For each message delivered on a subscription, AMPS counts the message against the subscription backlog until the message is explicitly acknowledged. In addition, when a queue specifies at-least-once delivery, AMPS retains the message in the queue until the message expires or until the message has been explicitly acknowledged and removed from the queue. From the point of view of the AMPS server, acknowledgment is implemented as a sow_delete from the queue with the bookmarks of the messages to remove. The AMPS Python client provides several ways to make it easier for applications to create and send the appropriate sow_delete.
The AMPS client allows you to specify that messages should be automatically acknowledged. When this mode is on, AMPS acknowledges the message automatically in the following cases:
Asynchronous Message Processing Interface - The message handler returns without throwing an exception.
Synchronous Message Processing Interface - The application requests the next message from the MessageStream.
AMPS batches acknowledgments created with this method, as described in the following section.
To enable automatic acknowledgment, use the set_auto_ack() method.
The AMPS Python client provides a convenience method, ack(), on delivered messages. When the application is finished with the message, the application simply calls ack() on the message. (This, in turn, provides the topic and bookmark to the ack() method of the client that received the message.)
For messages that originated from a queue with at-least-once semantics, this adds the bookmark from the message to the batch of messages to acknowledge. For other messages, this method has no effect.
The AMPS Python client automatically batches acknowledgments when either of the convenience methods is used. Batching acknowledgments reduces the number of round-trips to AMPS, which reduces network traffic and improves overall performance. AMPS sends the batch of acknowledgments when the number of acknowledgments exceeds a specified size, or when the amount of time since the last batch was sent exceeds a specified timeout.
You can set the number of messages to batch and the maximum amount of time between batches, as shown below:
The AMPS Python client is aware of the subscription backlog for a subscription. When AMPS returns the acknowledgment for a subscription that contains queues, AMPS includes information on the subscription backlog for the subscription. If the requested batch size is larger than the subscription backlog, the AMPS Python client adjusts the requested batch size to match the subscription backlog.
60East recommends tuning the batch size to improve application performance. A value of 1/3 of the smallest max_backlog value is a good initial starting point for testing. 60East does not recommend setting the batch size larger than 1/2 of the max_backlog value without testing to ensure that the application does not run out of messages to process while the acknowledgment is being sent to AMPS.