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Subject: Re: [sca-assembly] Summarizing the state of ASSEMBLY-235

Hi Mike,

I'm confused by the particular concern related to portability.

If I've built a composite that works with vendor X, using interface.vendorXIntfA and interface.vendorXIntfB, why would I have any expectation that I could take said composite and move it to a different SCA runtime, and have any expectation that it would work?

For that matter, just using interface.vendorXIntfA, and interface.vendorXIntfA, even though SCA 1.1 defines and allows compatibility between these two - even if they don't map to WSDL 1.1, I still wouldn't stand a chance of moving said composite away from vendorX.

Now, circling back to the first case, why do I need to know how vendorX determines compatibility between interface.vendorXIntfA & interface.vendorXIntfB? It should be up to the vendor to decide.

The key point here is that with my proposal, vendorX is not required to determine compatibility between instances of vendorXIntfA and vendorXIntfB by mapping to WSDL 1.1. They should be able to map to a different representation, and ought to choose that representation based on what best preserves the semantics. There are numerous ways in which a WSDL 1.1 mapping can drop semantics. That is, unless you start adding all sorts of extension elements to WSDL 1.1, at which point it just becomes yet another interface description that happens to use WSDL 1.1 for a portion of its representation.

As for a concrete example (completely made up) that you asked for:
  • interface.tibcoWSDL20 & interface.tibcoJMX
Why, oh why would I be expected to map both of these to WSDL 1.1 in order to determine compatibility, when I could just map the interface.tibcoJMX instance to interface.tibcoWSDL20?

Putting this in a chart:

WSDL 1.1
interface A
interface B
WSDL 1.1
directly compare
map to either map to either
map to either
map to either
directly compare
map to either map to either
interface A
map to either map to either directly compare map to either
interface B
map to either map to either map to either directly compare

The above is my proposal, whereas, the next table shows how the spec currently reads:

WSDL 1.1
interface A
interface B
WSDL 1.1
directly compare
map to WSDL 1.1 map to WSDL 1.1
map to WSDL 1.1
map to WSDL 1.1 directly compare map to WSDL 1.1 map to WSDL 1.1
interface A
map to WSDL 1.1 map to WSDL 1.1 directly compare map to WSDL 1.1
interface B
map to WSDL 1.1 map to WSDL 1.1 map to WSDL 1.1 directly compare

The difference lies not on the diagonal, but all the options around that. The problem is that interfaces A & B may have richer semantics than WSDL 1.1. The existing spec'd expectation potentially forces the provider of interface A to declare something incompatible when it isn't, or worse, declare something compatible when it isn't, because semantics can be lost when mapping to WSDL 1.1.

On top of all of that, it happens that there isn't actually a test you can define that normatively verifies any of this, except the top left corner, because SCA currently only defines interface.wsdl & interface.java.  If you want, we could further nail down that detail in my proposal. However, I don't see any value in mandating that interface A & interface B have to be converted to WSDL 1.1 before being compared, particularly since we can't test that.


On 2/17/11 7:47 AM, Mike Edwards wrote:
OF37A1BFF3.6B89D9D6-ON80257838.0057B4B3-8025783A.00561117@uk.ibm.com" type="cite">

I'm sorry that this has been left lonely and uncommented on.  We're clearly enjoying event processing too much.

There is a comment inline.  I think that it would also help if you could describe at least one concrete example of
a mapping that you'd like us to support.

Yours, Mike

From: Eric Johnson <eric@tibco.com>
To: OASIS SCA Assembly <sca-assembly@lists.oasis-open.org>
Date: 15/12/2010 01:26
Subject: [sca-assembly] Summarizing the state of ASSEMBLY-235

In a previous email I proposed something similar to the following change.  This time I tried to be more precise, so that this is more than just directional.

Change 6.2.1 #6, 6.2.2 #6, and 6.2.3 #6 in the following pattern:

Replace text that reads:
"for checking the compatibility of 2 remotable interfaces which are in different interface languages, both are mapped to WSDL 1.1 (if not already WSDL 1.1) and compatibility checking is done between the WSDL 1.1 mapped interfaces.

For checking the compatibility of 2 local interfaces which are in different interface languages, the method of checking compatibility is defined by the specifications which define those interface types, which must define mapping rules for the 2 interface types concerned."

... with the following ...

"The interfaces, whether local or remotable, must map onto a common interface description language, and that the two interfaces are compared on the basis of that common interface description language.  See section Comparing Interface Descriptions of Different Types for a discussion."

... and then add a section 6.2.4:

6.2.4 Comparing Interface Descriptions Of Different Types

A variety of interface descriptions for services exist. Examples of well-known types include XML-RPC, CORBA, REST, WSDL 1.1, WSDL 2.0, SNMP, and JMX.  Implementations ought to use the interface type mappings that best preserve the semantics of the underlying exchange.

To establish a basis of comparison between two different interface definition types, the implementation has to map one or both of the interface descriptions to a common definition type.  The implementation has to identify that common type, and ought to keep possible conversion errors to a minimum by eliminating spurious conversions, and selecting the form with the best semantic relevance.  For example, if one interface description type maps to WSDL 1.1, and the other interface type is WSDL 1.1, then the SCA implementation ought to compare on the basis of WSDL 1.1.  When neither interface type can directly convert to the other interface type in question, and conversion to WSDL 1.1 is possible, implementations SHOULD map both interface descriptions to WSDL 1.1.

The question is - who defines the mapping?

If I've got <interface.x.../> and <interface.y.../>, who says what the mapping between x and y is for some actual interfaces of each type?

Is this to be described in either or both of the specifications for x and y - or is it simply left to the SCA runtime to pick what it pleases?

I am somewhat concerned by the potential for lack of portability here, if runtimes are left free to choose what they will.  I think the current
formulation of the SCA spec aims to get consistency between runtime implementations.  How can we ensure this for the relaxed case?

Justifications for the above:

The specification already allows for the use of remotable interfaces defined using something other than WSDL 1.1.  For example, a Java JMX interface description can be marked remotable.  The existing rules only reject the notion of compatibility when the two interfaces being compared are of different types, but don't actually reject the notion of remotability being applied to said interface types.  As a possible example, XML-RPC can be represented by a variety of description languages.

The above change relaxes a constraint that imposes on an implementation the need to declare incompatibility where none may exist.  Specifically, by allowing additional scenarios to interoperate, the composers will be able to express interface definitions that more closely align to their implementation language, and the semantics of the underlying problem, rather than by restricting themselves to the subset of the interface description that maps to WSDL 1.1.

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