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Subject: Re: [sca-assembly] ISSUE 56: Need to clarify definition of BidirectionalInterfaces - Proposal text



Please see proposed additions in both the preamble and spec text, marked with <scn>....</scn>.

    Simon

Simon C. Nash, IBM Distinguished Engineer
Member of the IBM Academy of Technology
Tel. +44-1962-815156  Fax +44-1962-818999



Mike Edwards/UK/IBM@IBMGB

01/07/2008 14:23

To
"OASIS Assembly" <sca-assembly@lists.oasis-open.org>
cc
Subject
[sca-assembly] ISSUE 56: Need to clarify definition of Bidirectional Interfaces  - Proposal text






Folks,


Here is the proposal text for Issue 56


-----------------------------------------------------------------------------------------


Preamble.  In the Issue text, Tom Rutt posed 3 questions, which are not answered by the current specification,

each of which needs an answer - following that is formal specification text which is proposed to be added to

the specification.


1) Does every operation type invoked onto the "service interface"
direction have the potential to have correlated callback interface
operations in the opposite direction?


Answer:


In principle, any one of the forward interface operations may cause zero, one or many invocations on the callback

interface by the service provider.  For each of the forward service operations, any of the operations on the

callback interface may get invoked, in any order.


This is a case of "maximum flexibility, minimum information".  The service provider gets to use the callback interface

in any way that makes sense - but it gives the client a hard time in working out what will happen for a given invocation.

For the client to get more information, extra metadata is required which is not captured by SCA today.



2) Does every operation invocation invoked onto the callback interface
have to be considered as a callback for an operation invocation on the
service interface?


Answer:


Every invocation on an operation of the callback interface is considered to be the result of some operation

<scn> or combination of operations</scn>
invoked on the forward service interface.


3) How can it be specified (at design time) which operation types defined
in the callback interface description can be correlated with which
individual operation types defined in the service interface description?


Answer:

This kind of metadata about the service and the callback must be provided outside the standard SCA metadata.


It could be done through informal documentation.  It could be done by more formal means (one possibility might

be to use abstract BPEL to indicate the sequencing of messages).  SCA leaves this open to the creator of the

callback interface and the service that uses it.


The problem in the general case is to convey alternatives.  In the simplest case, one forward call results in exactly

one invocation of the callback interface (see Isseue 33 and the "long running request/response" pattern as an example

of this).  However, there are plenty of existing examples of asynchronous sequences much more complex than this.

Even ordering a book with Amazon has this potential for complex interactions - one request to order a set of books

results (usually) in at least 2 responses - one accepting the order and one indicating dispatch of the package.  However,

not all the order may get dispatched at once and a sequence of messages for each dispatch can occur.  It is also
possible that one or more books is not available for some lenghy period, in which case a callback message is

sent which gives the client the opportunity to cancel that part of the order.  OK, this is all done with email, but it is

possible to see that similar sequences could apply to order processing offered as Web services.



-----------------------------------------------------------------------------------------

Proposed text for the Assembly specification (CD-01), to be added after line 2333


In a bidirectional interface, the service interface can have more than one operation defined, and the callback interface can

also have more than one operation defined.

<scn>An operation on the callback interface may or may not be a direct result of some operation invoked on the forward service interface.</scn>
A single invocation of an operation on the service interface can cause

zero, one or many invocations of any of the operations on the callback interface

<scn>, and a single invocation of an operation on the callback interface can result from
one or many invocations of any of the operations on the service interface</scn>.

For a given invocation of a service operation,
which operations are invoked on the callback interface, the number of operations invoked and their sequence are not described
by SCA.  It is possible that this metadata about the bidirectional interface can be supplied through mechanisms outside SCA.  

For example, it might be provided as a written description attached to the callback interface.


-----------------------------------------------------------------------------------------



Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com





Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU












Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU








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