[rabbitmq-discuss] Message deleted at subscription?

[Julien Genestoux]

Thanks Matthias for this very helpful and complete answer. I was actually
not using AMQP, but STOMP... I guess the problem and the solution remain the
same.
Do you think I should rather switch from Stomp to Amqp?

Thanks,

--
Julien Genestoux
http://www.ouvre-boite.com
http://blog.notifixio.us

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On Sun, Jan 18, 2009 at 1:41 AM, Matthias Radestock <matthias at lshift.net>wrote:

> Julien,
>
> Julien Genestoux wrote:
>
>> A process puts messages in a queue every second roughly. Atfer some time,
>> I check the queue.
>>  > rabbitmqctl list_queues messages name consumers
>> Listing queues ...
>> 508    /queue/MyQueue    0
>>
>> Now, I start a consumer with the following code :
>>
>> queue_client.subscribe("/queue/FeedsToParse", {:durable => true,
>> "auto-delete".intern => false}) do |message|
>>  sleep 600
>>  puts "done!"
>> end
>>
>> As you can see, when starting this client, the number of elements in my
>> queue should decrease pretty slowly (1 every 10 minutes)... but here is what
>> happens if I check the queue again:
>>  > rabbitmqctl list_queues messages name consumers
>> Listing queues ...
>> 417    /queue/MyQueue    1
>>
>> So it seems that starting a consumer deletes some messages in the queue. I
>> have no idea why. Can anyone explain?
>>
>
> The 'messages' count is the sum of:
>
> - messages_ready - Number of ready messages (i.e. messages ready to be
> delivered to client)
>
> - messages_unacknowledged - Number of unacknowledged messages.
>
> - messages_uncommitted - Number of uncommitted messages (i.e. messages
> published in a transaction but not yet committed)
>
> Since your example does not involve transactions, only messages_ready +
> messages_unacknowledged come into play.
>
> On publication, messages_ready increases.
>
> When delivering messages to a client, messages_ready decreases and
> messages_unacknowledged increases by the same amount, so the sum remains the
> same.
>
> When an acknowledgement is received from the client,
> messages_unacknowledged decreases, and so does the sum.
>
> Now ...
>
> I am familiar with Aman's ruby client, but looking at the code and docs it
> appears that by default the subscribe method creates a consumer from which
> the server does not expect acknowledgements (and indeed there is no explicit
> ack in the code above). Here's what ruby client's docs say;
>
>    # == Options
>    # * :ack => true | false (default false)
>    # If this field is set to false the server does not expect
> acknowledgments
>    # for messages. That is, when a message is delivered to the client
>    # the server automatically and silently acknowledges it on behalf
>    # of the client. This functionality increases performance but at
>    # the cost of reliability. Messages can get lost if a client dies
>    # before it can deliver them to the application.
>
> So as soon as your code creates the subscription, the server will start
> sending it the queue's messages (resulting in messages_ready to decrease)
> and automatically acknowledge all such messages (resulting in
> messages_unacknowledged to remain unchanged), and thus the 'messages' count
> decreases. This is an asynchronous operation; so the client will receive a
> stream of messages without having to do anything. The stream is only paused
> by network/tcp congestion, and basic.qos (search the archives for
> discussions on the latter).
>
> If that is not what you want I suggest you change the flag to true and
> explicitly acknowledge messages in your code.
>
>
> Regards,
>
> Matthias
>
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