--- status: draft last_updated: 2026-04-30 --- # 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()`. ## 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: Type.Unknown(), }), "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()), }), } ``` ### 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} │ │ │ │<── call.responded ────────────────│ │ {requestId, output} │ ``` 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 ### `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 on `call.responded`, rejects on `call.error` or `call.aborted`) ### 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 `output` - **`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`. Used by handlers to send results back through the protocol. ### `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, ...)`. ## CallHandler `buildCallHandler` creates a function that bridges pubsub events to `OperationRegistry.execute()`. ```ts function buildCallHandler(config: CallHandlerConfig): CallHandler interface CallHandlerConfig { registry: OperationRegistry eventTarget?: EventTarget } type CallHandler = (event: CallRequestedEvent) => Promise ``` ### Handler Flow 1. Look up operation by `operationId` from the registry 2. If not found, throw `CallError(OPERATION_NOT_FOUND, ...)` 3. Check access control (see below) 4. Validate input with `validateOrThrow` 5. Execute operation handler 6. On success: the handler is expected to have published `call.responded` through whatever mechanism 7. On failure: `mapError` converts the thrown value to `CallError` The `CallHandler` is designed to be wired into a pubsub subscription: ```ts const callHandler = buildCallHandler({ registry, eventTarget }) pubsub.subscribe("call.requested", callHandler) ``` ## Access Control ### Enforcement Point `CallHandler` enforces `AccessControl` before dispatching to `registry.execute()`. Direct `registry.execute()` calls bypass access control — this is by design for trusted internal calls. ### Flow ``` call.requested event arrives with Identity → Look up operation's AccessControl → Check requiredScopes (caller has ALL?) → Check requiredScopesAny (caller has ANY?) → Check resourceType/resourceAction against identity.resources → All pass → proceed to execute → Any fail → throw CallError(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`: - **Direct mode**: `buildEnv({ registry, context })` — env functions call `registry.execute()` directly - **Call protocol mode**: `buildEnv({ registry, context, callMap })` — env functions call `callMap.call()`, publishing `call.requested` events with `parentRequestId` propagation `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 | Hub ↔ spoke bidirectional | `WebSocketEventTarget` (future) | Same protocol, same event shapes, same `PendingRequestMap` — different `eventTarget`. ## 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 from the registry, casts its handler to `AsyncGenerator`, and yields values. 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.