Patil, Sanjay wrote:
Message
I guess modifying an existing Subscription
makes life simpler by guaranteeing atomicity, and by avoiding creating
entirely new subscription when you really wanted to make a
certain small change.
(Mozilla just crashed on me, so I believe I lost a previous reply.
Here's another attempt).
The changes in question are small if you take the view that all
filtering and routing is done by the producer. But, despite some
wording in the introduction, we want to allow the producer to delegate,
either to a broker, or to a traditional system, or perhaps by some
other means. In this case, some or all of the filtering and routing
may be done in a distributed fashion.
This may or may not actually present practical problems. We'd need to
think it through. Taking the approach that subscription properties are
fixed over the lifetime of a subscription gets around any such
problems, at the expense of exposing simultaneity issues to the outside
world.
I'm not (yet) taking a side on this. At this point I'm just trying to
highlight the issues.
The concerns you raised below seem to be
relevant only if one were to take the alternative approach of canceling
a Subscription and creating a new one , which does not seem to be
necessary. Perhaps I am missing something!
On a side note -- If
we had a property such as EffectiveFrom on the Subscription, it would
have been possible to send two separate messages for Subscribe
and Cancel (actually SetTerminationTime) and still achieve atomicity
(assuming that there is sufficient time window available for the
Producer to receive and process both Subscribe and Cancel messages
before the new subscription becomes effective). I am not suggesting
that we need to work on such an alternative protocol for modifying
Subscriptions. It just struck to me as a generally valid use case for
having a property such as - EffectiveFrom.
Thanks,
Sanjay
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.
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