[rabbitmq-discuss] How to achieve HA consumers?

Wed Sep 5 14:51:52 BST 2012

Hi Matthias,

Thanks for your reply. I wrote some test code and found a wired problem.

The init code is like

 factory = new ConnectionFactory();
 connection = factory.newConnection(address);
 channel = connection.createChannel();
 channel.basicQos(1);

And the receive code is like

 QueueingConsumer consumer = new QueueingConsumer(channel);
 boolean autoAck = false; 

 Queue.DeclareOk declare = channel.queueDeclarePassive("LBCQueue");
 int consumerCount = declare.getConsumerCount();
 if (consumerCount > 0)
  return false;

 channel.basicConsume("LBCQueue", autoAck, consumer);

First I started the first consumer and didn't publish any message. 

Then I started the second consumer and the value of the variable declare is #method<queue.declare-ok>(queue=LBCQueue, message-count=0, consumer-count=1).
So far everything is OK.

Now I publish 1 messge into the queue and restart the second consumer. The value of the variable declare is #method<queue.declare-ok>(queue=LBCQueue, message-count=0, consumer-count=1).
So far everything is still OK.

Then I publish another message into the queue and restart the second consumer again. 

This time the value of declare is #method<queue.declare-ok>(queue=LBCQueue, message-count=1, consumer-count=0).
The message-count is correct but the consumer-count is changed from 1 to 0. I don't know why. Is it a bug?

johnson

From: Matthias Radestock
Date: 2012-09-05 15:48
To: johnson
CC: rabbitmq-discuss; Francesco Mazzoli
Subject: Re: [rabbitmq-discuss] How to achieve HA consumers?
Johnson,

On 05/09/12 05:32, johnson wrote:
> The channel.queueDeclarePassive method returns the consumer count of
>  the specified channel,

No it doesn't. queue.declarePassive returns the total number of active
consumers of the queue at that point in time. Regardless of what
connection/channel they were created on.

That should be pretty clear from the docs -
http://www.rabbitmq.com/amqp-0-9-1-reference.html#queue.declare-ok.consumer-count
- which make no mention of the count being connection/channel specific.

> So if I start two consumers [...] in two different machines [...] the
> channel.queueDeclarePassive method always return the consumer count
> == 1, not 2.

I am not sure what you are expecting to see here. If you start two
consumers, the count they will see prior to consuming will be 0 or 1.

As mentioned "You'd need to be mindful of races when consumers start 
up.", i.e. if both consumers start up near simultaneously then they will 
both see a count of 0.

> I also try to find a API that can retrieve the existing connection or
> channel. Then the second consumer can reuse the connection or channel
> the first consumer created. But it seems no such APIs.

A connection represents a point-to-point TCP connection. You can't 
"reuse" that from a third point.

Regards,

Matthias.
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