *messages.InitializeCheckout) error {
+			if m == nil {
+				return fmt.Errorf("InitializeCheckout: nil payload")
+			}
+			if m.OrderId == "" {
+				return fmt.Errorf("InitializeCheckout: missing orderId")
+			}
+
+			g.OrderReference = m.OrderId
+			g.PaymentStatus = m.Status
+			g.PaymentInProgress = m.PaymentInProgress
+			return nil
+		},
+		// No WithTotals(): monetary aggregates are unaffected.
+	)
+}

mutation_order_created.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_order_created.go
+//
+// Registers the OrderCreated mutation.
+//
+// This mutation represents the completion (or state transition) of an order
+// initiated earlier via InitializeCheckout / external Klarna processing.
+// It finalizes (or updates) the cart's order metadata.
+//
+// Behavior:
+//   - Validates payload non-nil and OrderId not empty.
+//   - Sets (or overwrites) OrderReference with the provided OrderId.
+//   - Sets PaymentStatus from payload.Status.
+//   - Marks PaymentInProgress = false (checkout flow finished / acknowledged).
+//   - Does NOT adjust monetary totals (no WithTotals()).
+//
+// Notes / Future Extensions:
+//   - If multiple order completion events can arrive (e.g., retries / webhook
+//     replays), this handler is idempotent: it simply overwrites fields.
+//   - If you need to guard against conflicting order IDs, add a check:
+//     if g.OrderReference != "" && g.OrderReference != m.OrderId { ... }
+//   - Add audit logging or metrics here if required.
+//
+// Concurrency:
+//   - Relies on the higher-level guarantee that Apply() calls are serialized
+//     per grain. If out-of-order events are possible, embed versioning or
+//     timestamps in the mutation and compare before applying changes.
+func init() {
+	RegisterMutation[messages.OrderCreated](
+		"OrderCreated",
+		func(g *CartGrain, m *messages.OrderCreated) error {
+			if m == nil {
+				return fmt.Errorf("OrderCreated: nil payload")
+			}
+			if m.OrderId == "" {
+				return fmt.Errorf("OrderCreated: missing orderId")
+			}
+
+			g.OrderReference = m.OrderId
+			g.PaymentStatus = m.Status
+			g.PaymentInProgress = false
+			return nil
+		},
+		// No WithTotals(): order completion does not modify pricing or taxes.
+	)
+}

mutation_registry.go

@@ -0,0 +1,301 @@
+package main
+
+import (
+	"fmt"
+	"reflect"
+	"sync"
+)
+
+// mutation_registry.go
+//
+// Mutation Registry Infrastructure
+// --------------------------------
+// This file introduces a generic registry for cart mutations that:
+//
+// 1. Decouples mutation logic from the large type-switch inside CartGrain.Apply.
+// 2. Enforces (at registration time) that every mutation handler has the correct
+//    signature: func(*CartGrain, *T) error
+// 3. Optionally auto-updates cart totals after a mutation if flagged.
+// 4. Provides a single authoritative list of registered mutations for
+//    introspection / coverage testing.
+// 5. Allows incremental migration: you can first register new mutations here,
+//    and later prune the legacy switch cases.
+//
+// Usage Pattern
+// -------------
+//   // Define your mutation proto message (e.g. messages.ApplyCoupon in messages.proto)
+//   // Regenerate protobufs.
+//
+//   // In an init() (ideally in a small file like mutations_apply_coupon.go)
+//   func init() {
+//       RegisterMutation[*messages.ApplyCoupon](
+//           "ApplyCoupon",
+//           func(g *CartGrain, m *messages.ApplyCoupon) error {
+//               // domain logic ...
+//               discount := int64(5000)
+//               if g.TotalPrice < discount {
+//                   discount = g.TotalPrice
+//               }
+//               g.TotalDiscount += discount
+//               g.TotalPrice -= discount
+//               return nil
+//           },
+//           WithTotals(), // we changed price-related fields; recalc totals
+//       )
+//   }
+//
+//   // To invoke dynamically (alternative to the current switch):
+//   if updated, err := ApplyRegistered(grain, incomingMessage); err == nil {
+//       grain = updated
+//   } else if errors.Is(err, ErrMutationNotRegistered) {
+//       // fallback to legacy switch logic
+//   }
+//
+// Migration Strategy
+// ------------------
+// 1. For each existing mutation handled in CartGrain.Apply, add a registry
+//    registration with equivalent logic.
+// 2. Add a test that enumerates all *expected* mutation proto types and asserts
+//    they are present in RegisteredMutationTypes().
+// 3. Once coverage is 100%, replace the switch in CartGrain.Apply with a call
+//    to ApplyRegistered (and optionally keep a minimal default to produce an
+//    "unsupported mutation" error).
+//
+// Thread Safety
+// -------------
+// Registration is typically done at init() time; a RWMutex provides safety
+// should late dynamic registration ever be introduced.
+//
+// Auto Totals
+// -----------
+// Many mutations require recomputing totals. To avoid forgetting this, pass
+// WithTotals() when registering. This will invoke grain.UpdateTotals() after
+// the handler returns successfully.
+//
+// Error Semantics
+// ---------------
+// - If a handler returns an error, totals are NOT recalculated (even if
+//   WithTotals() was specified).
+// - ApplyRegistered returns (nil, ErrMutationNotRegistered) if the message type
+//   is absent.
+//
+// Extensibility
+// -------------
+// It is straightforward to add options like audit hooks, metrics wrappers,
+// or optimistic concurrency guards by extending MutationOption.
+//
+// NOTE: Generics require Go 1.18+. If constrained to earlier Go versions,
+//       replace the generic registration with a non-generic RegisterMutationType
+//       that accepts reflect.Type and an adapter function.
+//
+// ---------------------------------------------------------------------------
+
+var (
+	mutationRegistryMu sync.RWMutex
+	mutationRegistry   = make(map[reflect.Type]*registeredMutation)
+
+	// ErrMutationNotRegistered is returned when no handler exists for a given mutation type.
+	ErrMutationNotRegistered = fmt.Errorf("mutation not registered")
+)
+
+// MutationOption configures additional behavior for a registered mutation.
+type MutationOption func(*mutationOptions)
+
+// mutationOptions holds flags adjustable per registration.
+type mutationOptions struct {
+	updateTotals bool
+}
+
+// WithTotals ensures CartGrain.UpdateTotals() is called after a successful handler.
+func WithTotals() MutationOption {
+	return func(o *mutationOptions) {
+		o.updateTotals = true
+	}
+}
+
+// registeredMutation stores metadata + the execution closure.
+type registeredMutation struct {
+	name         string
+	handler      func(*CartGrain, interface{}) error
+	updateTotals bool
+	msgType      reflect.Type
+}
+
+// RegisterMutation registers a mutation handler for a specific message type T.
+//
+// Parameters:
+//
+//	name    - a human-readable identifier (used for diagnostics / coverage tests).
+//	handler - business logic operating on the cart grain & strongly typed message.
+//	options - optional behavior flags (e.g., WithTotals()).
+//
+// Panics if:
+//   - name is empty
+//   - handler is nil
+//   - duplicate registration for the same message type T
+//
+// Typical call is placed in an init() function.
+func RegisterMutation[T any](name string, handler func(*CartGrain, *T) error, options ...MutationOption) {
+	if name == "" {
+		panic("RegisterMutation: name is required")
+	}
+	if handler == nil {
+		panic("RegisterMutation: handler is nil")
+	}
+
+	// Derive the reflect.Type for *T then its Elem (T) for mapping.
+	var zero *T
+	rtPtr := reflect.TypeOf(zero)
+	if rtPtr.Kind() != reflect.Ptr {
+		panic("RegisterMutation: expected pointer type for generic parameter")
+	}
+	rt := rtPtr.Elem()
+
+	opts := mutationOptions{}
+	for _, opt := range options {
+		opt(&opts)
+	}
+
+	wrapped := func(g *CartGrain, m interface{}) error {
+		typed, ok := m.(*T)
+		if !ok {
+			return fmt.Errorf("mutation type mismatch: have %T want *%s", m, rt.Name())
+		}
+		return handler(g, typed)
+	}
+
+	mutationRegistryMu.Lock()
+	defer mutationRegistryMu.Unlock()
+
+	if _, exists := mutationRegistry[rt]; exists {
+		panic(fmt.Sprintf("RegisterMutation: duplicate registration for type %s", rt.String()))
+	}
+
+	mutationRegistry[rt] = &registeredMutation{
+		name:         name,
+		handler:      wrapped,
+		updateTotals: opts.updateTotals,
+		msgType:      rt,
+	}
+}
+
+// ApplyRegistered attempts to apply a registered mutation.
+// Returns updated grain if successful.
+//
+// If the mutation is not registered, returns (nil, ErrMutationNotRegistered).
+func ApplyRegistered(grain *CartGrain, msg interface{}) (*CartGrain, error) {
+	if grain == nil {
+		return nil, fmt.Errorf("nil grain")
+	}
+	if msg == nil {
+		return nil, fmt.Errorf("nil mutation message")
+	}
+
+	rt := indirectType(reflect.TypeOf(msg))
+	mutationRegistryMu.RLock()
+	entry, ok := mutationRegistry[rt]
+	mutationRegistryMu.RUnlock()
+
+	if !ok {
+		return nil, ErrMutationNotRegistered
+	}
+
+	if err := entry.handler(grain, msg); err != nil {
+		return nil, err
+	}
+
+	if entry.updateTotals {
+		grain.UpdateTotals()
+	}
+
+	return grain, nil
+}
+
+// RegisteredMutations returns metadata for all registered mutations (snapshot).
+func RegisteredMutations() []string {
+	mutationRegistryMu.RLock()
+	defer mutationRegistryMu.RUnlock()
+	out := make([]string, 0, len(mutationRegistry))
+	for _, entry := range mutationRegistry {
+		out = append(out, entry.name)
+	}
+	return out
+}
+
+// RegisteredMutationTypes returns the reflect.Type list of all registered messages.
+// Useful for coverage tests ensuring expected set matches actual set.
+func RegisteredMutationTypes() []reflect.Type {
+	mutationRegistryMu.RLock()
+	defer mutationRegistryMu.RUnlock()
+	out := make([]reflect.Type, 0, len(mutationRegistry))
+	for t := range mutationRegistry {
+		out = append(out, t)
+	}
+	return out
+}
+
+// MustAssertMutationCoverage can be called at startup to ensure every expected
+// mutation type has been registered. It panics with a descriptive message if any
+// are missing. Provide a slice of prototype pointers (e.g. []*messages.AddItem{nil} ...)
+func MustAssertMutationCoverage(expected []interface{}) {
+	mutationRegistryMu.RLock()
+	defer mutationRegistryMu.RUnlock()
+
+	missing := make([]string, 0)
+	for _, ex := range expected {
+		if ex == nil {
+			continue
+		}
+		t := indirectType(reflect.TypeOf(ex))
+		if _, ok := mutationRegistry[t]; !ok {
+			missing = append(missing, t.String())
+		}
+	}
+	if len(missing) > 0 {
+		panic(fmt.Sprintf("mutation registry missing handlers for: %v", missing))
+	}
+}
+
+// indirectType returns the element type if given a pointer; otherwise the type itself.
+func indirectType(t reflect.Type) reflect.Type {
+	for t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+	return t
+}
+
+/*
+Integration Guide
+-----------------
+
+1. Register all existing mutations:
+
+   func init() {
+       RegisterMutation[*messages.AddItem]("AddItem",
+           func(g *CartGrain, m *messages.AddItem) error {
+               // (port logic from existing switch branch)
+               // ...
+               return nil
+           },
+           WithTotals(),
+       )
+       // ... repeat for others
+   }
+
+2. In CartGrain.Apply (early in the method) add:
+
+   if updated, err := ApplyRegistered(c, content); err == nil {
+       return updated, nil
+   } else if err != ErrMutationNotRegistered {
+       return nil, err
+   }
+
+   // existing switch fallback below
+
+3. Once all mutations are registered, remove the legacy switch cases
+   and leave a single ErrMutationNotRegistered path for unknown types.
+
+4. Add a coverage test (see docs for example; removed from source for clarity).
+5. (Optional) Add metrics / tracing wrappers for handlers.
+
+*/

mutation_remove_delivery.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_remove_delivery.go
+//
+// Registers the RemoveDelivery mutation.
+//
+// Behavior:
+//   - Removes the delivery entry whose Id == payload.Id.
+//   - If not found, returns an error.
+//   - Cart totals are recalculated (WithTotals) after removal.
+//   - Items previously associated with that delivery simply become "without delivery";
+//     subsequent delivery mutations can reassign them.
+//
+// Differences vs legacy:
+//   - Legacy logic decremented TotalPrice explicitly before recalculating.
+//     Here we rely solely on UpdateTotals() to recompute from remaining
+//     deliveries and items (simpler / single source of truth).
+//
+// Future considerations:
+//   - If delivery pricing logic changes (e.g., dynamic taxes per delivery),
+//     UpdateTotals() may need enhancement to incorporate delivery tax properly.
+func init() {
+	RegisterMutation[messages.RemoveDelivery](
+		"RemoveDelivery",
+		func(g *CartGrain, m *messages.RemoveDelivery) error {
+			if m == nil {
+				return fmt.Errorf("RemoveDelivery: nil payload")
+			}
+			targetID := int(m.Id)
+			index := -1
+			for i, d := range g.Deliveries {
+				if d.Id == targetID {
+					index = i
+					break
+				}
+			}
+			if index == -1 {
+				return fmt.Errorf("RemoveDelivery: delivery id %d not found", m.Id)
+			}
+
+			// Remove delivery (order not preserved beyond necessity)
+			g.Deliveries = append(g.Deliveries[:index], g.Deliveries[index+1:]...)
+			return nil
+		},
+		WithTotals(),
+	)
+}

mutation_remove_item.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_remove_item.go
+//
+// Registers the RemoveItem mutation.
+//
+// Behavior:
+//   - Removes the cart line whose local cart line Id == payload.Id
+//   - If no such line exists returns an error
+//   - Recalculates cart totals (WithTotals)
+//
+// Notes:
+//   - This removes only the line item; any deliveries referencing the removed
+//     item are NOT automatically adjusted (mirrors prior logic). If future
+//     semantics require pruning delivery.item_ids you can extend this handler.
+//   - If multiple lines somehow shared the same Id (should not happen), only
+//     the first match would be removed—data integrity relies on unique line Ids.
+func init() {
+	RegisterMutation[messages.RemoveItem](
+		"RemoveItem",
+		func(g *CartGrain, m *messages.RemoveItem) error {
+			if m == nil {
+				return fmt.Errorf("RemoveItem: nil payload")
+			}
+			targetID := int(m.Id)
+
+			index := -1
+			for i, it := range g.Items {
+				if it.Id == targetID {
+					index = i
+					break
+				}
+			}
+			if index == -1 {
+				return fmt.Errorf("RemoveItem: item id %d not found", m.Id)
+			}
+
+			g.Items = append(g.Items[:index], g.Items[index+1:]...)
+			return nil
+		},
+		WithTotals(),
+	)
+}

mutation_set_cart_items.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_set_cart_items.go
+//
+// Registers the SetCartRequest mutation. This mutation replaces the entire list
+// of cart items with the provided list (each entry is an AddRequest).
+//
+// Behavior:
+//   - Clears existing items (but leaves deliveries intact).
+//   - Iterates over each AddRequest and delegates to CartGrain.AddItem
+//     (which performs product lookup, creates AddItem mutation).
+//   - If any single addition fails, the mutation aborts with an error;
+//     items added prior to the failure remain (consistent with previous behavior).
+//   - Totals recalculated after completion via WithTotals().
+//
+// Notes:
+//   - Potential optimization: batch product lookups; currently sequential.
+//   - Consider adding rollback semantics if atomic replacement is desired.
+//   - Deliveries might reference item IDs that are now invalid—original logic
+//     also left deliveries untouched. If that becomes an issue, add a cleanup
+//     pass to remove deliveries whose item IDs no longer exist.
+func init() {
+	RegisterMutation[messages.SetCartRequest](
+		"SetCartRequest",
+		func(g *CartGrain, m *messages.SetCartRequest) error {
+			if m == nil {
+				return fmt.Errorf("SetCartRequest: nil payload")
+			}
+
+			// Clear current items (keep deliveries)
+			g.mu.Lock()
+			g.Items = make([]*CartItem, 0, len(m.Items))
+			g.mu.Unlock()
+
+			for _, it := range m.Items {
+				if it == nil {
+					continue
+				}
+				if it.Sku == "" || it.Quantity < 1 {
+					return fmt.Errorf("SetCartRequest: invalid item (sku='%s' qty=%d)", it.Sku, it.Quantity)
+				}
+				_, err := g.AddItem(it.Sku, int(it.Quantity), it.Country, it.StoreId)
+				if err != nil {
+					return fmt.Errorf("SetCartRequest: add sku '%s' failed: %w", it.Sku, err)
+				}
+			}
+			return nil
+		},
+		WithTotals(),
+	)
+}

mutation_set_delivery.go

@@ -0,0 +1,101 @@
+package main
+
+import (
+	"fmt"
+	"slices"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_set_delivery.go
+//
+// Registers the SetDelivery mutation.
+//
+// Semantics (mirrors legacy switch logic):
+//   - If the payload specifies an explicit list of item IDs (payload.Items):
+//   - Each referenced cart line must exist.
+//   - None of the referenced items may already belong to a delivery.
+//   - Only those items are associated with the new delivery.
+//   - If payload.Items is empty:
+//   - All items currently without any delivery are associated with the new delivery.
+//   - A new delivery line is created with:
+//   - Auto-incremented delivery ID (cart-local)
+//   - Provider from payload
+//   - Fixed price (currently hard-coded: 4900 minor units) – adjust as needed
+//   - Optional PickupPoint copied from payload
+//   - Cart totals are recalculated (WithTotals)
+//
+// Error cases:
+//   - Referenced item does not exist
+//   - Referenced item already has a delivery
+//   - No items qualify (resulting association set empty) -> returns error (prevents creating empty delivery)
+//
+// Concurrency:
+//   - Uses g.mu to protect lastDeliveryId increment and append to Deliveries slice.
+//     Item scans are read-only and performed outside the lock for simplicity;
+//     if stricter guarantees are needed, widen the lock section.
+//
+// Future extension points:
+//   - Variable delivery pricing (based on weight, distance, provider, etc.)
+//   - Validation of provider codes
+//   - Multi-currency delivery pricing
+func init() {
+	RegisterMutation[messages.SetDelivery](
+		"SetDelivery",
+		func(g *CartGrain, m *messages.SetDelivery) error {
+			if m == nil {
+				return fmt.Errorf("SetDelivery: nil payload")
+			}
+			if m.Provider == "" {
+				return fmt.Errorf("SetDelivery: provider is empty")
+			}
+
+			withDelivery := g.ItemsWithDelivery()
+			targetItems := make([]int, 0)
+
+			if len(m.Items) == 0 {
+				// Use every item currently without a delivery
+				targetItems = append(targetItems, g.ItemsWithoutDelivery()...)
+			} else {
+				// Validate explicit list
+				for _, id64 := range m.Items {
+					id := int(id64)
+					found := false
+					for _, it := range g.Items {
+						if it.Id == id {
+							found = true
+							break
+						}
+					}
+					if !found {
+						return fmt.Errorf("SetDelivery: item id %d not found", id)
+					}
+					if slices.Contains(withDelivery, id) {
+						return fmt.Errorf("SetDelivery: item id %d already has a delivery", id)
+					}
+					targetItems = append(targetItems, id)
+				}
+			}
+
+			if len(targetItems) == 0 {
+				return fmt.Errorf("SetDelivery: no eligible items to attach")
+			}
+
+			// Append new delivery
+			g.mu.Lock()
+			g.lastDeliveryId++
+			newId := g.lastDeliveryId
+			g.Deliveries = append(g.Deliveries, &CartDelivery{
+				Id:          newId,
+				Provider:    m.Provider,
+				PickupPoint: m.PickupPoint,
+				Price:       4900, // TODO: externalize pricing
+				Items:       targetItems,
+			})
+			g.mu.Unlock()
+
+			return nil
+		},
+		WithTotals(),
+	)
+}

mutation_set_pickup_point.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+// mutation_set_pickup_point.go
+//
+// Registers the SetPickupPoint mutation using the generic mutation registry.
+//
+// Semantics (mirrors original switch-based implementation):
+//   - Locate the delivery with Id == payload.DeliveryId
+//   - Set (or overwrite) its PickupPoint with the provided data
+//   - Does NOT alter pricing or taxes (so no totals recalculation required)
+//
+// Validation / Error Handling:
+//   - If payload is nil -> error
+//   - If DeliveryId not found -> error
+//
+// Concurrency:
+//   - Relies on the existing expectation that higher-level mutation routing
+//     serializes Apply() calls per grain; if stricter guarantees are needed,
+//     a delivery-level lock could be introduced later.
+//
+// Future Extensions:
+//   - Validate pickup point fields (country code, zip format, etc.)
+//   - Track history / audit of pickup point changes
+//   - Trigger delivery price adjustments (which would then require WithTotals()).
+func init() {
+	RegisterMutation[messages.SetPickupPoint](
+		"SetPickupPoint",
+		func(g *CartGrain, m *messages.SetPickupPoint) error {
+			if m == nil {
+				return fmt.Errorf("SetPickupPoint: nil payload")
+			}
+
+			for _, d := range g.Deliveries {
+				if d.Id == int(m.DeliveryId) {
+					d.PickupPoint = &messages.PickupPoint{
+						Id:      m.Id,
+						Name:    m.Name,
+						Address: m.Address,
+						City:    m.City,
+						Zip:     m.Zip,
+						Country: m.Country,
+					}
+					return nil
+				}
+			}
+			return fmt.Errorf("SetPickupPoint: delivery id %d not found", m.DeliveryId)
+		},
+		// No WithTotals(): pickup point does not change pricing / tax.
+	)
+}

pool-server.go

@@ -0,0 +1,351 @@
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"log"
+	"math/rand"
+	"net/http"
+	"strconv"
+	"time"
+
+	messages "git.tornberg.me/go-cart-actor/proto"
+)
+
+type PoolServer struct {
+	pod_name string
+	pool     GrainPool
+}
+
+func NewPoolServer(pool GrainPool, pod_name string) *PoolServer {
+	return &PoolServer{
+		pod_name: pod_name,
+		pool:     pool,
+	}
+}
+
+func (s *PoolServer) process(id CartId, mutation interface{}) (*messages.CartState, error) {
+	grain, err := s.pool.Apply(id, mutation)
+	if err != nil {
+		return nil, err
+	}
+	return ToCartState(grain), nil
+}
+
+func (s *PoolServer) HandleGet(w http.ResponseWriter, r *http.Request, id CartId) error {
+	grain, err := s.pool.Get(id)
+	if err != nil {
+		return err
+	}
+
+	return s.WriteResult(w, ToCartState(grain))
+}
+
+func (s *PoolServer) HandleAddSku(w http.ResponseWriter, r *http.Request, id CartId) error {
+	sku := r.PathValue("sku")
+	data, err := s.process(id, &messages.AddRequest{Sku: sku, Quantity: 1})
+	if err != nil {
+		return err
+	}
+	return s.WriteResult(w, data)
+}
+
+func ErrorHandler(fn func(w http.ResponseWriter, r *http.Request) error) func(w http.ResponseWriter, r *http.Request) {
+	return func(w http.ResponseWriter, r *http.Request) {
+		err := fn(w, r)
+		if err != nil {
+			log.Printf("Server error, not remote error: %v\n", err)
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte(err.Error()))
+		}
+	}
+}
+
+func (s *PoolServer) WriteResult(w http.ResponseWriter, result *messages.CartState) error {
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Access-Control-Allow-Origin", "*")
+	w.Header().Set("X-Pod-Name", s.pod_name)
+	if result == nil {
+
+		w.WriteHeader(http.StatusInternalServerError)
+
+		return nil
+	}
+	w.WriteHeader(http.StatusOK)
+	enc := json.NewEncoder(w)
+	err := enc.Encode(result)
+	return err
+}
+
+func (s *PoolServer) HandleDeleteItem(w http.ResponseWriter, r *http.Request, id CartId) error {
+
+	itemIdString := r.PathValue("itemId")
+	itemId, err := strconv.Atoi(itemIdString)
+	if err != nil {
+		return err
+	}
+	data, err := s.process(id, &messages.RemoveItem{Id: int64(itemId)})
+	if err != nil {
+		return err
+	}
+	return s.WriteResult(w, data)
+}
+
+type SetDelivery struct {
+	Provider    string                `json:"provider"`
+	Items       []int64               `json:"items"`
+	PickupPoint *messages.PickupPoint `json:"pickupPoint,omitempty"`
+}
+
+func (s *PoolServer) HandleSetDelivery(w http.ResponseWriter, r *http.Request, id CartId) error {
+
+	delivery := SetDelivery{}
+	err := json.NewDecoder(r.Body).Decode(&delivery)
+	if err != nil {
+		return err
+	}
+	data, err := s.process(id, &messages.SetDelivery{
+		Provider:    delivery.Provider,
+		Items:       delivery.Items,
+		PickupPoint: delivery.PickupPoint,
+	})
+	if err != nil {
+		return err
+	}
+	return s.WriteResult(w, data)
+}
+
+func (s *PoolServer) HandleSetPickupPoint(w http.ResponseWriter, r *http.Request, id CartId) error {
+
+	deliveryIdString := r.PathValue("deliveryId")
+	deliveryId, err := strconv.Atoi(deliveryIdString)
+	if err != nil {
+		return err
+	}
+	pickupPoint := messages.PickupPoint{}
+	err = json.NewDecoder(r.Body).Decode(&pickupPoint)
+	if err != nil {
+		return err
+	}
+	reply, err := s.process(id, &messages.SetPickupPoint{
+		DeliveryId: int64(deliveryId),
+		Id:         pickupPoint.Id,
+		Name:       pickupPoint.Name,
+		Address:    pickupPoint.Address,
+		City:       pickupPoint.City,
+		Zip:        pickupPoint.Zip,
+		Country:    pickupPoint.Country,
+	})
+	if err != nil {
+		return err
+	}
+	return s.WriteResult(w, reply)
+}
+
+func (s *PoolServer) HandleRemoveDelivery(w http.ResponseWriter, r *http.Request, id CartId) error {
+
+	deliveryIdString := r.PathValue("deliveryId")
+	deliveryId, err := strconv.Atoi(deliveryIdString)
+	if err != nil {
+		return err
+	}
+	reply, err := s.process(id, &messages.RemoveDelivery{Id: int64(deliveryId)})
+	if err != nil {
+		return err
+	}
+	return s.WriteResult(w, reply)
+}
+
+func (s *PoolServer) HandleQuantityChange(w http.ResponseWriter, r *http.Request, id CartId) error {
+	changeQuantity := messages.ChangeQuantity{}
+	err := json.NewDecoder(r.Body).Decode(&changeQuantity)
+	if err != nil {
+		return err
+	}
+	reply, err := s.process(id, &changeQuantity)
+	if err != nil {
+		return err
+	}
+	return s.WriteResult(w, reply)
+}
+
+func (s *PoolServer) HandleSetCartItems(w http.ResponseWriter, r *http.Request, id CartId) error {
+	setCartItems := messages.SetCartRequest{}
+	err := json.NewDecoder(r.Body).Decode(&setCartItems)
+	if err != nil {
+		return err
+	}
+	reply, err := s.process(id, &setCartItems)
+	if err != nil {
+		return err
+	}
+	return s.WriteResult(w, reply)
+}
+
+func (s *PoolServer) HandleAddRequest(w http.ResponseWriter, r *http.Request, id CartId) error {
+	addRequest := messages.AddRequest{}
+	err := json.NewDecoder(r.Body).Decode(&addRequest)
+	if err != nil {
+		return err
+	}
+	reply, err := s.process(id, &addRequest)
+	if err != nil {
+		return err
+	}
+	return s.WriteResult(w, reply)
+}
+
+func (s *PoolServer) HandleConfirmation(w http.ResponseWriter, r *http.Request, id CartId) error {
+	orderId := r.PathValue("orderId")
+	if orderId == "" {
+		return fmt.Errorf("orderId is empty")
+	}
+	order, err := KlarnaInstance.GetOrder(orderId)
+
+	if err != nil {
+		return err
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("X-Pod-Name", s.pod_name)
+	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Access-Control-Allow-Origin", "*")
+	w.WriteHeader(http.StatusOK)
+	return json.NewEncoder(w).Encode(order)
+}
+
+func (s *PoolServer) HandleCheckout(w http.ResponseWriter, r *http.Request, id CartId) error {
+	// Build checkout meta (URLs derived from host)
+	meta := &CheckoutMeta{
+		Terms:        fmt.Sprintf("https://%s/terms", r.Host),
+		Checkout:     fmt.Sprintf("https://%s/checkout?order_id={checkout.order.id}", r.Host),
+		Confirmation: fmt.Sprintf("https://%s/confirmation/{checkout.order.id}", r.Host),
+		Validation:   fmt.Sprintf("https://%s/validate", r.Host),
+		Push:         fmt.Sprintf("https://%s/push?order_id={checkout.order.id}", r.Host),
+		Country:      getCountryFromHost(r.Host),
+	}
+
+	// Get current grain state (may be local or remote)
+	grain, err := s.pool.Get(id)
+	if err != nil {
+		return err
+	}
+
+	// Build pure checkout payload
+	payload, _, err := BuildCheckoutOrderPayload(grain, meta)
+	if err != nil {
+		return err
+	}
+
+	// Call Klarna (create or update)
+	var klarnaOrder *CheckoutOrder
+	if grain.OrderReference != "" {
+		klarnaOrder, err = KlarnaInstance.UpdateOrder(grain.OrderReference, bytes.NewReader(payload))
+	} else {
+		klarnaOrder, err = KlarnaInstance.CreateOrder(bytes.NewReader(payload))
+	}
+	if err != nil {
+		return err
+	}
+
+	// Persist initialization state via mutation (best-effort)
+	if _, applyErr := s.pool.Apply(id, &messages.InitializeCheckout{
+		OrderId:           klarnaOrder.ID,
+		Status:            klarnaOrder.Status,
+		PaymentInProgress: true,
+	}); applyErr != nil {
+		log.Printf("InitializeCheckout apply error: %v", applyErr)
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	return json.NewEncoder(w).Encode(klarnaOrder)
+}
+
+func NewCartId() CartId {
+	id := time.Now().UnixNano() + rand.Int63()
+
+	return ToCartId(fmt.Sprintf("%d", id))
+}
+
+func CookieCartIdHandler(fn func(w http.ResponseWriter, r *http.Request, cartId CartId) error) func(w http.ResponseWriter, r *http.Request) error {
+	return func(w http.ResponseWriter, r *http.Request) error {
+		var cartId CartId
+		cartIdCookie := r.CookiesNamed("cartid")
+		if cartIdCookie == nil || len(cartIdCookie) == 0 {
+			cartId = NewCartId()
+			http.SetCookie(w, &http.Cookie{
+				Name:     "cartid",
+				Value:    cartId.String(),
+				Secure:   r.TLS != nil,
+				HttpOnly: true,
+				Path:     "/",
+				Expires:  time.Now().AddDate(0, 0, 14),
+				SameSite: http.SameSiteLaxMode,
+			})
+		} else {
+			cartId = ToCartId(cartIdCookie[0].Value)
+		}
+		return fn(w, r, cartId)
+	}
+}
+
+func (s *PoolServer) RemoveCartCookie(w http.ResponseWriter, r *http.Request, cartId CartId) error {
+	cartId = NewCartId()
+	http.SetCookie(w, &http.Cookie{
+		Name:     "cartid",
+		Value:    cartId.String(),
+		Path:     "/",
+		Secure:   r.TLS != nil,
+		HttpOnly: true,
+		Expires:  time.Unix(0, 0),
+		SameSite: http.SameSiteLaxMode,
+	})
+	w.WriteHeader(http.StatusOK)
+	return nil
+}
+
+func CartIdHandler(fn func(w http.ResponseWriter, r *http.Request, cartId CartId) error) func(w http.ResponseWriter, r *http.Request) error {
+	return func(w http.ResponseWriter, r *http.Request) error {
+		cartId := ToCartId(r.PathValue("id"))
+		return fn(w, r, cartId)
+	}
+}
+
+func (s *PoolServer) Serve() *http.ServeMux {
+	mux := http.NewServeMux()
+	//mux.HandleFunc("/", s.RewritePath)
+	mux.HandleFunc("OPTIONS /", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Access-Control-Allow-Origin", "*")
+		w.Header().Set("Access-Control-Allow-Methods", "GET, PUT, POST, DELETE")
+		w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
+		w.WriteHeader(http.StatusOK)
+	})
+
+	mux.HandleFunc("GET /", ErrorHandler(CookieCartIdHandler(s.HandleGet)))
+	mux.HandleFunc("GET /add/{sku}", ErrorHandler(CookieCartIdHandler(s.HandleAddSku)))
+	mux.HandleFunc("POST /", ErrorHandler(CookieCartIdHandler(s.HandleAddRequest)))
+	mux.HandleFunc("POST /set", ErrorHandler(CookieCartIdHandler(s.HandleSetCartItems)))
+	mux.HandleFunc("DELETE /{itemId}", ErrorHandler(CookieCartIdHandler(s.HandleDeleteItem)))
+	mux.HandleFunc("PUT /", ErrorHandler(CookieCartIdHandler(s.HandleQuantityChange)))
+	mux.HandleFunc("DELETE /", ErrorHandler(CookieCartIdHandler(s.RemoveCartCookie)))
+	mux.HandleFunc("POST /delivery", ErrorHandler(CookieCartIdHandler(s.HandleSetDelivery)))
+	mux.HandleFunc("DELETE /delivery/{deliveryId}", ErrorHandler(CookieCartIdHandler(s.HandleRemoveDelivery)))
+	mux.HandleFunc("PUT /delivery/{deliveryId}/pickupPoint", ErrorHandler(CookieCartIdHandler(s.HandleSetPickupPoint)))
+	mux.HandleFunc("GET /checkout", ErrorHandler(CookieCartIdHandler(s.HandleCheckout)))
+	mux.HandleFunc("GET /confirmation/{orderId}", ErrorHandler(CookieCartIdHandler(s.HandleConfirmation)))
+
+	mux.HandleFunc("GET /byid/{id}", ErrorHandler(CartIdHandler(s.HandleGet)))
+	mux.HandleFunc("GET /byid/{id}/add/{sku}", ErrorHandler(CartIdHandler(s.HandleAddSku)))
+	mux.HandleFunc("POST /byid/{id}", ErrorHandler(CartIdHandler(s.HandleAddRequest)))
+	mux.HandleFunc("DELETE /byid/{id}/{itemId}", ErrorHandler(CartIdHandler(s.HandleDeleteItem)))
+	mux.HandleFunc("PUT /byid/{id}", ErrorHandler(CartIdHandler(s.HandleQuantityChange)))
+	mux.HandleFunc("POST /byid/{id}/delivery", ErrorHandler(CartIdHandler(s.HandleSetDelivery)))
+	mux.HandleFunc("DELETE /byid/{id}/delivery/{deliveryId}", ErrorHandler(CartIdHandler(s.HandleRemoveDelivery)))
+	mux.HandleFunc("PUT /byid/{id}/delivery/{deliveryId}/pickupPoint", ErrorHandler(CartIdHandler(s.HandleSetPickupPoint)))
+	mux.HandleFunc("GET /byid/{id}/checkout", ErrorHandler(CartIdHandler(s.HandleCheckout)))
+	mux.HandleFunc("GET /byid/{id}/confirmation", ErrorHandler(CartIdHandler(s.HandleConfirmation)))
+
+	return mux
+}

proto/cart_actor.proto

@@ -3,66 +3,114 @@ syntax = "proto3";
 package messages;
 
 option go_package = "git.tornberg.me/go-cart-actor/proto;messages";
+
 import "messages.proto";
 
 // -----------------------------------------------------------------------------
-// Cart Actor gRPC API (oneof Envelope Variant)
+// Cart Actor gRPC API (Breaking v2 - Per-Mutation RPCs)
 // -----------------------------------------------------------------------------
-// This version removes the legacy numeric MutationType enum + raw bytes payload
-// approach and replaces it with a strongly typed oneof envelope. Each concrete
-// mutation proto is embedded directly, enabling:
-//   * Type-safe routing server-side (simple type switch on the oneof).
-//   * Direct persistence of MutationEnvelope messages (no custom binary header).
-//   * Elimination of the legacy message handler registry.
+// This version removes the previous MutationEnvelope + Mutate RPC.
+// Each mutation now has its own request wrapper and dedicated RPC method
+// providing simpler, type-focused client stubs and enabling per-mutation
+// metrics, auth and rate limiting.
 //
-// NOTE: Regenerate Go code after editing:
+// Regenerate Go code after editing:
 //   protoc --go_out=. --go_opt=paths=source_relative \
 //          --go-grpc_out=. --go-grpc_opt=paths=source_relative \
 //          proto/cart_actor.proto proto/messages.proto
+//
+// Backward compatibility: This is a breaking change (old clients must update).
 // -----------------------------------------------------------------------------
 
-// MutationEnvelope carries exactly one mutation plus metadata.
-// client_timestamp:
-//   - Optional Unix timestamp provided by the client.
-//   - If zero the server MAY overwrite with its local time.
-message MutationEnvelope {
-  string cart_id = 1;
-  int64 client_timestamp = 2;
-
-  oneof mutation {
-    AddRequest add_request = 10;
-    AddItem add_item = 11;
-    RemoveItem remove_item = 12;
-    RemoveDelivery remove_delivery = 13;
-    ChangeQuantity change_quantity = 14;
-    SetDelivery set_delivery = 15;
-    SetPickupPoint set_pickup_point = 16;
-    CreateCheckoutOrder create_checkout_order = 17;
-    SetCartRequest set_cart_items = 18;
-    OrderCreated order_completed = 19;
+// Shared reply for all mutation RPCs.
+message CartMutationReply {
+  int32 status_code = 1;          // HTTP-like status (200 success, 4xx client, 5xx server)
+  oneof result {
+    CartState state = 2;          // Updated cart state on success
+    string error = 3;             // Error message on failure
   }
+  int64 server_timestamp = 4;     // Server-assigned Unix timestamp (optional auditing)
 }
 
-// MutationReply returns a legacy-style status code plus a JSON payload
-// holding either the updated cart state (on success) or an error string.
-message MutationReply {
-  int32 status_code = 1;
+// Fetch current cart state without mutation.
+message StateRequest {
+  string cart_id = 1;
+}
 
-  // Exactly one of state or error will be set.
+message StateReply {
+  int32 status_code = 1;
   oneof result {
     CartState state = 2;
     string error = 3;
   }
 }
 
-// StateRequest fetches current cart state without mutating.
-message StateRequest {
+// Per-mutation request wrappers. We wrap the existing inner mutation
+// messages (defined in messages.proto) to add cart_id + optional metadata
+// without altering the inner message definitions.
+
+message AddRequestRequest {
   string cart_id = 1;
+  int64 client_timestamp = 2;
+  AddRequest payload = 10;
+}
+
+message AddItemRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  AddItem payload = 10;
+}
+
+message RemoveItemRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  RemoveItem payload = 10;
+}
+
+message RemoveDeliveryRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  RemoveDelivery payload = 10;
+}
+
+message ChangeQuantityRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  ChangeQuantity payload = 10;
+}
+
+message SetDeliveryRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  SetDelivery payload = 10;
+}
+
+message SetPickupPointRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  SetPickupPoint payload = 10;
+}
+
+message CreateCheckoutOrderRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  CreateCheckoutOrder payload = 10;
+}
+
+message SetCartItemsRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  SetCartRequest payload = 10;
+}
+
+message OrderCompletedRequest {
+  string cart_id = 1;
+  int64 client_timestamp = 2;
+  OrderCreated payload = 10;
 }
 
 // -----------------------------------------------------------------------------
-// CartState represents the full cart snapshot returned by state/mutation replies.
-// Replaces the previous raw JSON payload.
+// Cart state snapshot (unchanged from v1 except envelope removal context)
 // -----------------------------------------------------------------------------
 message CartState {
   string cart_id = 1;
@@ -76,7 +124,6 @@ message CartState {
   string payment_status = 9;
 }
 
-// Lightweight representation of an item in the cart
 message CartItemState {
   int64 id = 1;
   int64 source_item_id = 2;
@@ -105,38 +152,37 @@ message CartItemState {
   int32 stock = 25;
 }
 
-// Delivery / shipping entry
 message DeliveryState {
   int64 id = 1;
   string provider = 2;
   int64 price = 3;
   repeated int64 item_ids = 4;
-  PickupPoint pickup_point = 5;
+  PickupPoint pickup_point = 5; // Defined in messages.proto
 }
 
-// StateReply mirrors MutationReply for consistency.
-message StateReply {
-  int32 status_code = 1;
+// (CheckoutRequest / CheckoutReply removed - checkout handled at HTTP layer)
 
-  oneof result {
-    CartState state = 2;
-    string error = 3;
-  }
-}
-
-// CartActor exposes mutation and state retrieval for remote grains.
+// -----------------------------------------------------------------------------
+// Service definition (per-mutation RPCs + checkout)
+// -----------------------------------------------------------------------------
 service CartActor {
-  // Mutate applies a single mutation to a cart, creating the cart lazily if needed.
-  rpc Mutate(MutationEnvelope) returns (MutationReply);
+  rpc AddRequest(AddRequestRequest) returns (CartMutationReply);
+  rpc AddItem(AddItemRequest) returns (CartMutationReply);
+  rpc RemoveItem(RemoveItemRequest) returns (CartMutationReply);
+  rpc RemoveDelivery(RemoveDeliveryRequest) returns (CartMutationReply);
+  rpc ChangeQuantity(ChangeQuantityRequest) returns (CartMutationReply);
+  rpc SetDelivery(SetDeliveryRequest) returns (CartMutationReply);
+  rpc SetPickupPoint(SetPickupPointRequest) returns (CartMutationReply);
+  // (Checkout RPC removed - handled externally)
+  rpc SetCartItems(SetCartItemsRequest) returns (CartMutationReply);
+  rpc OrderCompleted(OrderCompletedRequest) returns (CartMutationReply);
 
-  // GetState retrieves the cart's current state (JSON).
   rpc GetState(StateRequest) returns (StateReply);
 }
 
 // -----------------------------------------------------------------------------
-// Future Enhancements:
-//   * Replace JSON state payload with a strongly typed CartState proto.
-//   * Add streaming RPC (e.g., WatchState) for live updates.
-//   * Add batch mutations (repeated MutationEnvelope) if performance requires.
-//   * Introduce optimistic concurrency via version fields if external writers appear.
+// Future enhancements:
+// * BatchMutate RPC (repeated heterogeneous mutations)
+// * Streaming state updates (WatchState)
+// * Versioning / optimistic concurrency control
 // -----------------------------------------------------------------------------

proto/cart_actor_grpc.pb.go

@@ -19,19 +19,36 @@ import (
 const _ = grpc.SupportPackageIsVersion9
 
 const (
-	CartActor_Mutate_FullMethodName   = "/messages.CartActor/Mutate"
-	CartActor_GetState_FullMethodName = "/messages.CartActor/GetState"
+	CartActor_AddRequest_FullMethodName     = "/messages.CartActor/AddRequest"
+	CartActor_AddItem_FullMethodName        = "/messages.CartActor/AddItem"
+	CartActor_RemoveItem_FullMethodName     = "/messages.CartActor/RemoveItem"
+	CartActor_RemoveDelivery_FullMethodName = "/messages.CartActor/RemoveDelivery"
+	CartActor_ChangeQuantity_FullMethodName = "/messages.CartActor/ChangeQuantity"
+	CartActor_SetDelivery_FullMethodName    = "/messages.CartActor/SetDelivery"
+	CartActor_SetPickupPoint_FullMethodName = "/messages.CartActor/SetPickupPoint"
+	CartActor_SetCartItems_FullMethodName   = "/messages.CartActor/SetCartItems"
+	CartActor_OrderCompleted_FullMethodName = "/messages.CartActor/OrderCompleted"
+	CartActor_GetState_FullMethodName       = "/messages.CartActor/GetState"
 )
 
 // CartActorClient is the client API for CartActor service.
 //
 // For semantics around ctx use and closing/ending streaming RPCs, please refer to https://pkg.go.dev/google.golang.org/grpc/?tab=doc#ClientConn.NewStream.
 //
-// CartActor exposes mutation and state retrieval for remote grains.
+// -----------------------------------------------------------------------------
+// Service definition (per-mutation RPCs + checkout)
+// -----------------------------------------------------------------------------
 type CartActorClient interface {
-	// Mutate applies a single mutation to a cart, creating the cart lazily if needed.
-	Mutate(ctx context.Context, in *MutationEnvelope, opts ...grpc.CallOption) (*MutationReply, error)
-	// GetState retrieves the cart's current state (JSON).
+	AddRequest(ctx context.Context, in *AddRequestRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
+	AddItem(ctx context.Context, in *AddItemRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
+	RemoveItem(ctx context.Context, in *RemoveItemRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
+	RemoveDelivery(ctx context.Context, in *RemoveDeliveryRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
+	ChangeQuantity(ctx context.Context, in *ChangeQuantityRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
+	SetDelivery(ctx context.Context, in *SetDeliveryRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
+	SetPickupPoint(ctx context.Context, in *SetPickupPointRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
+	// (Checkout RPC removed - handled externally)
+	SetCartItems(ctx context.Context, in *SetCartItemsRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
+	OrderCompleted(ctx context.Context, in *OrderCompletedRequest, opts ...grpc.CallOption) (*CartMutationReply, error)
 	GetState(ctx context.Context, in *StateRequest, opts ...grpc.CallOption) (*StateReply, error)
 }
 
@@ -43,10 +60,90 @@ func NewCartActorClient(cc grpc.ClientConnInterface) CartActorClient {
 	return &cartActorClient{cc}
 }
 
-func (c *cartActorClient) Mutate(ctx context.Context, in *MutationEnvelope, opts ...grpc.CallOption) (*MutationReply, error) {
+func (c *cartActorClient) AddRequest(ctx context.Context, in *AddRequestRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
 	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
-	out := new(MutationReply)
-	err := c.cc.Invoke(ctx, CartActor_Mutate_FullMethodName, in, out, cOpts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_AddRequest_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *cartActorClient) AddItem(ctx context.Context, in *AddItemRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_AddItem_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *cartActorClient) RemoveItem(ctx context.Context, in *RemoveItemRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_RemoveItem_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *cartActorClient) RemoveDelivery(ctx context.Context, in *RemoveDeliveryRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_RemoveDelivery_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *cartActorClient) ChangeQuantity(ctx context.Context, in *ChangeQuantityRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_ChangeQuantity_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *cartActorClient) SetDelivery(ctx context.Context, in *SetDeliveryRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_SetDelivery_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *cartActorClient) SetPickupPoint(ctx context.Context, in *SetPickupPointRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_SetPickupPoint_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *cartActorClient) SetCartItems(ctx context.Context, in *SetCartItemsRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_SetCartItems_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *cartActorClient) OrderCompleted(ctx context.Context, in *OrderCompletedRequest, opts ...grpc.CallOption) (*CartMutationReply, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(CartMutationReply)
+	err := c.cc.Invoke(ctx, CartActor_OrderCompleted_FullMethodName, in, out, cOpts...)
 	if err != nil {
 		return nil, err
 	}
@@ -67,11 +164,20 @@ func (c *cartActorClient) GetState(ctx context.Context, in *StateRequest, opts .
 // All implementations must embed UnimplementedCartActorServer
 // for forward compatibility.
 //
-// CartActor exposes mutation and state retrieval for remote grains.
+// -----------------------------------------------------------------------------
+// Service definition (per-mutation RPCs + checkout)
+// -----------------------------------------------------------------------------
 type CartActorServer interface {
-	// Mutate applies a single mutation to a cart, creating the cart lazily if needed.
-	Mutate(context.Context, *MutationEnvelope) (*MutationReply, error)
-	// GetState retrieves the cart's current state (JSON).
+	AddRequest(context.Context, *AddRequestRequest) (*CartMutationReply, error)
+	AddItem(context.Context, *AddItemRequest) (*CartMutationReply, error)
+	RemoveItem(context.Context, *RemoveItemRequest) (*CartMutationReply, error)
+	RemoveDelivery(context.Context, *RemoveDeliveryRequest) (*CartMutationReply, error)
+	ChangeQuantity(context.Context, *ChangeQuantityRequest) (*CartMutationReply, error)
+	SetDelivery(context.Context, *SetDeliveryRequest) (*CartMutationReply, error)
+	SetPickupPoint(context.Context, *SetPickupPointRequest) (*CartMutationReply, error)
+	// (Checkout RPC removed - handled externally)
+	SetCartItems(context.Context, *SetCartItemsRequest) (*CartMutationReply, error)
+	OrderCompleted(context.Context, *OrderCompletedRequest) (*CartMutationReply, error)
 	GetState(context.Context, *StateRequest) (*StateReply, error)
 	mustEmbedUnimplementedCartActorServer()
 }
@@ -83,8 +189,32 @@ type CartActorServer interface {
 // pointer dereference when methods are called.
 type UnimplementedCartActorServer struct{}
 
-func (UnimplementedCartActorServer) Mutate(context.Context, *MutationEnvelope) (*MutationReply, error) {
-	return nil, status.Errorf(codes.Unimplemented, "method Mutate not implemented")
+func (UnimplementedCartActorServer) AddRequest(context.Context, *AddRequestRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method AddRequest not implemented")
+}
+func (UnimplementedCartActorServer) AddItem(context.Context, *AddItemRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method AddItem not implemented")
+}
+func (UnimplementedCartActorServer) RemoveItem(context.Context, *RemoveItemRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method RemoveItem not implemented")
+}
+func (UnimplementedCartActorServer) RemoveDelivery(context.Context, *RemoveDeliveryRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method RemoveDelivery not implemented")
+}
+func (UnimplementedCartActorServer) ChangeQuantity(context.Context, *ChangeQuantityRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method ChangeQuantity not implemented")
+}
+func (UnimplementedCartActorServer) SetDelivery(context.Context, *SetDeliveryRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method SetDelivery not implemented")
+}
+func (UnimplementedCartActorServer) SetPickupPoint(context.Context, *SetPickupPointRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method SetPickupPoint not implemented")
+}
+func (UnimplementedCartActorServer) SetCartItems(context.Context, *SetCartItemsRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method SetCartItems not implemented")
+}
+func (UnimplementedCartActorServer) OrderCompleted(context.Context, *OrderCompletedRequest) (*CartMutationReply, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method OrderCompleted not implemented")
 }
 func (UnimplementedCartActorServer) GetState(context.Context, *StateRequest) (*StateReply, error) {
 	return nil, status.Errorf(codes.Unimplemented, "method GetState not implemented")
@@ -110,20 +240,164 @@ func RegisterCartActorServer(s grpc.ServiceRegistrar, srv CartActorServer) {
 	s.RegisterService(&CartActor_ServiceDesc, srv)
 }
 
-func _CartActor_Mutate_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
-	in := new(MutationEnvelope)
+func _CartActor_AddRequest_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(AddRequestRequest)
 	if err := dec(in); err != nil {
 		return nil, err
 	}
 	if interceptor == nil {
-		return srv.(CartActorServer).Mutate(ctx, in)
+		return srv.(CartActorServer).AddRequest(ctx, in)
 	}
 	info := &grpc.UnaryServerInfo{
 		Server:     srv,
-		FullMethod: CartActor_Mutate_FullMethodName,
+		FullMethod: CartActor_AddRequest_FullMethodName,
 	}
 	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
-		return srv.(CartActorServer).Mutate(ctx, req.(*MutationEnvelope))
+		return srv.(CartActorServer).AddRequest(ctx, req.(*AddRequestRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _CartActor_AddItem_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(AddItemRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(CartActorServer).AddItem(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: CartActor_AddItem_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(CartActorServer).AddItem(ctx, req.(*AddItemRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _CartActor_RemoveItem_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(RemoveItemRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(CartActorServer).RemoveItem(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: CartActor_RemoveItem_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(CartActorServer).RemoveItem(ctx, req.(*RemoveItemRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _CartActor_RemoveDelivery_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(RemoveDeliveryRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(CartActorServer).RemoveDelivery(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: CartActor_RemoveDelivery_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(CartActorServer).RemoveDelivery(ctx, req.(*RemoveDeliveryRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _CartActor_ChangeQuantity_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(ChangeQuantityRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(CartActorServer).ChangeQuantity(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: CartActor_ChangeQuantity_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(CartActorServer).ChangeQuantity(ctx, req.(*ChangeQuantityRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _CartActor_SetDelivery_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(SetDeliveryRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(CartActorServer).SetDelivery(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: CartActor_SetDelivery_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(CartActorServer).SetDelivery(ctx, req.(*SetDeliveryRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _CartActor_SetPickupPoint_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(SetPickupPointRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(CartActorServer).SetPickupPoint(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: CartActor_SetPickupPoint_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(CartActorServer).SetPickupPoint(ctx, req.(*SetPickupPointRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _CartActor_SetCartItems_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(SetCartItemsRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(CartActorServer).SetCartItems(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: CartActor_SetCartItems_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(CartActorServer).SetCartItems(ctx, req.(*SetCartItemsRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _CartActor_OrderCompleted_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(OrderCompletedRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(CartActorServer).OrderCompleted(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: CartActor_OrderCompleted_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(CartActorServer).OrderCompleted(ctx, req.(*OrderCompletedRequest))
 	}
 	return interceptor(ctx, in, info, handler)
 }
@@ -154,8 +428,40 @@ var CartActor_ServiceDesc = grpc.ServiceDesc{
 	HandlerType: (*CartActorServer)(nil),
 	Methods: []grpc.MethodDesc{
 		{
-			MethodName: "Mutate",
-			Handler:    _CartActor_Mutate_Handler,
+			MethodName: "AddRequest",
+			Handler:    _CartActor_AddRequest_Handler,
+		},
+		{
+			MethodName: "AddItem",
+			Handler:    _CartActor_AddItem_Handler,
+		},
+		{
+			MethodName: "RemoveItem",
+			Handler:    _CartActor_RemoveItem_Handler,
+		},
+		{
+			MethodName: "RemoveDelivery",
+			Handler:    _CartActor_RemoveDelivery_Handler,
+		},
+		{
+			MethodName: "ChangeQuantity",
+			Handler:    _CartActor_ChangeQuantity_Handler,
+		},
+		{
+			MethodName: "SetDelivery",
+			Handler:    _CartActor_SetDelivery_Handler,
+		},
+		{
+			MethodName: "SetPickupPoint",
+			Handler:    _CartActor_SetPickupPoint_Handler,
+		},
+		{
+			MethodName: "SetCartItems",
+			Handler:    _CartActor_SetCartItems_Handler,
+		},
+		{
+			MethodName: "OrderCompleted",
+			Handler:    _CartActor_OrderCompleted_Handler,
 		},
 		{
 			MethodName: "GetState",

proto/messages.pb.go

@@ -889,6 +889,102 @@ func (x *OrderCreated) GetStatus() string {
 	return ""
 }
 
+type Noop struct {
+	state         protoimpl.MessageState `protogen:"open.v1"`
+	unknownFields protoimpl.UnknownFields
+	sizeCache     protoimpl.SizeCache
+}
+
+func (x *Noop) Reset() {
+	*x = Noop{}
+	mi := &file_messages_proto_msgTypes[11]
+	ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+	ms.StoreMessageInfo(mi)
+}
+
+func (x *Noop) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*Noop) ProtoMessage() {}
+
+func (x *Noop) ProtoReflect() protoreflect.Message {
+	mi := &file_messages_proto_msgTypes[11]
+	if x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use Noop.ProtoReflect.Descriptor instead.
+func (*Noop) Descriptor() ([]byte, []int) {
+	return file_messages_proto_rawDescGZIP(), []int{11}
+}
+
+type InitializeCheckout struct {
+	state             protoimpl.MessageState `protogen:"open.v1"`
+	OrderId           string                 `protobuf:"bytes,1,opt,name=orderId,proto3" json:"orderId,omitempty"`
+	Status            string                 `protobuf:"bytes,2,opt,name=status,proto3" json:"status,omitempty"`
+	PaymentInProgress bool                   `protobuf:"varint,3,opt,name=paymentInProgress,proto3" json:"paymentInProgress,omitempty"`
+	unknownFields     protoimpl.UnknownFields
+	sizeCache         protoimpl.SizeCache
+}
+
+func (x *InitializeCheckout) Reset() {
+	*x = InitializeCheckout{}
+	mi := &file_messages_proto_msgTypes[12]
+	ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+	ms.StoreMessageInfo(mi)
+}
+
+func (x *InitializeCheckout) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*InitializeCheckout) ProtoMessage() {}
+
+func (x *InitializeCheckout) ProtoReflect() protoreflect.Message {
+	mi := &file_messages_proto_msgTypes[12]
+	if x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use InitializeCheckout.ProtoReflect.Descriptor instead.
+func (*InitializeCheckout) Descriptor() ([]byte, []int) {
+	return file_messages_proto_rawDescGZIP(), []int{12}
+}
+
+func (x *InitializeCheckout) GetOrderId() string {
+	if x != nil {
+		return x.OrderId
+	}
+	return ""
+}
+
+func (x *InitializeCheckout) GetStatus() string {
+	if x != nil {
+		return x.Status
+	}
+	return ""
+}
+
+func (x *InitializeCheckout) GetPaymentInProgress() bool {
+	if x != nil {
+		return x.PaymentInProgress
+	}
+	return false
+}
+
 var File_messages_proto protoreflect.FileDescriptor
 
 const file_messages_proto_rawDesc = "" +
@@ -997,7 +1093,12 @@ const file_messages_proto_rawDesc = "" +
 	"\acountry\x18\x06 \x01(\tR\acountry\"@\n" +
 	"\fOrderCreated\x12\x18\n" +
 	"\aorderId\x18\x01 \x01(\tR\aorderId\x12\x16\n" +
-	"\x06status\x18\x02 \x01(\tR\x06statusB.Z,git.tornberg.me/go-cart-actor/proto;messagesb\x06proto3"
+	"\x06status\x18\x02 \x01(\tR\x06status\"\x06\n" +
+	"\x04Noop\"t\n" +
+	"\x12InitializeCheckout\x12\x18\n" +
+	"\aorderId\x18\x01 \x01(\tR\aorderId\x12\x16\n" +
+	"\x06status\x18\x02 \x01(\tR\x06status\x12,\n" +
+	"\x11paymentInProgress\x18\x03 \x01(\bR\x11paymentInProgressB.Z,git.tornberg.me/go-cart-actor/proto;messagesb\x06proto3"
 
 var (
 	file_messages_proto_rawDescOnce sync.Once
@@ -1011,7 +1112,7 @@ func file_messages_proto_rawDescGZIP() []byte {
 	return file_messages_proto_rawDescData
 }
 
-var file_messages_proto_msgTypes = make([]protoimpl.MessageInfo, 11)
+var file_messages_proto_msgTypes = make([]protoimpl.MessageInfo, 13)
 var file_messages_proto_goTypes = []any{
 	(*AddRequest)(nil),          // 0: messages.AddRequest
 	(*SetCartRequest)(nil),      // 1: messages.SetCartRequest
@@ -1024,6 +1125,8 @@ var file_messages_proto_goTypes = []any{
 	(*RemoveDelivery)(nil),      // 8: messages.RemoveDelivery
 	(*CreateCheckoutOrder)(nil), // 9: messages.CreateCheckoutOrder
 	(*OrderCreated)(nil),        // 10: messages.OrderCreated
+	(*Noop)(nil),                // 11: messages.Noop
+	(*InitializeCheckout)(nil),  // 12: messages.InitializeCheckout
 }
 var file_messages_proto_depIdxs = []int32{
 	0, // 0: messages.SetCartRequest.items:type_name -> messages.AddRequest
@@ -1051,7 +1154,7 @@ func file_messages_proto_init() {
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: unsafe.Slice(unsafe.StringData(file_messages_proto_rawDesc), len(file_messages_proto_rawDesc)),
 			NumEnums:      0,
-			NumMessages:   11,
+			NumMessages:   13,
 			NumExtensions: 0,
 			NumServices:   0,
 		},

proto/messages.proto

@@ -93,3 +93,13 @@ message OrderCreated {
 		string orderId = 1;
 		string status = 2;
 }
+
+message Noop {
+		// Intentionally empty - used for ownership acquisition or health pings
+}
+
+message InitializeCheckout {
+        string orderId = 1;
+        string status = 2;
+        bool paymentInProgress = 3;
+}

remote-grain-pool._go

@@ -1,67 +0,0 @@
-// package main
-
-// import "sync"
-
-// type RemoteGrainPool struct {
-// 	mu     sync.RWMutex
-// 	Host   string
-// 	grains map[CartId]*RemoteGrain
-// }
-
-// func NewRemoteGrainPool(addr string) *RemoteGrainPool {
-// 	return &RemoteGrainPool{
-// 		Host:   addr,
-// 		grains: make(map[CartId]*RemoteGrain),
-// 	}
-// }
-
-// func (p *RemoteGrainPool) findRemoteGrain(id CartId) *RemoteGrain {
-// 	p.mu.RLock()
-// 	grain, ok := p.grains[id]
-// 	p.mu.RUnlock()
-// 	if !ok {
-// 		return nil
-// 	}
-// 	return grain
-// }
-
-// func (p *RemoteGrainPool) findOrCreateGrain(id CartId) (*RemoteGrain, error) {
-// 	grain := p.findRemoteGrain(id)
-
-// 	if grain == nil {
-// 		grain, err := NewRemoteGrain(id, p.Host)
-// 		if err != nil {
-// 			return nil, err
-// 		}
-// 		p.mu.Lock()
-// 		p.grains[id] = grain
-// 		p.mu.Unlock()
-// 	}
-// 	return grain, nil
-// }
-
-// func (p *RemoteGrainPool) Delete(id CartId) {
-// 	p.mu.Lock()
-// 	delete(p.grains, id)
-// 	p.mu.Unlock()
-// }
-
-// func (p *RemoteGrainPool) Process(id CartId, messages ...Message) (*FrameWithPayload, error) {
-// 	var result *FrameWithPayload
-// 	grain, err := p.findOrCreateGrain(id)
-// 	if err != nil {
-// 		return nil, err
-// 	}
-// 	for _, message := range messages {
-// 		result, err = grain.HandleMessage(&message, false)
-// 	}
-// 	return result, err
-// }
-
-// func (p *RemoteGrainPool) Get(id CartId) (*FrameWithPayload, error) {
-// 	grain, err := p.findOrCreateGrain(id)
-// 	if err != nil {
-// 		return nil, err
-// 	}
-// 	return grain.GetCurrentState()
-// }

remote_grain_grpc.go

@@ -67,8 +67,8 @@ func (g *RemoteGrainGRPC) GetId() CartId {
 	return g.Id
 }
 
-// Apply executes a cart mutation remotely using the typed oneof MutationEnvelope
-// and returns a *CartGrain reconstructed from the typed MutationReply (state oneof).
+// Apply executes a cart mutation via per-mutation RPCs (breaking v2 API)
+// and returns a *CartGrain reconstructed from the CartMutationReply state.
 func (g *RemoteGrainGRPC) Apply(content interface{}, isReplay bool) (*CartGrain, error) {
 	if isReplay {
 		return nil, fmt.Errorf("replay not supported for remote grains")
@@ -78,45 +78,100 @@ func (g *RemoteGrainGRPC) Apply(content interface{}, isReplay bool) (*CartGrain,
 	}
 
 	ts := time.Now().Unix()
-	env := &proto.MutationEnvelope{
-		CartId:          g.Id.String(),
-		ClientTimestamp: ts,
-	}
+
+	var invoke func(ctx context.Context) (*proto.CartMutationReply, error)
 
 	switch m := content.(type) {
 	case *proto.AddRequest:
-		env.Mutation = &proto.MutationEnvelope_AddRequest{AddRequest: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.AddRequest(ctx, &proto.AddRequestRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	case *proto.AddItem:
-		env.Mutation = &proto.MutationEnvelope_AddItem{AddItem: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.AddItem(ctx, &proto.AddItemRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	case *proto.RemoveItem:
-		env.Mutation = &proto.MutationEnvelope_RemoveItem{RemoveItem: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.RemoveItem(ctx, &proto.RemoveItemRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	case *proto.RemoveDelivery:
-		env.Mutation = &proto.MutationEnvelope_RemoveDelivery{RemoveDelivery: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.RemoveDelivery(ctx, &proto.RemoveDeliveryRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	case *proto.ChangeQuantity:
-		env.Mutation = &proto.MutationEnvelope_ChangeQuantity{ChangeQuantity: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.ChangeQuantity(ctx, &proto.ChangeQuantityRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	case *proto.SetDelivery:
-		env.Mutation = &proto.MutationEnvelope_SetDelivery{SetDelivery: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.SetDelivery(ctx, &proto.SetDeliveryRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	case *proto.SetPickupPoint:
-		env.Mutation = &proto.MutationEnvelope_SetPickupPoint{SetPickupPoint: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.SetPickupPoint(ctx, &proto.SetPickupPointRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	case *proto.CreateCheckoutOrder:
-		env.Mutation = &proto.MutationEnvelope_CreateCheckoutOrder{CreateCheckoutOrder: m}
+		return nil, fmt.Errorf("CreateCheckoutOrder deprecated: checkout is handled via HTTP endpoint (HandleCheckout)")
 	case *proto.SetCartRequest:
-		env.Mutation = &proto.MutationEnvelope_SetCartItems{SetCartItems: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.SetCartItems(ctx, &proto.SetCartItemsRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	case *proto.OrderCreated:
-		env.Mutation = &proto.MutationEnvelope_OrderCompleted{OrderCompleted: m}
+		invoke = func(ctx context.Context) (*proto.CartMutationReply, error) {
+			return g.client.OrderCompleted(ctx, &proto.OrderCompletedRequest{
+				CartId:          g.Id.String(),
+				ClientTimestamp: ts,
+				Payload:         m,
+			})
+		}
 	default:
 		return nil, fmt.Errorf("unsupported mutation type %T", content)
 	}
 
+	if invoke == nil {
+		return nil, fmt.Errorf("no invocation mapped for mutation %T", content)
+	}
+
 	ctx, cancel := context.WithTimeout(context.Background(), g.mutateTimeout)
 	defer cancel()
 
-	resp, err := g.client.Mutate(ctx, env)
+	resp, err := invoke(ctx)
 	if err != nil {
 		return nil, err
 	}
 
-	// Map typed reply
 	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
 		if e := resp.GetError(); e != "" {
 			return nil, fmt.Errorf("remote mutation failed %d: %s", resp.StatusCode, e)

synced-pool.go

@@ -426,20 +426,13 @@ func (p *SyncedPool) getGrain(id CartId) (Grain, error) {
 	return grain, nil
 }
 
-// Process applies mutation(s) to a grain (local or remote).
-func (p *SyncedPool) Process(id CartId, mutations ...interface{}) (*CartGrain, error) {
+// Apply applies a single mutation to a grain (local or remote).
+func (p *SyncedPool) Apply(id CartId, mutation interface{}) (*CartGrain, error) {
 	grain, err := p.getGrain(id)
 	if err != nil {
 		return nil, err
 	}
-	var res *CartGrain
-	for _, m := range mutations {
-		res, err = grain.Apply(m, false)
-		if err != nil {
-			return nil, err
-		}
-	}
-	return res, nil
+	return grain.Apply(mutation, false)
 }
 
 // Get returns current state of a grain (local or remote).