[rabbitmq-discuss] Multiple rabbitmq servers, one or more consumers (.NET)

Bryan Murphy bmurphy1976 at gmail.com
Mon Feb 14 16:56:34 GMT 2011

I'm working on some advanced HA strategies for our infrastructure.
One thing I'm experimenting with is setting up one or many consumers
that can poll from queues on multiple RabbitMQ servers.

The idea is simple, set up multiple redundant servers configured the
same.  Send messages round-robin to the various servers, and then poll
the servers for messages.  If a server goes off the reservation, pull
it from the list until it comes back online or is replaced with a new
one and keep processing messages from the remaining servers.

I know there are other strategies to do this, but I wanted to
experiment with a few different techniques to see what works best for
us and learn a little bit more about how the driver works.

I dug around in the .NET driver code and managed to pull together an
implementation that uses a SharedQueue and multiple
connections/models.  I had to extend QueueingBasicConsumer and
BasicDeliverEventArgs to add a few properties that allow me to map the
messages back to the appropriate connection/model for acking.

This appears to work great at first pass, however, I don't believe
this will work well for long running jobs in a load balanced scenario
(multiple consumers).  What I believe will happen is this (using a
BasicQos of prefetchSize=0, prefetchCount=1):

message 1 --> rabbitmq server 1 --> consumer 1 shared local queue
message 2 --> rabbitmq server 2 --> consumer 1 shared local queue

(BasicQos of 1 is per model, and there's a model per connection, which
means there's a model per rabbitmq server)


consumer 1 shared local queue --> message 1 --> message 1
consumer 1 shared local queue --> message 2 --> blocked on message 1

and consumer 2 (3, 4, 5, etc.) are sitting idle because message 2 has
been allocated to consumer 1.

Instead of using model.BasicConsume, I could use model.BasicGet with a
timeout to fetch messages, but I've found the performance of this
method to be dreadful in the past so I wanted to try this method

So, my basic question, is there a simple way I get message 2 to not
block on message 1 and instead get delivered to a different consumer,
or am I barking up the wrong tree here?


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