RE: ISSUE 3564: Optional Assertions may not be usable in all circumstances

I have reviewed the primer material on using wsp:Optional that Maryann and Umit proposed [1].  Other than a few minor edits (see attached) I think the proposal looks good.  The attached doc contains:

- Various small editorial cleanups.
- I believe the mtom assertion in Example 5-2 should be marked wsp:Optional="true" since it is referred to as an optional assertion.
- The first bullet point seems to indicate that you must engage an optional behavior.  I think the intention was to state that if a provider decides to support an optional assertion it needs to have some way of figuring out which alternative is being used, either by looking at the wire or by some other means:

NEW TEXT FOR BULLET #1:
*       The engagement of the optional behavior must be explicit on the wire or through some other mechanism so that the provider can determine that the optional behavior is engaged.

I suggest that we accept Maryann and Umit's text with these minor changes as satisfying proposal option C for this issue.

Daniel Roth

[1] http://lists.w3.org/Archives/Public/public-ws-policy/2006Sep/att-0054/ws-policy-assertionauthors-V1.html#optional-policy-assertion

-----Original Message-----
From: public-ws-policy-request@w3.org [mailto:public-ws-policy-request@w3.org] On Behalf Of Asir Vedamuthu
Sent: Friday, August 11, 2006 4:56 PM
To: Yalcinalp, Umit; public-ws-policy@w3.org
Subject: RE: ISSUE 3564: Optional Assertions may not be usable in all circumstances


> In this case, a client sending a message
> can not ensure that the outbound message will
> be sent using MTOM optimization, by engaging the
> capability as there are two alternatives.

Providers and requestors rely on wire messages that conform to prescribed data formats and protocols for interoperability. If a requestor wants to signal to a provider that the requestor stack chose to engage MTOM code the requestor may indicate this by sending an MTOM encoded message. Another possibility is to send an HTTP Accept Header [1][2][3]. That is,

Accept: application/soap+xml, Multipart/Related

[1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1
[2] http://www.w3.org/TR/soap12-part2/#httpmediatype
[3] http://www.w3.org/TR/soap12-mtom/#xop-serialization

This is protocol negotiation. This is not metadata.

> MTOM and Reliability are good examples that
> will be hindered if optionality is provided

The above description covers the MTOM case. The behavior indicated by the Reliability assertion manifests on the wire as a Create Sequence message.

> When a capability is only possible on message
> level subjects and expressed to apply to
> outbound messages only as optional assertions.

If there are policies associated with a message policy subject for an outbound message, a provider is informing potential requestors that they must be open to engaging any of the policy alternatives in the effective policy of the message policy subject for an outbound message. It is up to the provider to make the choice on the outbound message. A provider may choose an alternative based on an inbound message (as defined by protocol bits - as illustrated above). The choice is still at the provider's discretion. Providers should consider this when expressing multiple alternatives on outbound messages. BTW, this is a good point to highlight in the Primer.

Regards,

Asir S Vedamuthu
Microsoft Corporation


________________________________________
From: public-ws-policy-request@w3.org [mailto:public-ws-policy-request@w3.org] On Behalf Of Yalcinalp, Umit
Sent: Wednesday, August 02, 2006 3:32 PM
To: public-ws-policy@w3.org
Subject: ISSUE 3564: Optional Assertions may not be usable in all circumstances

Title: Optional Assertions may not be usable in all circumstances
Description: Typically providers express capabilities by attaching assertions
that are declared as "optional' on web service artifacts. Marking assertions
optional allow representation of capabilities which may or may not be engaged
although the capability is provided on the provider side. This is due to the
fact that the presence of an optional assertion leads into two separate
alternatives, one containing the assertion and one that does not.
There are certain cases where the client of a web service can not ensure the
engagement of an optional capability since the mechanism of choosing among the
alternatives can not be ensured as there is no explicit mechanism to enable
this. The following situations are detailed examples for this case:
(a) When a capability may be assigned to an endpoint thus governing both
inbound and outbound, but the incoming message can not be self describing to
ensure the engagement of the capability.
A typical example is to provide MTOM capability on an endpoint. Although the
presence of an optional MTOM assertion indicates that MTOM optimization is
possible, a client may not be able to engage MTOM. This situation will occur
when the inbound message does not require optimization (i.e. may be a normal
payload) and the outbound message may include an attachment and may provide
MTOM optimization per WSDL definition. In this case, a client sending a message
can not ensure that the outbound message will be sent using MTOM optimization,
by engaging the capability as there are two alternatives.
(b) When a capability is only possible on message level subjects and expressed
to apply to outbound messages only as optional assertions. Again, the client
can not engage the capability expressed by optional assertions, as the inbound
messages will not include additional information to engage the capability
expressed by the optional assertion that apply to outbound messages only.
Note that case (a) the scoping may be apply to the endpoint, but the definition
of messages may not allow the determination to be made based on the input
message. In case (b), the granularity of attachment directly may prevent the
determination. Therefore, there are two distinct cases.
Without the presence of additional metadata exchange between the client and the
provider, the engagement of optional assertions (more precisely the selection
of one of the alternatives), engagement of a capability is only possible when
the capability pertains to the input message and can be inferred. However, this
is also a limited view, because on message level policy subjects, it is also
not possible to disengage a capability on an outbound message.
Justification:
Non-uniform treatment of optional assertions leads to interoperability problems
as proven by the interop event [InteropPresentation]. It will lead providers to
(1) assume either a particular treatment of optional assertions (always enforce
or never use). Vendors may enforce an capability although an assertion is
expressed to be optional. For example, if a client who is not capable of using
MTOM were to use an endpoint with the capability and would always get MTOM
optimized messages back from a service, the client can not use the service
although the service "advertises" that MTOM is optional.
(2) never use optional assertions. This is a limiting factor for being able to
express capabilities that may be engaged but not required. MTOM and Reliability
are good examples that will be hindered if optionality is provided.
Proposal:
There are several ways to solve this problem. Below here are three sketches.
Complete proposals will need to be developed.
(A) Provide additional binding specific mechanisms (such as specific SOAP
headers) that allow clients to engage capabilities that may optionally apply to
a message exchange.
(B) b1. Disallow alternatives to exist for each subject after normalization as
a design time constraint. Hence, disallow optionality at the endpoint level.
    b2. Disallow optionality completely. (Very much against this option)
For b1, suggest in primer if a capability is to be expressed, always require
the provider two separate endpoints, one that requires and one that does not in
a WSDL. This is a design consideration that would need to be captured by
primer, etc. but goes against the current design of the framework. I believe
this is a very short term option and does not really yield to the usage of the
framework to its fullest.
C. Develop guidelines in primer about the utility of optionality and illustrate
when optional assertions may require additional support from the environment
(as in A)
My preference:
A, worst case C.
Option B. has two major drawbacks and is only a short term solution until a
full solution that addresses the framework intent is developed as mentioned in
(A). Some of the techniques may go into the primer but optionality should not
be disallowed. Further, this approach does not solve fine granular engagements
of capabilities on a message level (see b)in the description). It separates the
conceptional aspect (capability vs. constraint) from the framework and reduces
WS-Policy to express only constraints.
We should not hinder the framework at this stage and discourage optionality.
[InteropPresentation]
http://lists.w3.org/Archives/Public/public-ws-policy/2006Jul/0039.html

----------------------
Dr. Umit Yalcinalp
Architect
NetWeaver Industry Standards
SAP Labs, LLC
Email: umit.yalcinalp@sap.com Tel: (650) 320-3095
SDN: https://www.sdn.sap.com/irj/sdn/weblogs?blog=/pub/u/36238