Handling exceptions

All exceptions are located in the fedora_messaging.exceptions module.

When publishing

When calling fedora_messaging.api.publish(), the following exceptions can be raised:

  • ValidationError: raised if the message fails validation with its JSON schema. This only depends on the message you are trying to send, the AMQP server is not involved.
  • PublishReturned: raised if the broker rejects the message.
  • ConnectionException: raised if a connection error occurred before the publish confirmation arrived.

The ValidationError exception means you should fix either the schema (and maybe make a new version) or the message. No need to catch it, this should crash your app during development and testing.

Your app may handle the other two exceptions in whichever way is relevant. It should involve logging, and sending again or discarding may be valid options.

You already noticed the ValidationError being raised when you tried sending an invalid message in the previous chapter.

When consuming

Invalid messages according to the JSON schema are automatically rejected by the client.

The callback function can raise the following exceptions:

  • Nack: raise this to return the message to the queue
  • Drop: raise this to drop the message
  • HaltConsumer: raise this to shutdown the consumer and return the message to the queue.

Any other exception will bubble up in the consumer, shut it down, and return pending messages to the queue. Your app will have to handle the exception.

Modify the callback function to raise those exceptions and see what happens.

When returning Nack systematically, the consumer will just loop on that one message, as it is put back in the queue and delivered again forever.

Notice how raising HaltConsumer or another exception stops the consumer, but does not consume the message: it will be re-delivered on the next startup.