W3C home > Mailing lists > Public > public-ws-chor@w3.org > May 2003

Re: Co-ordination protocol and BPEL

From: Mark Little <mark.little@arjuna.com>
Date: Thu, 22 May 2003 14:37:51 +0100
Message-ID: <000701c32067$57a13990$b596f080@exhp>
To: "Furniss, Peter" <Peter.Furniss@choreology.com>, "Assaf Arkin" <arkin@intalio.com>, "Ricky Ho" <riho@cisco.com>
Cc: <public-ws-chor@w3.org>

> For the sequence case, unless the first message is explicitly defined as
> being the first of a conversation, and that definition says that a
> context on it is implicit to the rest of the conversation, I'd kind of
> assume that each message has it's own context.

Agreed and there are systems out there that do this by making context a
first-class entity that can be shipped around without an explicit
application message. Obviously you need an equivalent of "close", though
this can be implicit (if you receive another context) or could be on
transaction termination.

> Of course, these could
> always be the same context, meaning each of them are part of the same
> transaction.
>

[stuff deleted]

> Shouldn't the receiver/participant trust the "someone" to mean what they
> say too ? If they said commit in relation to the context they sent, then
> they want a commitment on what they've asked for so far. If the receiver
> can do that to be internally consistent, it should vote commit. It's up
> to the other end to decide what the totatlity is.

I agree and think "autonomous" commitment is important. Why rollback the
transaction if the ultimate result of the transaction is to commit anyway?

> > Since you haven't yet completed the sequence you can't compensate for
> > it. Instead you get a terminate fault and handle it. However,
> > since you
> > already did some activities you may have nested scopes with their own
> > compensation handlers which you would invoke from the event
> > handler of
> > the sequence.
>
> what you do to undo things is surely implementation detail - of no
> concern to the top-end, and not needing to be distinguished in the
> coordination protocol (contra ws-t). "cancel", "compensate" and
> "rollback" all mean "make it not so". The receiver must know its own
> state and work out how to revert, to the best of its ability.
>
>
> >
> > > 4) What if the second <receive> pass in a different context ?  (the
> > > only way to prevent this happening is to use the context
> > itself as the
> > > correlation set)
> >
> > Good question.
>
> if it was sent in a different context, it was meant in a different
> context (as Arkin says, you must mean what you say). If the receiver
> can't do the work separately, (and the requestor doesn't shortly confirm
> the first transaction), then this use isn't going to work. But generally
> it would be quite reasonable to do things step by step, confirming each
> one. And, depending what this is, it could be just as reasonable to send
> a sequence then confirm the effects of 1, 3 and 6 but cancel 2,4 and 5.
> (or as the requestor pleases).

Being able to run concurrent units of work and then confirm or cancel them
is certainly important.

>
>
>
> > In EJB if you have a session bean and you call it twice in
> > two different
> > transactions then the access would be serialized or you would get an
> > error. I don't exactly remember which case is true, but you won't get
> > two invocations from two transactions for the same bean if one
> > transaction has not completed yet.
> >
> > So I would tend to believe that the rule should be that all incoming
> > messages should either be part of the same scope, part of a
> > child scope
> > (if that activity is defined as part of a child scope), or not
> > associated with any scope. But it can't be part of a different scope
> > unless it observes a parent-child relationship.
>
> Parent-child would be common, but again, if the application+coordination
> protocol combination allows the sender to communicate a different wish,
> it is possible to obey it.
>
>
> > > Lets say I'm an airline company.  I don't want to reserve
> > the seat for
> > > a lengthy period and later cancel it when the passenger change his
> > > mind.  So my strategy is to only book the seat when the customer
> > > commits its purchase.  After that, I don't refund (so not
> > > compensatable from the passenger's perspective).
> > > The original state of the seat is "available".  When the passenger
> > > invoke my "TicketBookingService", the seat is marked
> > > "potentially-booked".  When the transaction commit
> > successfully, the
> > > seat status to be "booked".  After that, it is non-compensatable.
> > >
> > > How do I achieve this if I only have the CompensationHandler ?
>
> excellent example - you are getting pushed out of shape by a protocol
> that thinks it knows how the world always works, and this bit doesn't.
>
> The general case is
> perform provisional work such that you can later:
>
> make it not so, if so instructed
> make it finally so, if so instucted
>
> (slot in as many synonyms for provisional, make it not so, make it so as
> you please)
>
> so, I agree, you need at least two "handlers". Whether these are defined
> as mainstream and exception, or both as event handlers is mostly a
> matter of how the language works, I think.

I'm not convinced you need at least two: one definitely - the ability to
undo. If you talk to companies that do long running work and have done so
for years you'll find that that vast majority of work is never cancelled and
they use a very optimistic approach - the work is done and 1% of the time it
is compensated. However, I suppose a two handler approach would still fit
this since the confirm would be a  null-op in this case.

Mark.
Received on Thursday, 22 May 2003 09:38:26 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Saturday, 18 December 2010 01:00:17 GMT