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Subject: Re: [wsbpel] Issue - 229 - Fault handling and compensation handlingallows selective compensation of child scopes


Hi,  Alexandre and others,

I do support opening this issue.

On the other hand, I am not sure an error needs to be raised when reverse order invariant is not observed. The reverse order is the default compensation order, NOT the only compensation order.  Instead, we can issue a warning (both compile time and runtime).

Even though we decided not to add a "reversable" flag to <scope> construct  in Issue 10 for simplicity, scopes are often introduced to a process definition in reality which actually have not significance in the context of compensation (e.g. just to introduce local partnerLink, perform some non-business tasks that cannot and need not be be compensated). Those scopes should be safe to be ignore in the context of compensation.

Also, while there may be a logical order of dependence of forward-work among scopes, a logical order may not necessarily exist in the backward-work among scopes. For example, scope-A has two sub-scopes: one for PO creation, one for ShippingOrder creation. Creation of ShippingOrder depends on the PO-id from PO creation. However, when we want to cancellation of both PO and SO during compensation, the cancellation of PO does not necessarily depend on the cancellation of SO (depending on the argreed procotols among customer and vendors and shippers). In fact, one may cancel PO and SO in parallel.


Thanks!


Regards,
Alex Yiu




Alexandre Alves wrote: 
Hi Alastair,

Isn't it already possible to break this reverse order invariant by
having the user explicitly compensate named scopes:

<scope name="root">
    <faultHandlers>
        <catchAll>
          <compensate scope="a">
          <compensate scope="b"> <!-- does not follow reverse order -->
        </catchAll>
    </faultHandlers>
    <compensationHandler>
        <compensate scope="a">
    </compensationHandler>
    <sequence>
      <scope name="a">
      ...
      </scope>
      <scope name="b">
      ...
      </scope>
      <scope name="c">
      ...
      </scope>
</sequence>

If so, it seems to me that parameterized compensation handlers are
already needed. And this scenario is not very different from the
proposed solution n. 4:

	  <catchAll>
          <compensate scope="a">
          <compensate /> <!-- default compensation -->
        </catchAll>

I do think we should open this issue. 

I would volunteer that one possible solution to this problem could be to
(conditionally) raise a fault if the reverse order invariant is broken.

Best regards,
Alexandre

-----Original Message-----
From: Green, Alastair J. [mailto:Alastair.Green@choreology.com] 
Sent: Tuesday, September 27, 2005 10:48 AM
To: chris.keller@active-endpoints.com; Yuzo Fujishima;
wsbpel@lists.oasis-open.org
Subject: RE: [wsbpel] Issue - 229 - Fault handling and compensation
handling allows selective compensation of child scopes

I think I disagree with the premise of this issue, that a change is
needed in this area.

The default behaviour, as I recall, is that the compensation handlers of
children will be processed in reverse order of scope processing.

This facility allows an override, to permit selective and partial
reversals. 

If this is removed then the process designer is forced to create shared
state to parameterize compensation handlers such that they know the
context in which they have been called, which is more fragile,
error-prone and difficult to code than allowing parental dictation.

Unless I am missing something, there is no compulsion to use this
feature. It seems like a useful facility, and a case of caveat emptor.

Alastair 

-----Original Message-----
From: Chris Keller [mailto:chris.keller@active-endpoints.com] 
Sent: 27 September 2005 16:45
To: 'Yuzo Fujishima'; wsbpel@lists.oasis-open.org
Subject: RE: [wsbpel] Issue - 229 - Fault handling and compensation
handling allows selective compensation of child scopes

Hi Yuzo,

Interesting suggestion.

I have one question on your option.  If a user creates a catch block for
a
fault or a compensation handler that doesn't use <compensate/> will
compensation handling of child scopes be done before or after the user's
code or not at all?

One disadvantage of the suggestion is that if you have invokes not in a
scope of their own (or declaring a compensation handler of its own) then
the
user may need to interleave its compensation with that of child scopes.
That
requires the user to dictate the order of compensation which would be
prohibited by this option.  Of course we could say the user must add a
compensation handler directly to the invoke itself for appropriate
interleaving with child scopes, which is probably the lesser of evils.

- Chris

-----Original Message-----
From: Yuzo Fujishima [mailto:fujishima@bc.jp.nec.com] 
Sent: Monday, September 26, 2005 9:32 PM
To: wsbpel@lists.oasis-open.org
Subject: Re: [wsbpel] Issue - 229 - Fault handling and compensation
handling
allows selective compensation of child scopes

Chris,

I would like to propose the fifth option:

5. Disallow specifying the target scope for the compensate activity.
   I.e., legal:   <compensate/>
         illegal: <compensate scope="...*/>
   Have the compensation handler of the scope, not the caller,
   decide whether the compensation should be done.

Rationale:

I think the source of the problem is that
the current specification makes the caller of the compensation handlers
decide which scopes should be compensated while only each scope's
compensation handler knows how and if compensation needs to be done.

Yuzo

ws-bpel issues list editor wrote:
  
This issue has been added to the wsbpel issue list with a status of 
"received". The status will be changed to "open" if a motion to open
    
the 
  
issue is proposed and that motion is approved by the TC. A motion
    
could 
  
also be proposed to close it without further consideration. Otherwise
    
it 
  
will remain as "received".

The issues list is posted as a Technical Committee document to the
    
OASIS 
  
WSBPEL TC pages <http://www.oasis-open.org/apps/org/workgroup/wsbpel>
    
on 
  
a regular basis. The current edition, as a TC document, is the most 
recent version of the document entitled ** in the "Issues" folder of
    
the 
  
WSBPEL TC document list 
<http://www.oasis-open.org/apps/org/workgroup/wsbpel/documents.php> - 
the next posting as a TC document will include this issue. The list 
editor's working copy, which will normally include an issue when it is
    

  
announced, is available at this constant URL 
<http://www.choreology.com/external/WS_BPEL_issues_list.html>.


    Issue - 229 - Fault handling and compensation handling allows
    selective compensation of child scopes

*Status:* received
*Date added:* 26 Sep 2005
*Categories:* Compensation <#category_compensation>
*Date submitted:* 21 September 2005
*Submitter:* Chris Keller <mailto:chris.keller@active-endpoints.com>
*Description:* Currently fault handling and compensation handling
    
allows 
  
users to selectively compensate work done by child scopes. This can
    
lead 
  
to errors in the process especially as users change their processes
    
over 
  
time. In addition it does not seem to be in the spirit of the BPEL
    
fault 
  
handling and compensation handling model in general (as well as good 
modular programming practice). Take the following example:

<scope name="root">
   <faultHandlers>
       <catchAll>
         <compensate scope="a">
       </catchAll>
   </faultHandlers>
   <compensationHandler>
       <compensate scope="a">
   </compensationHandler>
   <sequence>
     <scope name="a">
     ...
     </scope>
     <scope name="b">
     ...
     </scope>
     <scope name="c">
     ...
     </scope>
</sequence>

If a fault is caused by scope "c" the catchAll will only compensate
    
"a" 
  
leaving "b" uncompensated. Let's assume programmer 1 created the
    
process 
  
and didn't bother to compensate "b" since "b" at that time didn't do
    
any 
  
real work. Now programmer 2 picks up that process later and adds real 
work and a compensation handler for it to scope "b" not realizing that
    

  
the catchAll will not compensate "b" and that the work will not be 
compensated.

Additionally if the real work added to scope "b" was accomplished by 
programmer 2 by adding a child scope "b1" to "b". Programmer 2 looking
    

  
at scope "b" may think that default compensation handling is in place 
and feel safe that their new work will be compensated. Not realizing 
that the scope "root" has selectively chosen not to compensate "b" and
    

  
thereby "b1" in the process.

Possible solutions:

   1. After user defined fault handling and compensation handling is
      executed default handling will execute to compensate all other
      completed child scopes left uncompensated.
   2. If after user defined fault handling and compensation handling
    
is
  
      executed there remains child scopes that have not been
    
compensated
  
      throw an bpws:missingCompensation exception.
   3. Do nothing and say selective compensation is legal and good. And
      add a note that users should take care when changing business
      processes to ensure that user defined fault handling and
      compensation handling call compensate on the child scopes
    
correctly.
  
   4. Same as 3 with one exception after a user calls <compensate
      name="..."> allow them to call <compensate/> which will
    
compensate
  
      all remaining child scopes in the default order. This would
      require changing the following text at the end of section
    
13.3.3:
  
      "Note that the <compensate/> activity in a fault or compensation
      handler attached to scope S causes the default-order invocation
    
of
  
      compensation handlers for completed scopes directly nested
    
within
  
      S. The use of this activity can be mixed with any other
      user-specified behavior except the explicit invocation of
      <compensate scope="Sx"/> for scope Sx nested directly within S.
      Explicit invocation of compensation for such a scope nested
    
within
  
      S disables the availability of default-order compensation, as
      expected." 


*Changes:* 26 Sep 2005 - new issue

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