pubsub/docs/architecture/event-targets.md

---
status: draft
last_updated: 2026-05-01
---

# Event Target Adapters

In-process, Redis, WebSocket, and Worker event targets. All implement `TypedEventTarget<TEvent>`.

## Interface Contract

Every adapter must implement:

| Method | Behavior |
|--------|----------|
| `addEventListener(type, callback)` | Register listener for event type. Callback receives `CustomEvent` with typed `detail` (an `EventEnvelope`). |
| `dispatchEvent(event)` | Send/dispatch event. Returns `boolean` (always `true` for non-cancelable events). |
| `removeEventListener(type, callback)` | Unregister listener. Clean up underlying subscription when no listeners remain for a topic. |

All adapters use the `EventEnvelope` format (`{ type, id, payload }`) as the serialization contract. Adapters that cross process boundaries (Redis, WebSocket, Iroh) serialize/deserialize the full envelope as JSON.

## In-Process (Default)

No adapter needed. `createPubSub` uses `new EventTarget()` by default. This works for single-process deployments where all pubsub participants share the same memory.

No explicit `InProcessEventTarget` class — the web standard `EventTarget` already implements the interface. Could be formalized later if a name makes the API clearer, but `new EventTarget()` is already the standard.

## Redis

**Import**: `@alkdev/pubsub/event-target-redis`
**Peer dep**: `ioredis@^5.0.0` (optional)

### `createRedisEventTarget`

```ts
function createRedisEventTarget<TEvent extends TypedEvent>(
  args: CreateRedisEventTargetArgs,
): TypedEventTarget<TEvent>;
```

### `CreateRedisEventTargetArgs`

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `publishClient` | `Redis \| Cluster` | Yes | ioredis client for publishing. Can share a connection. |
| `subscribeClient` | `Redis \| Cluster` | Yes | ioredis client for subscribing. Must be dedicated — Redis requires subscriber connections to only receive messages. |
| `serializer` | `{ stringify, parse }` | No | Custom serializer. Defaults to `JSON`. |

### How It Works

- `dispatchEvent` → `publishClient.publish(event.type, serializer.stringify(event.detail))`
- `addEventListener` → `subscribeClient.subscribe(topic)`, track callbacks per topic
- `removeEventListener` → remove callback; if no callbacks remain for topic, `subscribeClient.unsubscribe(topic)`
- On message: deserializes with `serializer.parse`, reconstructs `CustomEvent(channel, { detail: envelope })`

The `detail` of the `CustomEvent` dispatched to local listeners is the full `EventEnvelope` object (`{ type, id, payload }`).

### Channel Naming

Currently uses the topic string directly as the Redis channel name (e.g., `session.status:proj_123`). Architecture recommends `alk:events:{eventType}` prefix but this is not yet implemented. Should be configurable: `createRedisEventTarget({ ..., prefix: "alk:events:" })`.

### Limitations (Current)

- **No error handling** — connection failures, reconnection, and message parse errors are not handled
- **No channel prefix** — raw event types as channel names risk collision in shared Redis instances
- **No unsubscribe cleanup on client disconnect** — if the subscribe client disconnects, registered callbacks remain in the map but will never fire

### Test Coverage

No tests yet (test directory is empty). Previous alkhub had 5 Redis tests (publish path only, mocked ioredis).

## WebSocket

**Import**: `@alkdev/pubsub/event-target-websocket` (not yet implemented)
**Peer dep**: none (WebSocket is a web standard)

### Design (Spec from `spoke-runner.md`)

```ts
class WebSocketEventTarget implements TypedEventTarget<any> {
  private listeners = new Map<string, Set<(event: CustomEvent) => void>>()

  constructor(private ws: WebSocket) {
    ws.onmessage = (msg) => {
      const envelope = JSON.parse(msg.data as string) // { type, id, payload }
      const topic = `${envelope.type}:${envelope.id}`
      const event = new CustomEvent(topic, { detail: envelope })
      for (const listener of this.listeners.get(topic) ?? []) {
        listener(event)
      }
    }
  }

  dispatchEvent(event: CustomEvent): boolean {
    this.ws.send(JSON.stringify(event.detail)) // sends { type, id, payload }
    return true
  }

  addEventListener(type: string, listener: (event: CustomEvent) => void): void { ... }
  removeEventListener(type: string, listener: (event: CustomEvent) => void): void { ... }
}
```

### Key Properties

- **Bidirectional** — `dispatchEvent` sends over WS, `addEventListener` receives from WS
- **Per-connection** — hub creates one per spoke connection
- **JSON framing** — WebSocket provides native message boundaries (no length-prefix needed)
- **No native deps** — works in browsers and Node
- **Envelope serialization** — sends/receives the full `EventEnvelope` JSON (`{ type, id, payload }`)

### Gap: Reconnection

WebSocket connections drop. On reconnect, the spoke must re-register with the hub (same `hub.register` flow). The `WebSocketEventTarget` itself is per-connection — a new connection means a new event target instance. Reconnection logic belongs to the spoke lifecycle, not the event target.

## Worker

**Import**: `@alkdev/pubsub/event-target-worker` (not yet implemented)
**Peer dep**: none (Web Worker API is standard)

### Design

A `WorkerEventTarget` implementing `TypedEventTarget` over `postMessage`/`onmessage`. This enables `createPubSub` to work across Web Worker boundaries.

The worker message protocol uses the `EventEnvelope` format:

```json
{ "type": "call.responded", "id": "uuid-123", "payload": { "output": 42 } }
```

### Two-Sided Design

- **Main thread** (`WorkerPoolManager` side): dispatches typed messages to workers via `worker.postMessage()`, receives responses via `worker.onmessage`
- **Worker thread**: dispatches to main thread via `parentPort.postMessage()`, receives from main thread via `globalThis.onmessage`

Both sides wrap `postMessage`/`onmessage` to implement the `TypedEventTarget` interface:

```ts
// Main thread side
const workerEventTarget = createWorkerEventTarget(worker);

// Worker thread side
const mainEventTarget = createMainThreadEventTarget();
```

### Key Properties

- **Bidirectional** — both sides can publish and subscribe
- **Per-worker** — each worker gets its own event target
- **Structured clone** — Web Workers use structured clone algorithm for serialization, but JSON-serializable `EventEnvelope` ensures cross-platform compatibility
- **No native deps** — works in any environment with Web Worker support

### Relationship to Taskgraph / Operations

The worker event target enables distributed operation execution. Workers can subscribe to `call.requested` events and publish `call.responded` events through the event target, allowing `@alkdev/operations` to dispatch work to worker threads via the same pubsub transport.