RE: re - BTP Issue 108 (was RE: [business-transaction] Email votes -7 Issues - Ends Tues April 9 = RESULT)

[Peter Furniss <peter.furniss@choreology.com>]

Thanks - it helps to have something concrete.


> Before the suggested text addition I'll re-iterate that there is another
> solution to this problem and perhaps we should have this as a separate
> voteable option: that all cohesion inferiors must be atoms; that way it is
> not necessary for clients to know what participants (if any) were enrolled
> by a given service when it receives a context - the client controls atoms
> only. The identities of atoms are defined well enough in the BTP
> specification and how they are related to services at the
> application level
> is down to the application.

This would disallow the determination by the service of the division into
choosable atoms (strictly, inferiors).  The most obvious example is a
reverse auction/invitation to tender/request for proposal application:

buyer sends or announces its requirements, with a cohesion context

would-be sellers put together proposals that meet the requirement, or parts
of it. A seller might put in several proposals, with different terms, or
matching different parts of the requirement. Each of these proposals is
associated with a different enrolled inferior.

buyer selects a set of inferiors that, in aggregate, meet his requirements,
and confirms exactly those

(to make concrete - vary the home entertainment system example to use this
pattern. One supplier (Dixons ?) offer Panasonic TV, Phillips TV, Panasonic
VCR as individual items, and a better price for the two Panasonics if you
buy both from them.)

That scenario also shows the limitation in some of the solutions - the need
is not just to associate the inferior with originally invoked service, but
with the particular reply, and thus with the application semantics of that
reply. (which item(s) at what price from Dixons, not just that it was
Dixons)


> Assuming the above fails, then we need to provide a way for users of
> cohesions and services to have a standard way in which to use
> cohesion with
> non-atom inferiors. Because the service that receives a cohesion
> context is
> not necessarily the participant (inferior) that is enrolled with the
> cohesion coordinator this causes communication problems for the client
> (communication in the sense that the client no longer has an appropriate
> handle on the inferior in order for it, or something acting on its behalf,
> to refer to it during cohesion termination phases.)
>
> So, here's a suggested text addition (using some of the above):
>
> "There are two ways in which services may become associated with a BTP
> cohesion: when work is requested within the scope of an atom which is
> associated with a cohesion, or when work is requested within the
> scope of a
> cohesion. In the former case, the user has knowledge of the atom
> (e.g., it's
> name) and can thus control the work performed by the service within the
> scope of that atom transaction when the user later comes to terminate the
> cohesion (e.g., issuing PREPARE_INFERIORS giving the atom's name will
> implicitly prepare the inferior associated with the service). However, if
> the service is invoked without an associated atom, then this automatic
> handle is missing. The service is free to enlist one or more participants
> with the cohesion to control the work that it does; remember that the
> service and participant are two separate roles that need not be
> performed by
> the same actor, i.e., a service is not necessarily going to be a
> participant
> so the fact that the user has a handle on the service (with which
> to invoke
> operations) does not mean that it implicitly has a handle on the
> participant(s) for that service.

On re-reading, that's nearly the same as I said. I assume "name" there is
strictly "identifier".

> There are a number of ways in which this problem may be addressed.
> Unfortunately not all of these will result in interoperable services
> (services which can run with any implementation of BTP or be used
> in any BTP
> application). Therefore, in order to guarantee interoperability, BTP
> supports the following mechanism (note that this does not preclude
> implementers offering other ways to address the same problem):
>
> reverse-context-flow: when a BTP-tagged invocation is sent, the
> BTP context
> is associated with it in order that the receiving service knows which
> transaction to perform the work within. When the response to an invocation
> which was performed within a cohesion but outside of an atom is returned,
> there will be a reverse participant-context associated with the message
> which will describe the inferior(s) that were enrolled within
> that cohesion.
> It is the responsibility for the receiver to remember these inferior
> identifiers in order to later perform cohesion termination on the
> work done
> by the service."
>
> OK, so this actually requires more than a text change since it puts some
> requirements on the BTP "interceptor" and service and doesn't say how the
> user gets access to this context information (which will probably be
> implementation/API dependant).

Isn't this assuming the enrollments are in fact to be treated as atomic ? So
it wouldn't work for Dixon's multiple offers. But BTP already has mechanisms
which can ensure that the service's multiple participants are atomic - the
service creates and enrols a sub-coordinator, and then enrols its
participants with that. Otherwise it's a kind of "soft atom" and vulnerable
to error and confusion (c.f. if Dixon's TV + VCR combined offer is in fact
implemented as two leaf participants, they should reveal only one inferior
enrollment for that to the buyer, otherwise the buyer might try to break
apart the both-or-neither nature of the offer.)

> The alternative to the last paragraph of the text above is:
>
> "A service is responsible for ensuring that the identifiers that
> are used to
> enrol participants acting on its behalf are unique *and* have a component
> that unambiguously relates them to the service they are acting for, e.g.,
> the service URI. At any point during the cohesion's lifetime the
> client can
> call STATUSES to obtain the current status of all enlisted participants
> (inferiors) and can then use this information to terminate the cohesion."

Again this doesn't work for case above, because it isn't the service alone
that needs to be identified.

> I thought I'd give a couple of alternatives. The advantage of the
> first over
> the second is that it avoids remote invocations. The advantage of
> the second
> over the first is that it does not require any changes to BTP or services.



Had a rummage through the conceptual model, and the section "Control of
Inferiors" already covers much of this. (The first paragraph after figure 12
isn't my best prose, and has an omission - line 1012 should have "an
Inferior" after "whether". ) But the explanation of the problem and options
was carefully phrased to cover the various scenarios. It deliberately does
not define exactly how the ENROL and the application work are associated,
because that is an application issue, not one we can handle.  If we do need
more, I think it either goes in that section, or belongs in another
document. (assuming we don't decide we need to change anything normative, of
course).


Peter

------------------------------------------
Peter Furniss
Chief Scientist, Choreology Ltd
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