CODEWITHSUNDEEP
jmsibm-mq
Tate
Tate

Reputation: 53

Does WMQ topic save message itself?

If a publisher publish some messages to a WMQ topic, but the subsciber didn't take it, then where the messages are saved? is there any way to know the message count?

Upvotes: 3

Views: 68

Answers (2)

T.Rob
T.Rob

Reputation: 31832

As MQ is JMS compliant, the answer is mostly a JMS answer.

  1. If the subscription is not durable and no subscription is registered, the messages for that subscriber are discarded.
  2. If the subscription is durable, MQ creates a queue (or uses a predefined one if specified by the subscriber) to deliver the messages. The messages will collect there if the subscriber is not consuming them.
  3. The 3rd case as Dave points out int he comments is that the non-durable subscriber is holding the subscription open but not consuming the messages. Since a queue is created to receive these that queue depth can be queried to determine if there's a back-up.

Based on there being a queue for every subscription (durable or otherwise) just look in the durable subscriber's queue to determine the number of messages outstanding.

Please also see Publish/subscribe lifecycles in the MQ Knowledge Center for more description of the behavior and specification of durable subscriber queues.

Of course, if that queue fills up the behavior changes. Depending on the settings either the publishers block or the publications continue but the messages are routed to an exception queue (if specified), the DLQ, or discarded.

Thanks Dave Ware for the comments about non-durable subscriptions.

Upvotes: 4

David Ware
David Ware

Reputation: 301

I'm wondering from the question if you're asking if MQ keeps a store of all the messages published to a topic, independent of any registered subscriptions?

If that's the question, then no, it doesn't. When messages are published they are matched to each existing subscription and a copy is sent to each of their associated queues as T.Rob describes.

So the only queue depths to worry about are those of the subscriptions.

(There is a caveat in that MQ supports "retained publications", - it means MQ keeps just the most recent publication on that topic string for late subscriptions if you choose to do that).

I try to explain all this here (slides/video), which may help... http://www.slideshare.net/DavidWare1/ame-2271-mq-publish-subscribe-pdf

Upvotes: 2

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