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Well, I’ll let you go … basic.reject in RabbitMQ

Support for AMQP's basic.reject just landed on default. It's taken this long because we couldn't agree on a single set of semantics that followed the specification, was actually useful, and wasn't too complicated to implement.

First up, this is what RabbitMQ's basic.reject will do: if you supply requeue=false it will discard the message (this is in lieu of dead-lettering it, until we have a dead letter feature); if you supply requeue=true, it will release it back on to the queue, to be delivered again.

The first is useful from a error handling point of view; if your application cannot process a particular message, you can get rid of it. At the minute, it's semantically the same as just acking the message; but, given a dead-letter mechanism, it will mean unprocessable messages can be picked up elsewhere for diagnosis.

The second, with requeue=true, is useful for example if your application needs a "message lock" semantics. In this scenario, a consumer can be delivered a message, then decide not to deal with it after all, and place it back on the queue. Note that RabbitMQ doesn't take care to stop the same consumer getting the message again -- see below.

The AMQP 0-9-1 specification says a number of seemingly incompatible things about basic.reject. For a start, it says in the method description

The client MUST NOT use this method as a means of selecting messages to process.

and in the specification XML (but not in the generated PDF),

The server MUST NOT deliver the message to the same client within the context of the current channel. The recommended strategy is to attempt to deliver the message to an alternative consumer, and if that is not possible, to move the message to a dead-letter queue. The server MAY use more sophisticated tracking to hold the message on the queue and redeliver it to the same client at a later stage.

This seems to suggest that the server has to take care not to deliver the message to the same consumer twice, but consumers are not allowed to rely on this prohibition. This means basic.reject could either redeliver the message or dead-letter it, which makes it useless for the "message lock" scenario given above.

So, we have chosen to implement the simplest thing that is useful, which is to re-enqueue the message and treat it as though it were completely new. This means the consumer can receive again a message it has rejected.

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5 Responses to “Well, I’ll let you go … basic.reject in RabbitMQ”

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  2. Matthew Sackman Says:

    One comparison to make is with basic.recover. Basic.recover{requeue=true} is similar in this respect to basic.reject{requeue=true} in that it pushes messages back into the queue. However the important difference is that basic.recover{requeue=true} requeues all unacknowledged messages on the channel, whereas basic.reject{requeue=true} only requeues the indicated unacknowledged message.

  3. Vishnu Iyengar Says:

    Is there any requeue count or any such metadata that comes with the message. While the requeue=true is nice if a message cannot be dealt with at some point in time, there must be a way to prevent a message from looping through the system forever

  4. Michael Says:

    @Vishnu basic.deliver has a redelivery flag that will be set if the message has been delivered at least once already. In AMQP 0-8 and 0-9-1 (at least) it's a boolean rather than a count.

  5. Hussain Says:

    Also observed that , just by doing a reject on channel doesn't bring messages in Ready state, one needs to call abort on the channel.