Unsubscribe from a specific queue using node-amqplib on reconnect

Question

problem: remote systems reconnect to multiple nodes websocket server, for each system a dedicated queue in RabbitMQ is created/used. The queues should be automatically removed if no active connections exist. Websocket connect/disconnect events handlers are asynchronous, quite heavy, observed problem that a disconnect event handler finished after reconnect, making system inconsistent.

The main issue is with RabbitMQ queues - initial solution was to create unique queues for each connection and remove them on disconnect. Appeared to be heavy.

Second approach was to keep a dedicated queue per remote system (same queue name for any connection), the problem was that assertQueue added consumers for the same queue. Need to find way to remove stale queue consumers without removing the queue itself.

Answer 1

Solution is to store list of consumers per remote system and on disconnect event trigger cancel function with the olderst consumerTag, then update the list of queue consumers for the given remote system.

on remote system connect event

import { Replies } from "amqplib";

// bind callback function for queue-specific messages and store returned consumer description
const result: Replies.Consume = await channel.consume(queueName, this.onSomeMessage.bind(this)); 

// update consumers list for the connected remote system
const consumers: Array<string> | undefined = this.consumers.get(remoteId);
if (consumers === undefined) {
  const consumersList: Array<string> = new Array();
  consumersList.push(result.consumerTag);
  this.consumers.set(remoteId, consumersList);
} else {
  consumers.push(result.consumerTag);
}

on remote system disconnect event

// remove the oldest consumer in the list and update the list itself
// use cancel method of the amqp channel
const consumers = this.consumers.get(remoteId);
if (consumers === undefined) {  
  // shouldn't happen
  console.error(`consumers list for ${remoteId} is empty`);
} else {
  const consumerTag = consumers[0];
  await this.rxchannel.addSetup(async (channel: ConfirmChannel) => {
    await channel.cancel(consumerTag);
    consumers.shift();
  });
}

The code snippets are from some class' methods implementation (if you're wondering about "this").

Copyright notice (especially for German colleagues): the code from this answer can be used under Beerware (https://en.wikipedia.org/wiki/Beerware) or MIT license (whatever one prefers).