Re: [wsn] Simultaneity and the semantics of setting subscriptionproperties

[David Hull <dmh@tibco.com>]

Patil, Sanjay wrote:

 

Hi David,

 

Thanks for your specific example.  It kind of  helps me understand your problem, but is also raises another interesting question -- should the Subscriber really know so much about the relationships between the various brokers on the provider side?

 

Wouldn't it be a simpler model , where  the Subscriber is aware of a single SubscriptionManager that can handle all of its requests for Subscription updates. Now the SubscriptionManager itself may be aware of the different brokers and their interrelationships and may indeed choose to affect the Subscription update by deleting the Subscription with one broker and creating a new Subscription with another broker and also in an atomic manner somehow. However, in this case it will be an implementation specific issue.

 

Basically, I still think that the current Subscription update semantics may be sufficient at the specification level.

 

Thanks,

Sanjay

-----Original Message-----
From: David Hull [mailto:dmh@tibco.com]
Sent: Friday, Jun 18, 2004 10:37 AM
Cc: wsn@lists.oasis-open.org
Subject: Re: [wsn] Simultaneity and the semantics of setting subscription properties

Patil, Sanjay wrote:

 

Hi David, perhaps it's just me, but I still do not understand the issue here.

 

Specifically --

 a> how the capability to modify Subscriptions relates to where the filtering and routnigs are done, and

Suppose distribution is through a tree of brokers.   Each tree node knows what topics its children are interested in, and subscribes to those topics from its parent, and so on up to the original producer.  Suppose consumers A and B belong to broker 1 and consumer C belongs to broker 2. Both A and B are subscribed to topic foo, but nothing belonging to broker 2 is.

Now change the subscription for consumer A to point to consumer C.  We can't just move broker 1's foo subscription atomically to broker 2, because consumer B still needs foo.  So broker 2 has to open a new subscription (or add foo to the list of topics on its existing one).  Then broker 1 and broker 2 have to coordinate to be sure that any foo message both get goes to only one of consumer A and consumer C.  Just for fun, suppose that consumer B's subscription is canceled while this is happening.  Broker 1 still can't let go of the subscription until it knows that broker 2's subscription is established.

When a tree node has to know about nodes other than its parent and children, it's no longer in a tree.

Note that these issues arise from changing the destination of a subscription.  Changing the filtering or policy for an existing subscription, without changing the consumer, may not be a major issue.

 b> how making Subscriptions immutable solves the problem (which I hope to understand once you answer a>)

It doesn't so much solve the problem as put it out of scope and make it explicit that duplicate or possibly even dropped messages are a possibility.

 

Thanks,

Sanjay

-----Original Message-----
From: David Hull [mailto:dmh@tibco.com]
Sent: Friday, Jun 18, 2004 9:35 AM
Cc: wsn@lists.oasis-open.org
Subject: Re: [wsn] Simultaneity and the semantics of setting subscription properties

Patil, Sanjay wrote:

 

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

-----Original Message-----
From: David Hull [mailto:dmh@tibco.com]
Sent: Thursday, Jun 17, 2004 3:18 PM
To: wsn@lists.oasis-open.org
Subject: [wsn] Simultanaeity and the semantics of setting subscription properties

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.