A few days ago I blogged about a situation we had where some messages send to RabbitMQ got lost. I showed a possible solution when using MassTransit. We further investigated the issue and a colleague(thanks Stijn!) suggested another solution by using a specific RabbitMQ feature: Alternate Exchanges.
The documentation explains the feature like this:
It is sometimes desirable to let clients handle messages that an exchange was unable to route (i.e. either because there were no bound queues or no matching bindings). Typical examples of this are
- detecting when clients accidentally or maliciously publish messages that cannot be routed
- "or else" routing semantics where some messages are handled specially and the rest by a generic handler
Alternate Exchange ("AE") is a feature that addresses these use cases.
Whenever an exchange with a configured AE cannot route a message to any queue, it publishes the message to the specified AE instead. If that AE does not exist then a warning is logged. If an AE cannot route a message, it in turn publishes the message to its AE, if it has one configured. This process continues until either the message is successfully routed, the end of the chain of AEs is reached, or an AE is encountered which has already attempted to route the message.
That is exactly the kind of problem we had, so Alternate Exchanges sounds like the perfect solution for us.
You can specify an alternate exchange through the MassTransit configuration:
However the recommended way to configure this is through RabbitMQ policies. When using a policy we specify a pattern(through a regex) to define when the policy should apply and the exact rules of the policy.
Policies can be created directly from the commandline:
rabbitmqctl set_policy AE "^my-direct$" '{"alternate-exchange":"my-ae"}' --apply-to exchanges
Or through the Management plugin:
- Go to the Management UI of your RabbitMQ cluster
- Click on Policies on the right
- Go to the Add / update a policy section. Specify a name, a regex pattern, the scope it should apply to and add one or more definitions. In our case we choose the Alternate exchange and specify the name of our exchange.
- Click on Add / update policy to save the policy.
- Now for every exchange that is created that matches the pattern an alternate-exchange is automatically configured.
That’s it!