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

[Mark McKeown]

Hi folks,
         I have read the WS-Naming strawman and been trying to follow
the discussion on resilient references on the list. Putting EPRs inside
EPRs seems a bit strange to me - the following seems simpler (the
obvious inspiration is tinyurl)  ...

<wsa:EndpointReference>
  <wsa:Address>mailto:mark.mckeown at manchester.ac.uk</wsa:Address>
  <ws-name:Handle>http://www.thegrid.com/a4dfg</ws-name:Handle>
</wsa:EndpointReference>

Where "Handle" is used as an identifier/name and is mandated to be
a HTTP/S URI. To resolve the Handle a client does a HTTP GET on the
URI to get a new EPR, (client gets 404 if resolution is not supported,
410 if the service/resource is gone/terminated).

I can see a number of advantages to this:

*) It is simple and easy to implement.

*) The Cache-Control headers in HTTP can be used to pass on information
   to the client to indicate how long it can cache an EPR for - this
   is an optional optimization.

*) If-Modified-Since HTTP header could be supported so a client will only
   retieve a new EPR if the one it has is stale - again an optional
   optimization.

*) The "Handle" is easy to pass around between users - it can be pasted
   into a mail message and the reciever can use HTTP GET to retrieve the
   EPR.

*) HTTPS can be used when trust is important.

*) The service developer can make his EPRs "resilient" whenever he wants,
   ie he creates an EPR with a "Handle" but only later makes the "Handle"
   URI live.

Effectively the low level HTTP protocol could be used to bootstrap the
higher level SOAP protocol. Perhaps there are strong reasons why this
approach was not considered?

cheers
Mark


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Mark Mc Keown                            RSS
Mark.McKeown at man.ac.uk 	                 Manchester Computing
+44 161 275 0601     		         University of Manchester
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On Fri, 10 Jun 2005, Mark Morgan wrote:

> > 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.
>
>