The final issue listed in Appendix A "Issues" in the draft spec that I would like to discuss is A.3.1 "Delivery delay".
This raises two separate issues which I think it's best to discuss separately. This email covers the first issue.
A.3.1 "Delivery delay" states:
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Delivery delay is a new feature added in JMS 2.0 and is described in section 4.12 "Delivery delay".
There is an unresolved issue regarding the delivery of messages to topics which have a delivery delay specified. This is
whether the decision to add a message to a subscription (whether durable or non-durable) should be made (a) when the
message reaches its delivery time or (b) when the message is sent.
Note that this is a question about deciding which subscriptions a message is added to, not about when the message would
actually be delivered. In both cases the message wouldn't be delivered to a consumer until the delivery time has been
reached.
If (a) were adopted it would require the JMS provider to keep all messages in the server until their delivery time is
reached, even if there were no durable or non-durable subscriptions in existence. This might even be needed for
non-persistent messages. Adopting (b) does not require this and by not requiring a message to be held separately from a
subscription is closer to the way in which JMS providers work now. Forcing a JMS provider to implement (a) might
potentially add a significant burden simply to cater for an unimportant edge case.
In this specification option (b) has been chosen, and section 4.12 "Delivery delay" states that if a message is
published to a topic, it will only be added to a durable or non-durable subscription on that topic if the subscription
exists at the time the message is sent and continues to exist at the time the specified message delivery time is reached.
However there were differing views on this within the JMS 2.0 expert group and further views are invited.
In addition...[second issue snipped]
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So the question we need to review is whether the decision to add a message to a subscription (whether durable or
non-durable) should be made (a) when the message reaches its delivery time or (b) when the message is sent.
When we last discussed this (in July 2012) I proposed (b) for the reasons stated above. This was supported by Nick
Wright. The only other comment received was from Rüdiger zu Dohna, who proposed that instead of choosing either (a) or
(b) we left this "unspecified".
My feeling is that this is too important to be left unspecified as it relates to the fundamentals of how message with
delivery delay are delivered to topics. Given the views expressed, and that no-one actually opposed (b), we should go
ahead with (b), which is the wording currently in the draft spec.
Any objections? If you have any, please say so now and give your reasons.
Thanks,
Nigel