Skip to content

Latest commit

 

History

History
73 lines (56 loc) · 3 KB

mandatory_message_handling.md

File metadata and controls

73 lines (56 loc) · 3 KB
Raw

Mandatory Message Handling

When we publish a message with the mandatory flag on, this means that the broker must notify the publisher if the message is not routed to any queue via the basic.return AMQP command. In this guide we will see how the channel handles mandatory messages.

Tracking Mandatory Messages

Note: Implementation was changed in 3.8.0, removing the dtree strucutre described below.

Mandatory messages are tracked in the mandatory field of the channel's state record. Messages are tracked using our own dtree data structure. As explained in that module documentation, entries on the dual-index tree are stored using a primary key, a set of secondary keys, and a value. In the case of tracking mandatory messages we have:

  • primary key: the MsgSeqNo assigned to the message by the channel
  • secondary keys: the list of queue pids where the message was routed
  • value: the actual message

Keep in mind that delivering the message to queues is an asynchronous operation, this means that in the deliver_to_queues/2 function inside the channel, we just know to which queues the message was routed to, but this doesn't mean it had arrived there. Therefore only when a queue has accepted the message, we can forget about it and consider it as routed.

Forgetting About Mandatory Messages

Once a queue has received a message, in other words, the message was successfully routed to the queue, the queue process will cast the following message back to the channel pid:

{mandatory_received, MsgSeqNo}

The channel will then proceed to use the MsgSeqNo to forget about the mandatory message it was tracking, by deleting it from the mandatory dtree from the channel state.

Keep in mind that mandatory messages only require that a return is sent in case the message is unroutable, so it's safe to forget about it once the message has been routed to a queue.

Sending Returns

If a mandatory message cannot be routed to any queue then we need to send basic.returns back to the publisher. This is done in two different places, responding to different situations.

The first one is the obvious one, if the message is not routed to any queue, then we can safely send a basic.return back. Take a look at the function rabbit_channel:process_routing_mandatory/5 for more details.

The other situation arises when a queue where we had just publishes messages crashes before it is able to receive the message. As explained in the deliver to queues guide, we monitor the QPids where the message was delivered. If the monitor reports that the queue has crashed, then we will send basic.return for all the messages that were delivered to the queues that crashed. Take a look at the function rabbit_channel:handle_publishing_queue_down/3 for more information.

Related guides