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Subject: Issue - 168 - Proposal To Vote
I propose we adopt the "less magic" approach described in the issue
description. This means that the semantics of process instantiation
would be as follows:
1. The arrival of message that matches an activity marked with
createInstance=yes/rendezvous (and not matching an existing process
instance) causes a new process instance to be created. This message is
termed the "instantiating message" for that process instance. The
createInstance=yes/rendezvous activity that was used to justify the
instantiation is termed the "instantiating activity" for that process
instance.
2. Once a process instance is created, all its activities are executed
in the order dictated by the structure of the process.
3. When a <receive> or <pick> activity with createInstance=yes is
executed, the message "received" will be the "instantiating message" of
the process instance.
4. When a <receive> and <pick> activity with createInstance=rendezvous
is executed, the message "received" will be either:
a ) the "instantiating message" if said activity is the
"instantiating activity"
b ) some other message matching the correlation key from the
"instantiating message" if said activity is not the "instantiating
activity"
Key changes to text:
6.4:
OLD: This is done by setting the createInstance attribute of such an
activity to "yes". When a message is received by such an activity, an
instance of the business process is created if it does not already exist
(see Providing Web Service Operations and Pick).
NEW: This is done by setting the createInstance attribute of such an
activity to "yes". When a message that matches such an activity is
received, an instance of the business process is created if it does not
already exist (see Providing Web Service Operations and Pick).
OLD: To be instantiated, each business process must contain at least one
such "start activity." This must be an initial activity in the sense
that there is no basic activity that logically precedes it in the
behavior of the process.
NEW: To be instantiated, each business process must contain at least one
such "start activity." ----strike---
11.4:
OLD: A receive activity annotated in this way MUST be an initial
activity in the process, that is, the only other basic activities may
potentially be performed prior to or simultaneously with such a receive
activity MUST be similarly annotated receive activities.
NEW: -----strike----
13.5.3:
OLD: If the event handler is associated with the global process scope,
the event handler is enabled as soon as the process instance is created.
The process instance is created when the first receive activity that
provides for the creation of a process instance (indicated via the
createInstance attribute set to "yes") has received and processed the
corresponding message. This allows the alarm time for a global alarm
event to be specified using the data provided within the message that
creates a process instance, as shown in the following example:
NEW: If the event handler is associated with the global process scope,
the event handler is enabled as soon as the process instance is created.
Note: alarm time for a global alarm event /cannot/ be specified using
the data provided within the message that creates a process instance!
On Mon, 2004-10-04 at 17:14, 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 the TC accepts it
> as identifying a bug in the spec or decides it should be accepted
> specially. Otherwise it will be closed without further consideration
> (but will be marked as "Revisitable")
>
> The issues list is posted as a Technical Committee document to the
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> Issue - 168 - Semantics of instance creation
> Status: received
> Date added: 4 Oct 2004
> Categories: State management
> Date submitted: 30 September 2004
> Submitter: Maciej Szefler
> Description: Discussions of issue 81 : Are start activities that
> aren't createInstance activities legal? have brought to light a
> certain deficiency of clarity in the current specification with
> respect to issue of instance creation. The present spec makes various
> vague and somewhat contradictory statements as to how createInstance
> activities should be handled.
>
> On the one hand, the spec suggests that process creation is "implicit"
> and that the createInstance flag is merely an annotation that defines
> which message events cause an instance to be created and that once
> created the process instance processes all activities in the same
> manner largely oblivious to the value of that annotation.
>
> On the other hand, the spec restricts the set of activities that are
> "initial" activities, and establishes exceptional semantics (for
> process-level event handlers) that could be construed to imply that
> createInstance activities are actually activated before any other
> activities, irrespective of their actual location in the process.
>
> I posit that the former interpretation provides a concise and
> manageable view of the instance creation process. By making the spec
> consistent with it we can define execution semantics of a single
> process instance without reference to instance creation. We can handle
> instance creation simply and separately by stipulating that a process
> instance is created when a message event that would match one of the
> createInstance activities is received. This message event is
> "allocated" to that activity, so that when that activity is actually
> activated (in the normal course of process instance evaluation) it
> will receive the said event.
>
> The major implication of this model on execution semantics is the
> elimination of the notion of "initiate" activities. This concept
> becomes unnecessary. One might object on the basis that without the
> initiate activity restrictions the following process would be
> perfectly legal:
> <sequence>
> <invoke .../>
> <receive createInstance="yes" .../>
> </sequence>
> Such a process certainly seems objectionable. However, the details of
> normal execution semantics would make such a process unlikely. That is
> to say, the <invoke> would need to use a message variable (for the
> request), and that variable could not have been initialized unless
> some activity preceded the <invoke>. One might then object with the
> following:
> <sequence>
> <receive createInstance="no" .. var="foo"/>
> <invoke ... inVar="foo"/>
> <receive createInstance="yes" ..>
> </sequence>
> However, in this process the first receive is invalid unless a
> correlation set is used. But in order to use the correlation set, it
> first needs to be initialized, and the only way to do that is with an
> invoke or a receive/pick that precedes it, so you're back to needing a
> <receive> to precede the <invoke>. This receive would have to have
> createInstance="yes" lest it run into the same problem. But if this
> receive had createInstance="yes" then the same annotation on the
> second <receive> would be invalid.
>
> Now, one might get cleverer still and object based on the following
> somewhat convoluted process:
> <sequence>
> <assign>
> <copy>
> <to variable="foo"/>
> <from> literal </from>
> </copy>
> </assign>
> <invoke ... inVar="foo">
> <correlation name="cset1" initiate="yes" pattern="in"/>
> </invoke>
> <receive createInstance="no" ..>
> <correlation name="cset1" initiate="no" />
> </receive>
> <receive createInstance="yes" ..>
> </sequence>
> However the above construct would result in ALL process instances
> having the same correlation set value, which does not make any sense.
>
> But one could still object by changing the pattern to "out" on the
> invoke, and asserting that the partner generates unique output
> messages for each invocation thereby yielding unique correlation keys.
> But even this very brink of the edge case forces us to change nothing
> in the semantics. The only significant implication is that in certain
> unlikely circumstances, the implementation might have to handle
> <invoke>s and non-createInstance <receive> before it has a chance to
> offload the createInstance message to the createInstance <receive>
> (i.e. it needs to provide a "memory" for the message that created the
> instance). The only plausible use case for this kind of behavior is
> for initialization of static content.
>
> Finally, adopting uniform execution semantics would lead us to
> elimination of the exceptional language in the spec that requires that
> process-level alarm handlers can use data that would normally only be
> valid after a receive activity completes. This is not so onerous, as
> it is possible to move a process-level event handler into a scope
> following the initial receives.
> Changes: 4 Oct 2004 - new issue
>
> ______________________________________________________________________
>
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