Simultanaeity and the semantics of setting subscription properties

[David Hull <dmh@tibco.com>]

As far as I can tell, and correct me if I'm wrong, the main reason to allow a subscription to be modified, as opposed to closing it out and creating a new one, is atomicity.  The clearest example of this I can see is useNotify (though I'm at a loss for a use case).  If I change useNotify, then I know that I will receive every message produced exactly once, and at some unspecified time in the future the consumer will start seeing messages in the other format.  On the other hand, if I first create a new subscription and then delete the old, the consumer may get some set of messages twice, once in the old format and once in the new.

We need to be careful about simultaneity here.  About the only things we can count are that a message arrives after it is sent, and that for a given connection, messages arrive in the order they are sent.  But if two different sources send to the same sink, the exact interleaving of arrivals is undefined.  In this context I am further assuming that if the NotificationProducer receives (Subscribe new, Cancel old), any notifications produced between these events will go to both subscribers.  But this is not certain, and we need to nail this down.

Now let's consider the various properties that can be set:

• ConsumerReference:  Atomicity means that every notification sent (assuming no other properties are changed) will go to either the new or the old consumer, but not both.
• TopicExpression: Every notification sent to the intersection of the two topic sets will arrive, regardless of when the switch actually takes effect.  I don't believe there are any guarantees for topics outside the intersection.
• UseNotify: See above.
• Precondition/Selector.  Again, atomicity only matters for the intersection.
• SubscriptionPolicy: This may depend on the actual policies involved, but the situation should be essentially similar to useNotify.

So there are at least two questions here:  Can or should we require any guarantees for the case where the same producer hears a new subscription creation and then an old one canceled?  In other words, does arrival time at the producer matter in the context of notifications sent to a consumer?

If so, how important are the atomicity guarantees for setting properties?  The worst side effect seems to be the possibility of duplicate messages.  And if guaranteeing ordering of subscriptions/cancellations is not feasible, how feasible are the atomicity guarantees for setting properties?

My gut feeling right now is that significantly changing a subscription's properties in-flight is tantamount to canceling it and creating a new one, and it would be nice to reflect that without losing useful atomicity properties.  In any case, the key semantic to look at is the ordering of events in an inherently loosely-ordered system.

Hope that makes sense.