---
status: draft
last_updated: 2026-04-30
---

# Event Target Adapters

In-process, Redis, and WebSocket 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`. |
| `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. |

## 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)`

### Channel Naming

Currently uses raw event type as 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

5 tests in alkhub (publish path only, mocked ioredis). No tests for subscription-receive path, unsubscribe cleanup, or error handling.

## 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 { type, payload } = JSON.parse(msg.data as string)
      const event = new CustomEvent(type, { detail: payload })
      for (const listener of this.listeners.get(type) ?? []) {
        listener(event)
      }
    }
  }

  dispatchEvent(event: CustomEvent): boolean {
    this.ws.send(JSON.stringify({ type: event.type, payload: event.detail }))
    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

### 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.

### Gap: Hub-Side Architecture

The hub needs per-connection event target + `PendingRequestMap` creation on accept, cleanup on disconnect. This is a hub architectural concern, not a pubsub concern. See `@alkdev/alkhub_ts/docs/architecture/spoke-runner.md`.