[rabbitmq-discuss] Message redelivering doubt with one and with more than one consumers.

[Alexandru Scvorţov]

Hi,

> In a scenario with one consumer as I saw in my tests, the consumer
> received messages with the Redelivered=true AMQP header atribute.
> The consumer was based in the classes provided by the .NET API and as
> I remember the consumer-tag is random generated by the server.

Sounds good.

> When there is a only one consumer (with a random consumer-tag name)
> and a message is not acked when the consumer is reconnected (so has
> another random tag name) that message is redelivered?

Yes, un-ack'd messages will be redelivered to the consumer when it
reconnects.  Be aware, though, they will be redelivered *after* the
messages still on the queue (i.e. the undelivered ones) are delivered.

So, if the following messages are published to the queue:
  1 2 3 4 5 6

And the following are delivered but not ack'd by the consumer:
  1 2 3

If the consumer crashes and reconnects, the messages delivered in order
will be:
  4 5 6 1 2 3

> Based on my experience I think this is true but don't understand that
> "If that same consumer then reconnects and creates a new subscription
> to the same queue"...

That's just another way of saying it reconnects and starts consuming
again.

> By the was my consumer always tries to create the queue and the
> bindings every time it connects to RabbitMQ.

That's perfect.  You should always do that.

> And also I'm wondering how this behavior acts when there are more than
> one consumer (same queue and same bindings)...

Messages are delivered one at a time to consumers in round-robin order.
If a message is delivered and the consumer dies, the message is
republished to the queue and a consumer will eventually get it (note
that it may be the same client if it reconnects).

> Under what circumstances an unack message wouldn't be redelivered again?

In none of the circumstances we were talking about.  A client can
basic.reject{requeue=false} a message, in which case it will not be
requeued.

Hope this helps.

Cheers,
Alex

On Thu, Feb 10, 2011 at 10:26:53AM +0100, Alfonso Pantoja wrote:
> Hi,
> 
> on the RabbitMQ FAQ page I see the response to
> 
> "Can you explain the cases where a message might be delivered to a
> consumer more than once?"
> the following:
> 
> "If a message is delivered to a consumer, and that consumer then dies
> (or closes the channel which has the subscription to the queue, or
> closes the connection itself) without acking the message, then
> RabbitMQ will reinject the message into the queue. If that same
> consumer then reconnects and creates a new subscription to the same
> queue, it's possible it'll receive the same message again. This is 'at
> least once' delivery and is very deliberate design to ensure that
> messages are not lost in transit."
> 
> 
> So my doubt is:
> 
> In a scenario with one consumer as I saw in my tests, the consumer
> received messages with the Redelivered=true AMQP header atribute.
> The consumer was based in the classes provided by the .NET API and as
> I remember the consumer-tag is random generated by the server.
> 
> When there is a only one consumer (with a random consumer-tag name)
> and a message is not acked when the consumer is reconnected (so has
> another random tag name) that message is redelivered?
> Based on my experience I think this is true but don't understand that
> "If that same consumer then reconnects and creates a new subscription
> to the same queue"...
> By the was my consumer always tries to create the queue and the
> bindings every time it connects to RabbitMQ.
> 
> And also I'm wondering how this behavior acts when there are more than
> one consumer (same queue and same bindings)...
> 
> Under what circumstances an unack message wouldn't be redelivered again?
> 
> Thank you in advance.
> 
> Regards,
> 
> Alfonso
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> The consumer can also call basic.recover which tells rabbit to resend
> all the messages sent to the consumer for which rabbit has not
> received an ack for. This basically amounts to the consumer saying "I
> know you sent me some messages, but I've forgotten what they are.
> Could you resend them all again?".
> _______________________________________________
> rabbitmq-discuss mailing list
> rabbitmq-discuss at lists.rabbitmq.com
> https://lists.rabbitmq.com/cgi-bin/mailman/listinfo/rabbitmq-discuss