Retained publications
By default, after a publication is sent to all interested subscribers it is discarded. However, a publisher can specify that a copy of a publication is retained so that it can be sent to future subscribers who register an interest in the topic.
Delete publications after they have been sent to all interested subscribers is suitable for event information, but is not always suitable for state information. By retaining a message, new subscribers do not have to wait for information to be published again before they receive initial state information. For example, a subscriber with a subscription to a stock price would receive the current price straight away, without waiting for the stock price to change (and hence be republished).
The queue manager can retain only one publication for each topic, so the existing retained publication of a topic is deleted when a new retained publication arrives at the queue manager. However, the deletion of the existing publication might not occur synchronously with the arrival of the new retained publication. Therefore, wherever possible, have no more than one publisher sending retained publications on any topic.
Subscribers can specify that they do not want to receive retained publications by using the MQSO_NEW_PUBLICATIONS_ONLY subscription option. Existing subscribers can ask for duplicate copies of retained publications to be sent to them.
There are times when you might not want to retain publications, even for state information:
- If all subscriptions to a topic are made before any publications are made on that topic, and we do not expect, or do not allow, new subscriptions, there is no need to retain publications because they are delivered to the complete set of subscribers the first time they are published.
- If publications occur frequently, such as every second, a new subscriber (or a subscriber recovering from a failure) receives the current state almost immediately after their initial subscription, so there is no need to retain these publications.
- If the publications are large, you might end up needing a considerable amount of storage space to store the retained publication for each topic. In a multiple queue manager environment, retained publications are stored by all queue managers in the network that have a matching subscription.
When deciding whether to use retained publications, consider how subscribing applications recover from a failure. If the publisher does not use retained publications, the subscriber application might need to store its current state locally.
To ensure that a publication is retained, use the MQPMO_RETAIN put-message option. If this option is used and the publication cannot be retained, the message is not published and the call fails with MQRC_PUT_NOT_RETAINED.
If a message is a retained publication, this is indicated by the MQIsRetained message property. The persistence of a message is as it was when it was originally published.
Parent topic: Publishers and publications
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