Friday, October 22, 2010

durable subscribers, persistent store, persistent delievery

In our Front Office app, we have 100+ destinations (mostly topics) in a single weblogic JMS broker. Weblogic web console shows the number of DURABLE subscribers on each topic -- I see all zeros. Therefore, after a restart, broker doesn't need to de-serialize from disk the "pending" messages. However, I believe we do have a persistent store -- probably a basic, common requirement in FO trading apps.

Q: when is the persistent store cleared?

Note if there's at least one durable subscriber and there's any pending message for her, then broker must persist it forever until either expired or delivered. (Expiration could be disabled.) Therefore durable subscribers ==> (imply) ==> persistent store on broker, but not conversely.

Persistent delivery (http://download.oracle.com/javaee/1.4/api/javax/jms/DeliveryMode.html) is unrelated to durable subscription (DS).

PD covers producer => broker.
DS covers broker => consumer.

Also,
PD is a producer setting
DS is a subscriber setting

"A message is guaranteed to be delivered once and only once ... if the delivery mode of the message is PERSISTENT and if the destination has a sufficient message retention policy" Therefore, PD doesn't always guarantee exactly-once.

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