Cancel Subscription

Overview

Every events store subscription opens a long-lived stream to the broker — a background handler that keeps pulling delivered events until you tell it to stop. Calling sub.cancel() on the subscription handle is how you release that stream deliberately: shutting down a worker, rotating consumers, or tearing down a process without leaking connections or leaving a dangling stream on the server.

Internally, cancel() sends a close signal that unwinds the receive loop and detaches from the broker-side subscription registration; it's safe to call from inside the onEvent callback itself, since the SDK finishes the current invocation before stopping the stream.

Gotchas: cancelling only stops this subscriber — the channel keeps storing every event published afterward, so nothing is lost, and a fresh subscription with StartFromSequence or StartFromFirst picks up exactly where this one left off. cancel() doesn't guarantee the stream has fully torn down the instant it returns, so don't assume zero more callbacks the moment you call it if precise synchronization matters.

Prerequisites

• KubeMQ server running on localhost:50000
• Node.js SDK installed (npm install kubemq-js)

Code

import { KubeMQClient, EventStoreStartPosition, createEventStoreMessage } from 'kubemq-js';

async function main() {
  const client = await KubeMQClient.create({
    address: 'localhost:50000',
    clientId: 'js-events-store-cancel-subscription-client',
  });

  try {
    let count = 0;
    const sub = client.subscribeToEventsStore({
      channel: 'js-events-store.cancel-subscription',
      startFrom: EventStoreStartPosition.StartFromNew,
      onEvent: (event) => {
        count++;
        console.log(`Received #${count}:`, new TextDecoder().decode(event.body));
        if (count >= 2) sub.cancel();
      },
      onError: (err) => {
        console.error('Error:', err.message);
      },
    });

    for (let i = 1; i <= 5; i++) {
      await client.sendEventStore(
        createEventStoreMessage({
          channel: 'js-events-store.cancel-subscription',
          body: `msg-${i}`,
        }),
      );
    }

    await new Promise((r) => setTimeout(r, 1000));
    console.log('Subscription cancelled after', count, 'messages');
  } finally {
    await client.close();
  }
}

main().catch(console.error);

How It Works

• StartFromNew subscribes without replaying history — only events published after this subscription is registered are delivered.
• The onEvent callback increments a counter and calls sub.cancel() when 2 messages have been received, stopping further delivery mid-stream.
• Calling cancel() from within the event callback is safe; the SDK completes the current invocation and then stops the stream.
• Events 3, 4, and 5 are still persisted on the server and can be replayed by a future subscriber using StartFromSequence or StartFromFirst.

Related

• Pattern overview
• Node.js SDK Reference
• Persistent Pub/Sub
• Consumer Group