[rabbitmq-discuss] Sequential message processing guarantee

[Alexandru Scvorţov]

Hi Yogesh,

In the absence of consumer failures, RabbitMQ will deliver messages from
a queue in order.

So, if messages 1, 2, 3 reach a queue in order, RabbitMQ will deliver
them to consumers in the same order (1, 2, 3).

But, if consumers fail before acknowledging the messages (or if they
reject the messages), those messages will requeued at the end of the
queue.  So, if the consumer that got message 1 rejects it, the new
order of messages will be 2, 3, 1.

This all deals with message *delivery*.

> There are multiple consumer threads consuming the messages.
> Though multiple threads are consuming the messages, I observed that
> messages still get processed sequentially.
> Is that the case and if NOT, what is the way to guarantee sequential
> processing of messages on one queue?

The library makes no guarantees about the order in which you process
messages, only about the order in which they're delivered.

If you want to process all the messages on a queue in order, only
consume from one thread (doing basic.get from one thread like you
suggested in the other email would work, but would also be highly
inefficient).

Does this answer your question?

Cheers,
Alex

On Sun, Jan 22, 2012 at 09:38:40AM -0800, Yogesh Ketkar wrote:
> Running the code below, gives
> channel.basicConsume(QUEUE_NAME, autoAck, CONSUMER_TAG,
>     	new DefaultConsumer(channel)  {
>         @Override
>         public void handleDelivery(String consumerTag, Envelope
> envelope, BasicProperties properties, byte[] body) {
>                 System.out.println(Thread.currentThread().getName());
>          }
> }
> 
> o/p like
> pool-1-thread-1
> pool-1-thread-2
> pool-1-thread-3
> etc
> 
> There are multiple consumer threads consuming the messages.
> Though multiple threads are consuming the messages, I observed that
> messages still get processed sequentially.
> Is that the case and if NOT, what is the way to guarantee sequential
> processing of messages on one queue?
> 
> regards, Yogesh
> 
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