--- status: stable last_updated: 2026-05-16 --- # Call Protocol PendingRequestMap, CallHandler, call≡subscribe semantics, event types, error model, and access control. ## Overview The call protocol is the unified transport layer for all operation invocations. It provides a single event-based mechanism that works the same whether the call is local (in-process), remote (hub↔spoke over websocket), or streamed (subscription). It is built on `@alkdev/pubsub`. At the protocol level, `call` and `subscribe` are the same thing with different consumption patterns: - **`call`**: Publish `call.requested`, subscribe to `call.responded:{requestId}`, resolve on first response → `Promise` - **`subscribe`**: Publish `call.requested`, subscribe to `call.responded:{requestId}`, yield each response → `AsyncIterable` Both use the same event types, the same `requestId` correlation, and the same `PendingRequestMap`. `call` is semantically `subscribe().next()`. All responses are wrapped in `ResponseEnvelope` — see [response-envelopes.md](response-envelopes.md) for the full envelope type system. ## Event Types All communication flows through typed events. The event map is defined as `CallEventMap` using TypeBox schemas, compatible with `@alkdev/pubsub`'s `PubSubPublishArgsByKey`. ### `CallEventMap` ```ts const CallEventMap = { "call.requested": Type.Object({ requestId: Type.String(), operationId: Type.String(), input: Type.Unknown(), parentRequestId: Type.Optional(Type.String()), deadline: Type.Optional(Type.Number()), identity: Type.Optional(Type.Object({ id: Type.String(), scopes: Type.Array(Type.String()), resources: Type.Optional(Type.Record(Type.String(), Type.Array(Type.String()))), })), }), "call.responded": Type.Object({ requestId: Type.String(), output: ResponseEnvelopeSchema, }), "call.aborted": Type.Object({ requestId: Type.String(), }), "call.error": Type.Object({ requestId: Type.String(), code: Type.String(), message: Type.String(), details: Type.Optional(Type.Unknown()), }), } ``` `call.responded.output` uses `ResponseEnvelopeSchema` (defined in [response-envelopes.md](response-envelopes.md)). This means every response through the call protocol carries `data` and `meta` with source-discriminated metadata. Handlers do not construct this envelope manually — `CallHandler` wraps handler return values automatically. ### Request Correlation Every call has a unique `requestId` (UUID). Nested calls include `parentRequestId` to track the call chain. Responses and errors match to requests by `requestId`. ### Event Flow ``` Caller Handler │ │ │─── call.requested ───────────────>│ │ {requestId, operationId, │ │ input, identity, deadline} │ │ │ handler returns value │ │ CallHandler wraps in ResponseEnvelope │<── call.responded ────────────────│ │ {requestId, │ │ output: ResponseEnvelope} │ ``` On error: ``` │<── call.error ────────────────────│ │ {requestId, code, message, │ │ details} │ ``` On abort (caller cancels): ``` │─── call.aborted ─────────────────>│ │ {requestId} │ ``` ### Identity The `identity` field in `call.requested` carries the caller's security context through the call chain. Derived from keypal's `ApiKeyMetadata` — `scopes` maps directly, `resources` uses key format `"type:id"` with scope arrays. Checked by `CallHandler` against the operation's `AccessControl`. ## PendingRequestMap `PendingRequestMap` manages in-flight requests and provides the `call()` interface. It wraps `@alkdev/pubsub` internally. ### Construction ```ts const callMap = new PendingRequestMap(eventTarget?) ``` - Creates an internal `PubSub` using `createPubSub` - If `eventTarget` is provided, passes it to `createPubSub` for transport-level event routing (Redis, WebSocket, etc.) - Wires subscription handlers for `call.responded`, `call.error`, and `call.aborted` to route events back to waiting callers - Subscriptions use empty-string id (`subscribe("call.responded", "")`) to receive all events of each type. Events are unwrapped from `EventEnvelope` via `.payload` ### `call(operationId, input, options?)` ```ts async call( operationId: string, input: unknown, options?: { parentRequestId?: string; deadline?: number; identity?: Identity }, ): Promise ``` 1. Generate `requestId` via `crypto.randomUUID()` 2. Create a `PendingRequest` with `resolve`/`reject` from a new Promise 3. If `deadline` is set, start a timeout timer that rejects with `TIMEOUT` 4. Store `PendingRequest` in the internal map 5. Publish `call.requested` event with all fields 6. Return the Promise (resolves with `ResponseEnvelope` on `call.responded`, rejects on `call.error` or `call.aborted`) The resolved value is a `ResponseEnvelope` — consumers access typed data via `envelope.data` and source metadata via `envelope.meta`. Use `unwrap(envelope)` as a convenience for the common case where only `data` is needed. ### Internal Subscription Wiring On construction, three async loops subscribe to pubsub topics: - **`call.responded`**: Look up `PendingRequest` by `requestId`, clear timer if set, resolve with the `ResponseEnvelope` from `output` field. The envelope is already validated by `respond()`'s `isResponseEnvelope()` guard (or created by `CallHandler`'s wrapping logic), so no additional validation is needed at this point. - **`call.error`**: Look up `PendingRequest`, clear timer, reject with `CallError(code, message, details)` - **`call.aborted`**: Look up `PendingRequest`, clear timer, reject with `CallError(ABORTED, ...)` ### `respond(requestId, output)` Publishes `call.responded`. The `output` parameter must be a `ResponseEnvelope` — `isResponseEnvelope()` is checked and a non-envelope value throws. This enforces the invariant that all call protocol responses carry source metadata. In practice, `respond()` is called by `CallHandler` after wrapping the handler's return value. Direct calls to `respond()` with raw values are rejected. ### `emitError(requestId, code, message, details?)` Publishes `call.error`. Used by handlers to send errors. ### `abort(requestId)` Looks up the `PendingRequest`, clears its timer, publishes `call.aborted`, rejects the Promise with `CallError(ABORTED, ...)`. ### `subscribe(operationId, input, options?)` ```ts subscribe( operationId: string, input: unknown, options?: { parentRequestId?: string; deadline?: number; identity?: Identity }, ): AsyncIterable ``` The subscription counterpart to `call()`. Uses the same event types (`call.requested`, `call.responded`, `call.aborted`, `call.error`) and the same `requestId` correlation, but yields each `call.responded` event instead of resolving after the first one. 1. Generate `requestId` via `crypto.randomUUID()` 2. Create a `Repeater` (from `@alkdev/pubsub`) keyed by `requestId` 3. Publish `call.requested` event with all fields 4. Return the `Repeater` as `AsyncIterable` 5. On each `call.responded:{requestId}` event: push `responded.output` to the repeater 6. On `call.error:{requestId}`: push the error (the Repeater throws from the iterator, then closes) 7. On `call.aborted:{requestId}`: close the repeater (iterator completes) 8. On consumer iteration stop: publish `call.aborted` and clean up This enables remote subscriptions over any `EventTarget` transport. The consumer iterates: ```ts for await (const envelope of pendingRequestMap.subscribe("opensemaphore.events", { repo: "alkdev/operations" }, { identity })) { // envelope is a ResponseEnvelope per SSE event } ``` The handler on the other side must be an `AsyncGenerator` (i.e., a `SubscriptionHandler`). The `CallHandler` on the hub side calls `subscribe()` internally and publishes each yielded envelope as `call.responded`. ### Internal Routing: `call()` vs `subscribe()` per `requestId` `PendingRequestMap` must track whether a given `requestId` belongs to a `call()` or a `subscribe()` to correctly route `call.responded` events. The internal data structure is: ```ts type PendingEntry = | { type: "call"; promise: PendingRequest } // single-resolution Promise | { type: "subscribe"; repeater: Repeater } // multi-yield AsyncIterable ``` When `call.responded:{requestId}` arrives: - If `type: "call"` → resolve the promise and delete the entry - If `type: "subscribe"` → push `responded.output` to the repeater (entry persists until the stream ends) When `call.error:{requestId}` arrives: - If `type: "call"` → reject the promise and delete the entry - If `type: "subscribe"` → push the error to the repeater (Repeater throws), then close the repeater and delete the entry When `call.aborted:{requestId}` arrives: - If `type: "call"` → reject the promise and delete the entry - If `type: "subscribe"` → close the repeater (iterator completes), then delete the entry Orphaned events (arriving after the consumer has stopped iterating or the entry was already deleted) are silently ignored — no error, no warning, no side effects. #### Relationship to `subscribe()` direct function | Aspect | `subscribe()` (direct) | `PendingRequestMap.subscribe()` | |--------|----------------------|-------------------------------| | Transport | In-process (calls handler directly) | Remote (via pubsub EventTarget) | | Handler access | Registry lookup in same process | Hub side: registry lookup, handler call | | Event routing | None — direct AsyncGenerator | `call.requested` → handler → `call.responded` per yield | | Use case | Local subscriptions | Cross-process subscriptions (WebSocket, Redis, etc.) | | Cleanup | Generator `return()` | Publish `call.aborted` + repeater stop | Both return `AsyncIterable` — same consumption pattern, different routing. ### Subscription Lifecycle #### Error Propagation | Scenario | `subscribe()` (direct) | `PendingRequestMap.subscribe()` | |----------|----------------------|-------------------------------| | Handler throws before first yield | `CallError` propagates to caller | `call.error` published, Repeater throws, consumer's `for await` catches the error | | Handler throws mid-stream | Error propagates from generator, `finally` block runs | `call.error` published, Repeater throws, consumer's `for await` catches the error | | Pre-handler errors (ACCESS_DENIED, VALIDATION_ERROR) | `CallError` thrown from `subscribe()` | `CallHandler` catches and publishes `call.error`, Repeater throws to consumer | | `call.error` event arrives (remote) | N/A (in-process) | Repeater receives error, consumer's `for await` sees it | #### Cleanup | Scenario | `subscribe()` (direct) | `PendingRequestMap.subscribe()` | |----------|----------------------|-------------------------------| | Consumer stops iteration (`break`) | `generator.return()` called in `finally` | `call.aborted` published, Repeater closed, hub's `CallHandler` receives abort and calls `generator.return()` | | Consumer's `for await` completes naturally | Generator's `return()` called if `finally` block has cleanup | Repeater stops, no `call.aborted` published | | Transport disconnect | N/A (in-process) | Hub may continue until timeout/closure; no heartbeat specified yet (see open questions) | #### Abortion When a consumer calls `pendingRequestMap.abort(requestId)` for a subscription: 1. `call.aborted:{requestId}` is published to the pubsub 2. The Repeater is closed (consumer's `for await` loop ends) 3. The hub-side `CallHandler` receives `call.aborted`, calls `generator.return()` on the subscription handler (triggers `finally` block for stream cleanup) When the direct `subscribe()` consumer breaks out of iteration, the generator's `finally` block runs automatically — no `call.aborted` event needed (in-process, no transport). ## CallHandler `buildCallHandler` creates a function that bridges pubsub events to `OperationRegistry.execute()` or `subscribe()`. It delegates to `execute()` for QUERY/MUTATION operations and to `subscribe()` for SUBSCRIPTION operations. ```ts function buildCallHandler(config: CallHandlerConfig): CallHandler interface CallHandlerConfig { registry: OperationRegistry callMap?: PendingRequestMap } type CallHandler = (event: CallRequestedEvent) => Promise ``` ### Handler Flow 1. Construct `OperationContext` from the event (`requestId`, `parentRequestId`, `identity` — `trusted` is NOT set, remote calls always run access control) 2. Look up the operation spec via `registry.getSpec(operationId)` 3. If spec not found → publish `call.error` with `OPERATION_NOT_FOUND` 4. If spec type is `QUERY` or `MUTATION`: a. Call `registry.execute(operationId, input, context)` — this performs all validation, access control, and result pipeline b. On success: publish `call.responded` via `callMap.respond(requestId, envelope)` c. On failure: `mapError` converts the thrown value to `CallError`, publish `call.error` 5. If spec type is `SUBSCRIPTION`: a. Call `subscribe(registry, operationId, input, context)` — this checks access control and wraps yields in `ResponseEnvelope` b. For each yielded envelope: publish `call.responded` via `callMap.respond(requestId, envelope)` c. On generator completion: iterator ends naturally d. On generator error: `mapError` converts the error, publish `call.error` e. On `call.aborted:{requestId}`: call `generator.return()` to clean up the subscription **Note on ADR-006**: [ADR-006](decisions/006-unified-invocation-path.md) specifies that `CallHandler` should be a "thin adapter that calls `registry.execute()`." For QUERY/MUTATION operations, this is the case. For SUBSCRIPTION operations, `CallHandler` must call `subscribe()` directly because `execute()` is designed for single-return operations (it `await`s the handler return value). Making `execute()` aware of SUBSCRIPTION type and routing to `subscribe()` internally would conflate two execution models in one function. The explicit dispatch in `CallHandler` is the correct design — `execute()` handles single-return operations, `subscribe()` handles streaming operations, and `CallHandler` routes between them based on `spec.type`. **Handling pre-generator errors**: Both `registry.execute()` and `subscribe()` can throw before reaching the handler (e.g., `OPERATION_NOT_FOUND`, `ACCESS_DENIED`, `VALIDATION_ERROR`). `CallHandler` wraps the entire invocation in a try/catch, so these pre-generator errors are caught and published as `call.error`. For subscriptions, this means errors during access control, validation, or spec/handler lookup are reported via `call.error`, not silently swallowed. **Key change**: In the pre-envelope model, handlers were responsible for publishing `call.responded` themselves (the handler return value was discarded). In the envelope model, `CallHandler` owns wrapping and publishing. Handler return values are captured and wrapped. This ensures every response goes through the envelope pipeline — no raw values can bypass it. ### Subscription Handling Detail For `SUBSCRIPTION` operations, the `CallHandler` iterates the async generator and publishes each yield as `call.responded`. This means a single `call.requested` event can produce **multiple** `call.responded` events with the same `requestId`. The `PendingRequestMap.subscribe()` method consumes this stream. The `call.aborted` event terminates the subscription. When the hub-side `CallHandler` receives `call.aborted:{requestId}`, it calls `generator.return()` on the active subscription handler. This triggers the generator's `finally` block (stream cleanup, resource release). ### MCP and OpenAPI Handlers Adapter handlers (from `from_mcp` and `from_openapi`) return pre-built `ResponseEnvelope` instances via `mcpEnvelope()` and `httpEnvelope()` factory functions. When `CallHandler` detects `isResponseEnvelope()` on the result, it passes through without re-wrapping. This means adapter metadata (HTTP status codes, MCP `isError` flags) is preserved. For MCP results with `meta.isError: true`, the handler still returns an envelope — the error is represented as data, not thrown. Only thrown exceptions trigger `call.error`. ## Access Control ### Enforcement Points Access control is enforced in two places: 1. **`registry.execute()`** — Checks `accessControl` on every invocation. Skips access control when `context.trusted === true` (nested calls from `buildEnv()`). When `requiredScopes` is non-empty and no `identity` is present, rejects with `ACCESS_DENIED`. 2. **`subscribe()`** — Checks `accessControl` when called. Skips access control when `context.trusted === true`. Same default-deny logic as `execute()`. 3. **`CallHandler`** — Delegates to `registry.execute()`, which performs access control. `CallHandler` does NOT set `trusted` on the context — remote calls always run access control because trust does not cross process boundaries. ### Flow ``` invoke execute(operationId, input, context) → if context.trusted → skip access control → if requiredScopes/requiredScopesAny/resourceType non-empty and no identity → ACCESS_DENIED → else check identity against accessControl → all pass → proceed to execute → any fail → ACCESS_DENIED ``` ### `checkAccess` Implementation ```ts function checkAccess(accessControl: AccessControl, identity: Identity): boolean ``` 1. If `requiredScopes` is non-empty, verify `identity.scopes` contains every entry (AND) 2. If `requiredScopesAny` is non-empty, verify `identity.scopes` contains at least one entry (OR) 3. If `resourceType` and `resourceAction` are set, verify `identity.resources["{resourceType}:{resourceId}"]` includes `resourceAction` 4. Return `true` if all applicable checks pass Note: Access control without an `identity` in the `CallRequestedEvent` is **allowed** — unauthenticated calls are permitted if the `AccessControl` check passes (e.g., operations with empty `requiredScopes`). ## Error Model The call protocol uses a unified error model. Both infrastructure and domain errors flow through `CallError`. ### `CallError` ```ts class CallError extends Error { readonly code: CallErrorCode // InfrastructureErrorCode | string readonly details?: unknown } ``` ### Infrastructure Error Codes Reserved codes produced by `CallHandler` and `PendingRequestMap`: | Code | When | Details | |------|------|---------| | `OPERATION_NOT_FOUND` | No operation matches `operationId` | `{ operationId: string }` | | `ACCESS_DENIED` | Missing scopes | `{ requiredScopes?: string[] }` | | `VALIDATION_ERROR` | Input fails `inputSchema` check | Wrapped from `Value.Errors` | | `TIMEOUT` | Deadline exceeded | `{ deadline: number }` | | `ABORTED` | Call cancelled | — | | `EXECUTION_ERROR` | Handler threw, no `errorSchemas` match | `{ message: string }` | | `UNKNOWN_ERROR` | Non-Error thrown | `{ raw: string }` | ### Domain Error Propagation Operations declare their possible errors via `errorSchemas` on `IOperationDefinition`. When a handler throws, `mapError` matches the thrown error against declared schemas — falls back to `EXECUTION_ERROR` if no match. `errorSchemas` is the contract between operation and callers about what errors it might produce. No `errorSchemas` = safe default with `EXECUTION_ERROR` wrapper. ### `mapError` Resolution 1. If already a `CallError`, return as-is 2. If `Error` instance and `errorSchemas` provided, check if `error.message` includes any declared error code → return `CallError(code, message, error)` 3. If `Error` instance, return `CallError(EXECUTION_ERROR, error.message, error)` 4. Otherwise, return `CallError(UNKNOWN_ERROR, String(error), { raw: String(error) })` ## Nested Call Wiring Routing is an env construction concern, not a separate protocol layer. `buildEnv` creates the `OperationEnv`: - **Unified mode**: `buildEnv({ registry, context })` — env functions call `registry.execute()` directly, returning `Promise`. The context is propagated with `trusted: true` so nested calls skip redundant access control checks. `parentRequestId` enables call graph reconstruction and abort cascading — every nested call includes it. ## Transport Mapping The call protocol is transport-agnostic. The `PubSub` event target determines how events move: | Transport | Use Case | EventTarget impl | |-----------|----------|-----------------| | In-process | Local hub operations | Browser `EventTarget` (default) | | Redis | Cross-process events | `RedisEventTarget` (from `@alkdev/pubsub`) | | WebSocket client | Spoke → Hub bidirectional | `WebSocketClientEventTarget` (from `@alkdev/pubsub`) | | WebSocket server | Hub → Spoke fan-out | `WebSocketServerEventTarget` (from `@alkdev/pubsub`) | Same protocol, same event shapes, same `PendingRequestMap` — different `eventTarget`. Both `call()` and `subscribe()` work over any transport — the only difference is consumption pattern (single-resolution Promise vs. AsyncIterable). ### WebSocket Topology The WebSocket event targets from `@alkdev/pubsub` carry call protocol events between hub and spoke processes: ``` Spoke process Hub process │ │ │ createPubSub({ │ createPubSub({ │ eventTarget: │ eventTarget: │ WebSocketClientET │ WebSocketServerET │ }) │ }) │ │ │ pendingRequestMap │ │ .call(op, input) ── ws ──────────────> │ CallHandler → execute() │ .subscribe(op, input) ── ws ─────────> │ CallHandler → subscribe() │ │ │ <── call.responded ────────────────── ws │ │ <── call.responded ────────────────── ws │ (multiple for subscribe) │ <── call.error ────────────────────── ws │ ``` The WebSocket client adapter handles `__subscribe`/`__unsubscribe` control events automatically — when `PendingRequestMap` subscribes to `call.responded:{requestId}`, the client adapter sends `__subscribe` for that topic, and the server adapter routes only matching events to that spoke. See [ADR-003](../../../@alkdev/pubsub/docs/architecture/decisions/003-subscription-control-protocol.md) in `@alkdev/pubsub`. ## Subscribe (Direct) The `subscribe()` function provides direct in-process subscription consumption: ```ts async function* subscribe( registry: OperationRegistry, operationId: string, input: unknown, context: OperationContext, ): AsyncGenerator ``` Gets the operation spec and checks access control (same default-deny logic as `execute()` — rejects with `ACCESS_DENIED` when `requiredScopes` is non-empty and no `identity` is present; skips check when `context.trusted`). Then casts the handler to `AsyncGenerator` and yields each value wrapped in `ResponseEnvelope`. If a yielded value `isResponseEnvelope()`, it passes through (e.g., for adapter handlers). Otherwise, `localEnvelope(value, operationId)` wraps it with a fresh `timestamp` per yield. Properly cleans up with `generator.return()` in a `finally` block. Use `subscribe()` for in-process consumption. Use `PendingRequestMap.call()` for cross-transport invocation that resolves after one event. For cross-transport streaming, use `PendingRequestMap.subscribe()` to yield multiple events. ### Handler Separation The `subscribe()` function looks up both spec and handler separately from the registry: 1. `registry.getSpec(operationId)` — throws if spec not found 2. `registry.getHandler(operationId)` — throws if handler not found This allows spec-only registration for scenarios where handlers are provided separately (e.g., ujsx host interpretation, dynamic handler injection). ## References - [response-envelopes.md](response-envelopes.md) — `ResponseEnvelope` types, factory functions, detection, and integration points - [ADR-005](decisions/005-response-envelopes.md) — Design rationale for response envelopes - [api-surface.md](api-surface.md) — Public API surface (types and signatures) - [adapters.md](adapters.md) — MCP and OpenAPI adapter internals