SSETarget

The @oselvar/ssetarget module exposes a simple API for dispatching Server-Sent Events to EventSource clients. Events are persisted, allowing clients to receive previous events - with support for the Last-Event-ID header.

Dispatch events:

const eventStore = new MemoryEventStore(); // Or use RedisEventStore
const sse = new SSETarget("/sse", eventStore);

await sse.dispatchEvent({
  type: "started",
});

Serve Server-Sent Events:

function handleRequest(req: Request) {
  return sse.fetch(req);
}

Cloudflare Workflow events

The library was originally developed to emit Server-Sent Events about the progress of Cloudflare Workflows.

This allows EventSource clients to be notified as workflow steps are started and completed (or failed).

See the ./src/examples directory and ./wrangler.toml for details.

Try out the example

Start the example:

# Terminal 1
npm start

Start a workflow:

curl -v -X POST http://localhost:9875

Listen to events (using the ID from the Location header)

curl http://localhost:9875/cbd3f7a0-e9ee-422d-8d33-9db7383dba2c/sse

The workflow will run some steps and emit SSEs, then wait for a workflow event. Trigger one to make the workflow resume:

curl -X POST http://localhost:9875/cbd3f7a0-e9ee-422d-8d33-9db7383dba2c/event