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Subject: RE: [ws-caf] Session Scenario (Re: [ws-caf] Context example assignments)
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The
sessions you describe here are similar to the Conversations we defined in Web
Services Management Framework (WSMF). In WSMF we did not try to define how
sessions were created or maintained - but we recognized the need to manage them
and defined interfaces to do this.
We
defined a Conversation as one services view of a set of related messages. This
definition allows a conversation to exist without defining a formal language for
it such as BPEL, and it allows a service to be aware of related messages,
without necessarily knowing all of the other participating services in the
conversation.
As you
explained WS-Context offers a nice way to define the identity of the context
through the context identifier. This same ID could be used by a manageability
consumer to monitor and control the session resource (context). In WSMF we
defined a number of metrics that a manager might want to monitor as well as the
ability to terminate the Conversation if it proved
troublesome.
The
OASIS WSDM TC has also discussed manageability representations and interfaces
for sessions.
More
information about WSMF and Conversations can be found at:
The
Conversation managed object is described in the "web services management" spec,
and there is a use case in the "overview".
Regarding the scenario you suggest: I think there is an alternative set
of steps:
1)
client makes first-contact with a service
2)
service calls begin, receives begun message, getting back a context
id
3)
service optionally calls other services, passing the context
id
4) any
response the service makes to the client will include the context
id
5) if
the client makes subsequent calls to the service, it must include the context
id
6) if
the service can recognize that the session is complete, it can send the complete
message, if not the client should do so.
These
steps work somewhat like a web session might. For instance, a shopping site
might have a shopping cart that needs to be maintained as context. In a web
session, the client is not responsible for creating or maintaining the context,
only in passing it back to the service. Usually the service will recognize when
a session is complete and can terminate the session on its own. At least a
service will need to recognize that a client is not coming back and timeout the
session.
Bryan
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:
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