Hi DK,
Thank you for your feedback on this proposal.
[1]
I just remind people that we should try to tackle 6.3 and 6.4 first
before proceeding to 6.2. Because those new two subissues are necessary
foundation and clarification for early completion.
[2]
Few explanatory notes on the current proposal:
- When I was designing the 6.2 together with Ivana, I intentionally
added the "countCompletedBranchesOnly" attribute to differentiate a
successfully completed <scope> branch vs a faulted-then-exited
<scope> branch. (Using that to achieve similar goals of
<ignoreFault> idea from Ivana)
- That implies, I personally still expect that all branches are
scope-based when <branches> condition is used, even after we say
"yes" to "partial termination of a scope" in 6.3. That needs static
analysis reinforcement.
- **If** we want to change from <branch> construct to a
number-based XPath function, we should have two versions of functions
as well to keep differentiation feature.
[ e.g. bpws:countCompletedBranches() vs bpws:countFinishedBranches() ].
And, we need to detect usage of those branch-counting functions within
any XPath expression and:
- ban any usage outside completion condition (... ? ... to avoid
the ambiguity in the case of nested early-completion <flow> )
- when used within completion condition, we need to enforce all
branches are scope-based.
- However, I have some hesitation moving from a BPEL
<branches> construct to an XPath function. Because:
- Currently, we enforce both static time analysis and runtime
error checking of bpws:invalidBranchCondition Say, we have 5 branch in
total and varX = 10:
<completionConditon> bpws:countCompletedBranches() > $varX
</completionCondition>
The above XPath problematic expression would be tougher for us to
detect. It can be detected only when we can intercept the ">" or
">=" execution in XPath.
Also, people may try to use those XPath functions in a funky way.
<completionConditon> not (foo:bar(bpws:countCompletedBranches()))
</completionCondition>
These cases make the above error detection much more difficult.
- Currently the <branches> semantics are AT-LEAST N
branches, NOT EXACTLY N branches. People may misuse the XPath function
as follows (saying we have 5 branch in total):
<completionConditon> bpws:countCompletedBranches() = 3
</completionCondition>
This condition may be able to be evaluated to true at runtime, if
people are lucky enough. And, if it is evaluated to true at runtime, it
may give users a false impression that we are having an EXACT-N-branch
semantics.
I guess this is the classical trade-off between flexibility of features
and whether we can trust users would not screw themselves up. Even
though I have hestitation, it would be good to hear more discussion on
this trade-off during F2F.
Thanks!
Regards,
Alex Yiu
Dieter Koenig1 wrote:
Hi Ivana, I would like to propose a simplification of this resolution
proposal for 6.2 (as shown below; this basically folds the "branches"
element into the completion condition as well). This could be considered an
amendment (hopefully friendly :-) of your proposal.
Kind Regards
DK
===============================================
In this proposal, just an optional completionCondition element is added to
a flow. It contains a boolean expression. The boolean expression is
evaluated each time a directly nested activity finishes. If the boolean
expression evaluates to true, the remaining running activities in the flow
are terminated (as described in 13.4.4. Semantics of Activity Termination).
<flow standard-attributes> <completionCondition expressionLanguage="anyURI"?>
... boolean-expression ...
</completionCondition>?
<links>?</flow>
In order to provide the "N out of M" termination semantics, the boolean
condition may use the XPath extension function
bpws:getTerminatedActivities() to determine the number of directly nested
activities that have already terminated.
Example:
<completionCondition>
bpws:getTerminatedActivities() >= 3
</completionCondition>?
"Trickovic,
Ivana"
<ivana.trickovic@ To
sap.com> <wsbpel@lists.oasis-open.org>
cc
01.04.2005 22:07
Subject
[wsbpel] Issue 6.2 - Proposal For
Vote
Motivation
===========
The current semantics of the flow activity is that it completes when all
its (directly) nested activities have completed, either successfully or
unsuccessfully. However, there are scenarios where it is necessary to
have ability to complete the flow activity before all its nested
activities complete in order to speed up the process execution. For
example, a process waits in parallel for 3 reviews of a paper. If 2
positive reviews are received the process may continue with the
execution without waiting for the last, third response.
The completion condition of may have the following flavors:
* Wait for N out of M nested activities to complete
* Wait until after boolean condition C evaluates to true
The completion condition is interesting for a flow activity enclosing
identical nested activities and for the parallel for-each activity
(still under discussion).
Proposal
=========
Syntax:
<flow standard-attributes>
standard-elements
<links>?
<link name="ncname">+
</links>
<completionCondition>?
activity+
</flow>
<completionCondition>
<branches expressionLanguage="URI"?
countCompletedBranchesOnly="yes|no"?>
an-integer-expression
</branches>?
<booleanExpression expressionLanguage="URI"?>
a-boolean-expression
</booleanExpression>?
</completionCondition>
Semantics:
(1) The completionCondition element is an optional element of the flow
activity. Default behavior of the flow activity is that it waits for all
its nested activities to complete.
(2) There are two kinds of completion condition:
A> <booleanExpression>: A boolean condition operating upon process
variables. It is evaluated at the end of execution of each nested
activity.
B> <branches>: An integer value expression which is used to define
condition of flavor N out of M. It is evaluated at the end of execution
of each nested activity. This condition has "at least N out of M"
semantics. (The exact N out of M condition semantics involve resolving
racing condition among nested activities.)
(3) Both conditions (<branches> and <booleanExpression>) may be
specified at the same time. They will be evaluated at the end of
execution of each nested activity. If at least one condition evaluates
to true the <flow> activity completes successfully terminating all
remaining running nested activities. If both conditions are specified,
the <branches> will be evaluated first. If the boolean condition is
specified the evaluation of the condition is done in a serialized
fashion with respect to the nested activities directly enclosed in the
flow activity.
(4) If the integer value evaluated from the <branches> expression is
larger than the number of nested activities in the <flow>, then
bpws:invalidBranchCondition fault MUST be thrown. Note that the number
of branches may be known only during runtime in some cases. Static
analysis should be encouraged to detect this erroneous situation at
design time when possible. (E.g. when the branches expression is a
constant.)
(5) <branches> expression has an optional attribute
"countCompletedBranchesOnly". Its default value is "no". If
countCompletedBranchesOnly is "no", it means the BPEL processor will
count branches which have completed (either successfully or
unsuccessfully). If countCompletedBranchesOnly is "yes", it means the
BPEL processor will count branches which have completed successfully
only.
(6) If flow activity specifies a completionCondition element the
completion condition is evaluated each time a nested activity completes.
If the completion condition evaluates to true the flow activity
completes successfully. All still running nested activities will be
terminated.
(7) Standard BPEL termination semantics applies to running nested
activities when the completion condition is met. The termination of
running nested activities follows the termination semantics defined in
the specification (see section 13.4.4 Semantics of Activity
Termination).
(8) If all nested activities of the flow activity have been completed
but the completion condition evaluates to false the
"bpws:completionConditionFailure" MUST be thrown by the flow activity.
(end)
----------------
Ivana
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