consuming-messages.md

RedisSMQ / Docs / Consuming Messages

Consuming Messages

A Consumer instance can be used to receive and consume messages from one or multiple queues.

To consume messages from a queue, the Consumer Class class provides the Consumer.consume() method which allows to register a message handler.

A message handler is a function that receives a delivered message from a given queue.

Message handlers can be registered at any time, before or after a consumer has been started.

In contrast to producers, consumers are not automatically started upon creation. To start a consumer use the Consumer.run() method.

To stop consuming messages from a queue and to remove the associated message handler from your consumer, use the Consumer.cancel() method.

To shut down completely your consumer and tear down all message handlers, use the Consumer.shutdown() method.

'use strict';

const { Consumer } = require('redis-smq');

const consumer = new Consumer();

const messageHandler = (msg, cb) => {
  console.log('Message payload', msg.body);
  cb(); // acknowledging the message
};

consumer.consume('test_queue', messageHandler, (err) => {
  if (err) console.error(err);
});

consumer.run((err) => {
  if (err) console.error(err);
});

Alternatively we can also register a message handler strictly after that a consumer has been started.

'use strict';

const { Consumer } = require('redis-smq');

const consumer = new Consumer();

consumer.run((err) => {
  if (err) console.error(err);
  else {
    const messageHandler = (msg, cb) => {
      console.log('Message payload', msg.body);
      cb(); // acknowledging the message
    };
    consumer.consume('test_queue', messageHandler, (err) => {
      if (err) console.error(err);
    });
  }
});

Message Acknowledgement

Once a message is received, to acknowledge it, you invoke the callback function without arguments, as shown in the example above.

Message acknowledgment informs the MQ that the delivered message has been successfully consumed.

If an error occurred while processing a message, you can unacknowledge it by passing in the error to the callback function.

By default, unacknowledged messages are re-queued and delivered again unless message retry threshold is exceeded.

Delivered messages that couldn't be processed or can not be delivered to consumers are moved to a system generated queue called dead-letter queue (DLQ).

By default, RedisSMQ does not store acknowledged and dead-lettered messages for saving disk and memory spaces, and also to increase message processing performance.

If you need such feature, you can enabled it from your configuration object.

See Consumer Class for more details.