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Chapter 9. Message delivery

9.1. Sending messages
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To send a message, override the on_sendable event handler and call the Sender.send() method. The sendable event fires when the Sender has enough credit to send at least one message.

Example: Sending messages

class ExampleHandler(MessagingHandler):
    def on_start(self, event):
        conn = event.container.connect("amqp://example.com")
        sender = event.container.create_sender(conn, "jobs")

    def on_sendable(self, event):
        message = Message("job-content")
        event.sender.send(message)
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For more information, see the send.py example.

9.2. Tracking sent messages
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When a message is sent, the sender can keep a reference to the delivery object representing the transfer. After the message is delivered, the receiver accepts or rejects it. The sender is notified of the outcome for each delivery.

To monitor the outcome of a sent message, override the on_accepted and on_rejected event handlers and map the delivery state update to the delivery returned from send().

Example: Tracking sent messages

def on_sendable(self, event):
    message = Message(self.message_body)
    delivery = event.sender.send(message)

def on_accepted(self, event):
    print("Delivery", event.delivery, "is accepted")

def on_rejected(self, event):
    print("Delivery", event.delivery, "is rejected")
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9.3. Receiving messages
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To receive a message, create a receiver and override the on_message event handler.

Example: Receiving messages

class ExampleHandler(MessagingHandler):
    def on_start(self, event):
        conn = event.container.connect("amqp://example.com")
        receiver = event.container.create_receiver(conn, "jobs")

    def on_message(self, event):
        print("Received message", event.message, "from", event.receiver)
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For more information, see the receive.py example.

9.4. Acknowledging received messages
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To explicitly accept or reject a delivery, use the Delivery.update() method with the ACCEPTED or REJECTED state in the on_message event handler.

Example: Acknowledging received messages

def on_message(self, event):
    try:
        process_message(event.message)
        event.delivery.update(ACCEPTED)
    except:
        event.delivery.update(REJECTED)
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By default, if you do not explicity acknowledge a delivery, then the library accepts it after on_message returns. To disable this behavior, set the auto_accept receiver option to false.

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