RabbitMQ - Message order of delivery

I need to choose a new Queue broker for my new project.

This time I need a scalable queue that supports pub/sub, and keeping message ordering is a must.

I read Alexis comment: He writes:

> "Indeed, we think RabbitMQ provides stronger ordering than Kafka"

I read the message ordering section in rabbitmq docs:

> "Messages can be returned to the queue using AMQP methods that feature > a requeue > parameter (basic.recover, basic.reject and basic.nack), or due to a channel > closing while holding unacknowledged messages...With release 2.7.0 and later > it is still possible for individual consumers to observe messages out of > order if the queue has multiple subscribers. This is due to the actions of > other subscribers who may requeue messages. From the perspective of the queue > the messages are always held in the publication order."

If I need to handle messages by their order, I can only use rabbitMQ with an exclusive queue to each consumer?

Is RabbitMQ still considered a good solution for ordered message queuing?

Well, let's take a closer look at the scenario you are describing above. I think it's important to paste the documentation immediately prior to the snippet in your question to provide context:

> Section 4.7 of the AMQP 0-9-1 core specification explains the > conditions under which ordering is guaranteed: messages published in > one channel, passing through one exchange and one queue and one > outgoing channel will be received in the same order that they were > sent. RabbitMQ offers stronger guarantees since release 2.7.0. > > Messages can be returned to the queue using AMQP methods that feature > a requeue parameter (basic.recover, basic.reject and basic.nack), or > due to a channel closing while holding unacknowledged messages. Any of > these scenarios caused messages to be requeued at the back of the > queue for RabbitMQ releases earlier than 2.7.0. From RabbitMQ release > 2.7.0, messages are always held in the queue in publication order, even in the presence of requeueing or channel closure. (emphasis added)

So, it is clear that RabbitMQ, from 2.7.0 onward, is making a rather drastic improvement over the original AMQP specification with regard to message ordering.

With multiple (parallel) consumers, order of processing cannot be guaranteed.
The third paragraph (pasted in the question) goes on to give a disclaimer, which I will paraphrase: "if you have multiple processors in the queue, there is no longer a guarantee that messages will be processed in order." All they are saying here is that RabbitMQ cannot defy the laws of mathematics.

Consider a line of customers at a bank. This particular bank prides itself on helping customers in the order they came into the bank. Customers line up in a queue, and are served by the next of 3 available tellers.

This morning, it so happened that all three tellers became available at the same time, and the next 3 customers approached. Suddenly, the first of the three tellers became violently ill, and could not finish serving the first customer in the line. By the time this happened, teller 2 had finished with customer 2 and teller 3 had already begun to serve customer 3.

Now, one of two things can happen. (1) The first customer in line can go back to the head of the line or (2) the first customer can pre-empt the third customer, causing that teller to stop working on the third customer and start working on the first. This type of pre-emption logic is not supported by RabbitMQ, nor any other message broker that I'm aware of. In either case, the first customer actually does not end up getting helped first - the second customer does, being lucky enough to get a good, fast teller off the bat. The only way to guarantee customers are helped in order is to have one teller helping customers one at a time, which will cause major customer service issues for the bank.

I hope this helps to illustrate the problem you are asking about. It is not possible to ensure that messages get handled in order in every possible case, given that you have multiple consumers. It doesn't matter if you have multiple queues, multiple exclusive consumers, different brokers, etc. - there is no way to guarantee a priori that messages are answered in order with multiple consumers. But RabbitMQ will make a best-effort.

Message ordering is preserved in Kafka, but only within partitions rather than globally. If your data need both global ordering and partitions, this does make things difficult. However, if you just need to make sure that all of the same events for the same user, etc... end up in the same partition so that they are properly ordered, you may do so. The producer is in charge of the partition that they write to, so if you are able to logically partition your data this may be preferable.

I think there are two things in this question which are not similar, consumption order and processing order.

Message Queues can -to a degree- give you a guarantee that messages will get consumed in order, they can't, however, give you any guarantees on the order of their processing.

The main difference here is that there are some aspects of message processing which cannot be determined at consumption time, for example:

• As mentioned a consumer can fail while processing, here the message's consumption order was correct, however, the consumer failed to process it correctly, which will make it go back to the queue, and until now the consumption order is still intact but we don't know how the processing order is now

• If by "processing" we mean that the message is now discarded and finished processing completely, then consider the case when your processing time is not linear, in other words processing one message takes longer than another, so if message 3 takes longer in processing than anticipated, then messages 4 and 5 might get consumed and finish processing before message 3 does

So even if you managed to get the message back to the front of the queue (which by the way violates the consumption order) you still cannot guarantee that all messages before the next message have finished processing.

If you want to ensure the processing order then:

1. Have only 1 consumer instance at all times
2. Or don't use a messaging queue and do the processing in a synchronous blocking method, which might sound bad but in many cases and business requirements is completely valid and sometimes even critical

There are proper ways to guarantuee the order of messages within RabbitMQ subscriptions.

If you use multiple consumers, they will process the message using a shared ExecutorService. See also ConnectionFactory.setSharedExecutor(...). You could set a Executors.newSingleThreadExecutor().

If you use one Consumer with a single queue, you can bind this queue using multiple bindingKeys (they may have wildcards). The messages will be placed into the queue in the same order that they were received by the message broker.

For example you have a single publisher that publishes messages where the order is important:

try (Connection connection2 = factory.newConnection();
        Channel channel2 = connection.createChannel()) {
    // publish messages alternating to two different topics
    for (int i = 0; i < messageCount; i++) {
        final String routingKey = i % 2 == 0 ? routingEven : routingOdd;
        channel2.basicPublish(exchange, routingKey, null, ("Hello" + i).getBytes(UTF_8));
    }
}

You now might want to receive messages from both topics in a queue in the same order that they were published:

// declare a queue for the consumer
final String queueName = channel.queueDeclare().getQueue();

// we bind to queue with the two different routingKeys
final String routingEven = "even";
final String routingOdd = "odd";
channel.queueBind(queueName, exchange, routingEven);
channel.queueBind(queueName, exchange, routingOdd);
channel.basicConsume(queueName, true, new DefaultConsumer(channel) { ... });

The Consumer will now receive the messages in the order that they were published, regardless of the fact that you used different topics.

There are some good 5-Minute Tutorials in the RabbitMQ documentation that might be helpful: https://www.rabbitmq.com/tutorials/tutorial-five-java.html