[rabbitmq-discuss] Shunting messages from one queue to another

[Tom Anderson]

Hi,

We have a queue which is used to send messages to an app, which then 
processes them, and inserts some data into a database. Perhaps people 
will have seen similar setups before :). Sometimes, the app cannot 
process the message for some reason, and will use basic.reject with 
requeue=false to discard it. We have a dead-letter queue, so discarded 
messages end up there.

Sometimes (actually, always), failures to process are not really due to 
errors in the messages, but in the processing code in the application. 
For example, we had a case where a zero in a particular field in a 
message led to an infinity after some calculations, which then caused 
database inserts to blow up. As another, we have an ongoing epidemic of 
database deadlocks causing transient failures to insert.

Our response to these problem is to fix the bug in the processing code, 
and then re-run the messages. We do this by prodding the upstream 
message-sending system to repeat all the relevant messages. This is 
manual and error-prone (it involves deleting documents from a MongoDB 
server by hand, then bouncing an app server >_<), and also rather 
inefficient.

What i would like to do instead is move the discarded messages from the 
dead-letter queue back to the main queue. In fact, we may want to move 
messages from the dead-letter queue to a holding queue, and from there 
back to the main queue, so that an infinite loop is never possible. What 
is the best way to do this?

We could write code of our own that talks to RabbitMQ to move the 
messages. I would like to avoid doing this if possible, because every 
line of code we write is a line of code we can get wrong.

We could use the shovel plugin to do this - in some ways this seems 
ideal. However, shovels are configured in RabbitMQ's configuration files 
as essentially permanent features, which we don't want; we want to be 
able to run a shovel-like process just once, rather than constantly. 
Also, shovels seem to be geared towards moving messages between queues 
in different brokers, whereas in our case, both queues are in a single 
broker. Would it still be sensible to use a shovel?

If not, are there any other utilities in the RabbitMQ ecosystem which we 
could use for this?

Thanks in advance,
tom

-- 

Tom Anderson | Developer | +44 20 7826 4312 | timgroup.com 
<http://timgroup.com/>

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