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Subject: Session Scenario (Re: [ws-caf] Context example assignments)

This is meant to suggest a simple, illustrative use case for context. It is in no sense meant to be normative about anything.

In many scenarios, there is a desire for web services to manage context at an infrastructure level to support interaction models that maintain state between interactions. Today, this is achieved with several proprietary models. One is to piggyback on the HTTP session of a servlet engine (or equivalent). Obviously, this is undesireable as it couples the service's conversational semantics to the application protocol used to shuttle SOAP messages around. There are also proprietary specs (SOAPConversations) that provide stateful context for interactions with a web service. This discussion proposes a way to achieve session-oriented services using WS-Context. To my way of thinking, WS-Context provides us with an ideal framework for a session protocol that can be used to managed client specific-sessions or fully distributed multi-service sessions. I presume the latter would rely on WS-Coordination Framework to provide coordinated termination across multiple session-oriented services, so I won't dwell on that aspect here.

For the purposes of our discussions a session may be described as the temporally-bounded maintenance of resources on behalf of a client or activity. Sessions may involve one or more services, though a service will typically require that clients initiate a session through a particular port, as you might expect, this example treats that port as described by the ActivityService abstract wsdl defined in WS-Context. This means that clients may begin and end sessions, and that sessions may be timed out by the infrastructure.. How sessions are managed or restricted and how resources are maintained between interactions is an implementation detail of a service.

As we know from the F2F, WS-Context provides a lightweight mechanism for allowing one or more Web services to share a common context. In the present case, a context is used to represent a session, where sessions are modeled as an activity. For the case of a simple client-server model, there may be a one-to-one correspondence between the "stateful" service and an activity service. Or it may be the case that the initial interaction with a service returns as session context in some cases without client demarcation. This would be very much similar to the way web browsers and web servers manage session cookies. I think this helps to illustrate the flexibile structuring mechanisms that WS-Context allows.

To support a session protocol, the context as defined in the Schema for WS-Context would probably need to be extended or augmented. While, services may be able to use the context identifier as an opaque session identifier to correlate requests to sessions, it might be that services need to provide their own session identifiers within an activity. I can also imagine that services may externalize their state to the context and expect it to be present in the context in subsequent interactions to facilitate loose coupling and minimize resource consumption in the stateful service (in some cases, this would mesh very nicely with the "pass by reference" model in WS-Context, as it would allow the ContextManager to act as a single point of truth for the state). Both mechanisms might need to be used within the same context. I'm not clear whether the best way to extend the context is through schema extension or using extensibility elements directly. CAF seems to follow the model of schema extension, so this would probably be the way to go.

Anyway, the client should be responsible for submitting contexts as headers during request interactions once a session has been initiated, though how this might occur is an implementation detail. You can imagine that the request association with a session context could be entirely managed by a web services platform.

A Scenario

1 )To start a session, the client application invokes begin on the ActivityService supported by the session oriented service.
2)  The begin method will return a session context in the begun reply.
3) The context is included as a header on subsequent requests to the session-oriented service. During interactions the "stateful" service maintains conversational state.
4)  To terminate a session, the client invokes the complete method on the ActivityService. This invalidates the session. Sessions may be invalidated due to internal errors, administrative intervention, resource conservation heuristics, timeout, etc.

I should note that there is a semantic mismatch between the CompletionStatus mechanism and the requirements for session. Session will not be able to make sense of FAIL_ONLY or FAIL. It's not clear to me how this should be handled, but I suspect it will be common case that with many protocols.

Also, note that while the scenario of a client to single session oriented service is a degenerate case of multiparty sessions, I assume that the session mechanism may be shared directly by the ActivityService and the session oriented service in the interaction I describe. More complicated interaction models can be supported in WS-Coordination Framework is introduced.

Newcomer, Eric wrote:
For Monday's concall it would be great to have some of the context examples circulated to the email list far enough in advance to be able to discuss them on the call.
In no particular order:
-- User Context example, Jan Alexander
-- Security context example, Eric Yuan
-- Database/File/Device example, Eric Newcomer
-- Cluster group membership example, Mark Little
-- Session state example, Greg Pavlik
I think that's it, someone let me know if I've forgotten to include them.  Apologies for the delay in getting the reminder out.
Eric Newcomer
Chief Technology Officer
IONA Technologies
200 West Street Waltham, MA 02451 USA
Tel: (781) 902-8366
Fax: (781) 902-8009
Making Software Work Together TM

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