[rabbitmq-discuss] Sequential message processing guarantee

[Yogesh Ketkar]

Thanks Alex.

I think, for sequential message processing, rather than this pattern
while(true) {
    GetResponse res = channel.basicGet(QUEUE_NAME, false);
    if(res != null) {
        // process and ack message
   }
}

using channel.basicConsume on the channel which is created like this
ExecutorService es = Executors.newSingleThreadExecutor();
Connection connection = factory.newConnection(es);
final Channel channel = connection.createChannel();

is probably a better approach.
Any comments on this?

regards, Yogesh

2012/1/23 Alexandru Scvorţov <alexandru at rabbitmq.com>

> 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
> >
> > _______________________________________________
> > rabbitmq-discuss mailing list
> > rabbitmq-discuss at lists.rabbitmq.com
> > https://lists.rabbitmq.com/cgi-bin/mailman/listinfo/rabbitmq-discuss
>
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