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Using conflation

You can configure how and when conflation is applied to different topics.

Conflation policies

Conflation policies control how conflation is applied to a topic. You can set conflation policy for a topic with a topic property.

These are the available conflation policies:
  • off
  • conflate (default)
  • unsubscribe
  • always

"off" disables all conflation for the topic. Topic updates will never be merged or discarded.

"conflate" automatically conflates topic updates when back pressure is detected by the server (that is, when the outgoing message queue exceeds the maximum allowed size in bytes or number of messages).

"unsubscribe" automatically unsubscribes the topic when back pressure is detected by the server, with a BACK_PRESSURE message. The unsubscription is not persisted to the cluster, so if a session fails over to a different server it will be resubscribed to the topic. This policy is useful for topics that are not essential to the application, and can be discarded in back pressure situations without affecting the main function of the application.

"always" automatically conflates topic updates as they are queued for the session. This policy ensures only the latest update is queued for the topic, minimising the server memory and network bandwidth used by the session, but potentially increasing the processor cost of conflation.

If no policy is set, the "conflate" policy is applied.

Conflation process

The conflation process considers the value and delta updates in the queue, and the current topic value (unless DONT_RETAIN_LAST_VALUE is enabled). It reduces the queued updates to a single value or a composite delta, whichever requires the fewest bytes to send.

Under the default "conflate" policy, no conflation is applied until there is a new message to send to a session with a full queue. The whole queue is then conflated, topic by topic. If conflation is not enough to bring the queue size under the configured limit, the server will close the session.