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Please pardon me for entering this thread a few weeks later, but
stepping out side of REST, this reminds me of some of the emerging Web
Services specs. More comments at end.
"K. Ari Krupnikov" wrote:
> David Megginson <firstname.lastname@example.org> writes:
> > K. Ari Krupnikov wrote:
> > > Suppose I want to build a RESTful interface to an ACID system. I want
> > > to allow the user to incrementally change the state of the system, but
> > > only "commit" the changes when he is satisfied with their sum. I
> > > imagine a user GETting a resource, POSTing some changes to it which
> > > may have side effects on other resources, GETting other resources to
> > > observe the side effects, and eventually committing to the changes or
> > > rejecting them.
> > If the scope of the potential side effects is small and well-defined,
> > you might be able to reduce everything to a single exchange: get all
> > of the potentially-affected state to the client in a single XML
> > document (or collection of XML documents), allow the client to make
> > changes locally until satisfied, then post the modified state back to
> > the server. Since modifications live on the client side until
> > committed, there is no need to come up with any separate URL scheme.
> You are talking about a very thick client here. It's not how much
> state there is this application (not much), it's how much logic there
> is (a lot). The server already knows how to do this logic. I don't
> want to duplicate the logic on the client. I want to use a browser,
> client-side. A full-blown Java client is a future possibility, but
> again, I'd like to keep the business logic in it to a minimum. I
> definitively don't want three separate codebases for business logic,
> different browsers) and one in a Java client.
> Client does presentation, server does business logic. Is that not in
> line with REST?
> > This approach would work for, say, a movie database, where the user
> > might check out a full movie description to change the director's
> > name, then check the entire description in again. It might not work
> > so well for, say, a geographical database, where a modification to a
> > shoreline may also require corrections to landuse data, roads and
> > railroads, and so on, in no easily-predictable way.
> > It is entirely possibly that your project is one in the second
> > category, but I wouldn't give up on the simpler approach until it were
> > proven inadequate. Stupid-and-easy usually wins: moving 50K of XML
> > each way once can be much more efficient than many 100 byte
> > transactions.
> It's not about reducing redundant traffic, it's about reducing
> redundant logic. One node should know how to do a process, be the
> final authority and bear the ultimate responsibility for that process.
Stepping away from REST for a second (but acknowledging that REST was
your main intent), this is the essence of service-oriented architectures
(SOAs) - shared (non-redundant) business logic encapsulated as shared
services that can be published, discovered, and bound/invoked (at design
time and/or run time). The coordination aspects you discuss above are
covered by emerging specifications such as IBM/Microsoft/BEA's
WS-Coordination/WS-Transaction (WS-Transaction is now segmented into 2
specs, WS-AtomicTransaction and WS-BusinessActivity) , WS-Resource
Framework (WSRF)  from the Globus Alliance/IBM/HP, and OASIS
Composite Application Framework . References  involve the
specification of a "coordinator" that coordinates the actions of
multiple Web services for requirements such as 2-phase commits, error
notification, etc. Reference  addresses interations of Web services
with stateful resources.
It is important to note that none of these specifications are "open
standards" (in OASIS and W3C-"like" terms) - the one closest to becoming
an open standards at this point is WS-CAF.
Booz | Allen | Hamilton
Strategy and Technology Consultants to the World
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