Resilient References? (Re: [ogsa-wg] ogsa London f2f minutes uploaded)

[Mark Morgan]

> when Mark writes: "...the general idea we have is to in fact 
> possibly have a method who's signature is EPR resolve( [EPR] 
> ) Where the parameter EPR is optional..."
> 
> Does this "optional" mean that the schema for the input 
> message parameter of the resolve operation will have an 
> optional "EPR"; it should either be empty or consist of a single EPR?

Borrowing the pseudo syntax used in the "Web Services Base Notification 1.2
Working Draft 03, 21 June 2004" and replacing the given template for the
NotificationMessage syntax with appropriate naming elements, what I mean is
this:


	<ogsa-naming:Resolve>
		<ogsa-naming:ResolutionMessage>
			<ogsa-naming:TargetEPRHint>?
				wsa:EndpointReference
			</ogsa-naming:TargetEPRHint>
		</ogsa-naming:ResolutionMessage>
	</ogsa-naming:Resolve>

> If so, what does the "optional" mean for the implementors of 
> that porttype/operation "EPR resolve( [EPR] )"?
> Does the service implementation have to cater for both options?

No, it doesn'.  It can implement the empty parameter one, or both the empty
parameter one and the one that takes an EPR as a parameter.  See below for
explanation.

> Or can it just return an error for the one it doesn't implement?

No, this would not be correct I don't believe.

> And then what does the "optional" mean for the client's processing?
> If the schema indicates that the client could either call the 
> resolve operation with or without the original EPR, how does 
> it know which option to choose? Should it just try the one it likes?

The important points to recall here are:
	1) There are two EPRs involved in this communication conversation.
One is the obvious one listed as an optional parameter.  For clarity, let's
call that parameter the "Hint EPR".  The other EPR is the implicit one which
would be indicated in the SOAP headers to which the communication is taking
place.  Let's call that EPR the "Resolver EPR".
	2) The Resolver EPR is created with whatever information the
implementor of the resolution services feels is appropriate.  It's up to
that service to generate an EPR with enough information to do the
resolution.
	3) The Hint EPR is just that -- a hint.  It should not be considered
the end all source of information for resolution.

What we imply by all of this is that a resolution service should be able to
deal with a resolution message that does not include any EPR as a parameter
by virtue of the fact that the Resolution EPR to which the resolution
messages are addressed is created by that service with enough information to
completely carry out the resolution.  If an EPR is given as a parameter to
that resolution service, the service MAY choose to use that hint to more
efficiently resolve the endpoint in question, but is not required to do so.
In otherwords, a resolution service implementor may choose to implement code
that uses the optional EPR parameter if available, but MAY also choose to
ignore it.

>From the client perspective, it should be the case that the client may
equally choose to send either an empty message (since this is guaranteed to
work), or one with an EPR (since a service may choose to ignore this
parameter).  In this way the client is free to pick either one -- the
parameterless one if it feels inclined to go with no hints, or the message
with an EPR if the client feels like this extra hint may be useful.

So, in what cases would the EPR parameter be useful should the client decide
to send it and should the service decide to use it?  Well, in the general
case, we have found that it is tremendously useful (bordering on necessary)
sometimes for a client to indicate to a resolution service that that client
has already tried to communicate with a given binding (EPR) and that the
resolution service might want to go to extra efforts (i.e., instead of just
looking the binding up in a cache or table) to determine whether or not a
more suitable EPR exists.

-Mark

> Thanks, Frank.