Re: [wsn] Subscription leasing, garbage collection and scalability

[Steve Graham <sggraham@us.ibm.com>]

Hi David:
ok so lets examine a scenario.

The scenario is that the NP does not
wish to particpate in scheduled termination (leasing, for whatever reason,
is something it simply doesn't want to get involved with).  So, in
this case, the NP will rely on some other means to destroy subscriptions,
either the good graces of the subscribers to let the NP know when they
are done, or some other means that results in the equivalent of the destroy
message being processed on a subscription WS-Resource.

So, this would then boil down to the
subscribe request including an initial termination time of xsi:nil (If
the value is xsi:nil, then the intent of the service requestor is to specify
there is no scheduled termination time for the WS-Resource.).
This would be the only value of InitialTerminationTime that the NP would
accept.  Any other value would cause the subscription request message
to be rejected and a fault response generated.

Now, how the heck does the subscriber
know to do this?  Darn good question.  Perhaps if we consider
the following additional resourceProperty, we may have a solution?

- add the following to BaseN (resource
Properties for the NP):
  <xsd:element name=”UsesScheduledTermination”
type=”xsd:boolean”
         
           minOccurs=”1” maxOccurs=”1”
default="true" />

and the following normative text:
/wsnt:UsesScheduledTermination

Indicates if the NotificationProducer uses the ScheduledTermination time mechanism for subscription WS-Resource lifetime management. If the value of this resource property is "true", then the subscriber MUST include a time value for the InitialTerminationTime component in any Subscribe request message (xsi:nil is not accepted). If the value of this resource property is "false" then the only value of InitialTerminationTime that will be accepted on the Subscribe request is xsi:nil.

Some policy assertion or type-specific extension of the interface could indicate to the requestor the "other" mechanism of subscription resource management.

thoughts?

(btw, I am out of the office as of 05/24/04 and will not be back until 06/01/04).  I will weigh in on this email thread when I am back on line.

sgg
++++++++
Steve Graham
(919)254-0615 (T/L 444)
STSM, On Demand Architecture
Member, IBM Academy of Technology
<Soli Deo Gloria/>
++++++++

David Hull <dmh@tibco.com> 05/24/2004 09:26 AM

To:        Steve Graham/Raleigh/IBM@IBMUS         cc:        wsn@lists.oasis-open.org, wsrf@lists.oasis-open.org         Subject:        Re: [wsn] Subscription leasing, garbage collection and scalability

Steve Graham wrote:

David:
ok, make a proposal.  Perhaps we should consider incorporating any proposal here back into WSRL.  Potentially, the solution you suggest may be more broadly applicable than just subscriptions?
My thinking was that WS-RL would be the place to talk more specifically about other solutions.  Sessions imply some means of tracking object dependencies, so that when the managed object (session) goes away, the dependent objects (subscriptions) go away too.  Definitely bigger than a breadbox.

Similarly, WS-RF may want to deal with the concept of a parent object carrying default values for properties of dependent objects.  Being able to deal with this generically might be a good argument for the proprieties-based approach.

For WS-BN, the more important consideration is to stay out of the way so that whatever emerges from WS-RL can be leveraged, and that existing systems that already provide GC functionality will play seamlessly.

For the latter, all we need is to make sure leasing is optional.  As we discussed in Chicago, it is effectively optional now, but in an indirect way.  Perhaps a better approach is

• Make the initialTerminationTime element optional.  If it is absent, GC is to be handled by some other (unspecified) means.
• If the producer requires leasing, it should fault if the initialTerminationTime is missing
• Similarly, if the producer does not support leasing, it should fault if the initialTerminationTime is present.
• Otherwise, follow the existing semantics.

sgg 

++++++++
Steve Graham
(919)254-0615 (T/L 444)
STSM, On Demand Architecture
Member, IBM Academy of Technology
<Soli Deo Gloria/>
++++++++

David Hull <dmh@tibco.com> 05/24/2004 08:52 AM

To:        wsn@lists.oasis-open.org        cc:                Subject:        [wsn] Subscription leasing, garbage collection and scalability

I believe the upshot of the previous exchange is, if you expect a 
subscriber to make a lot of subscriptions, the lease time had better be
long or indefinite.  In fact, from the producer's point of view, if
you 
expect there to be a lot of subscriptions, period, the lease time had 
better be long or indefinite.

In other words, the only method that WS-BN provides for garbage 
collection of subscriptions should be effectively disabled precisely 
when it is most likely to matter.

I think the problem here is that WS-BN addresses subscription leasing 
and not garbage collection per se.  This doesn't mean that it should
try 
to provide all possible means of garbage collection, only that it should
specifically point out a need, allow for as wide a range of solutions as
is feasible, and point to subscription leasing (a la WS-RL) as one 
solution.  This is essentially the same road we went down with topics.

The more I consider leasing of individual subscriptions, the more 
potential technical issues I see.  Obviously, none of them is 
insurmountable -- they have all been addressed and solved in other 
contexts.  But each carries weight for the potential implementor.
 I'm 
willing to drill down on these a bit if needed, but I'm hoping we can 
avoid this and take a higher-level decision that leasing of individual
subscriptions, while useful in some contexts, should not be required or
be the only possible GC mechanism.

With that provisional assumption, the immediate question becomes how can
we allow for session-based garbage collection without adding undue 
weight to the WS-BN framework.  Sessions have their own technical
issues 
and implementation weight.