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Subject: i090 - What is Offer good for anyway?
The use of Offer is quite often referred to as “just
an optimization” which is the principal grounds that it is being argued
that it should be removed from the spec. While we agree it can be viewed as
just an optimization if you are looking at this from the perspective of having
long living RM sequences established between the client and server, there is
another view that makes it more useful and one use case that can not be
realized without it. First let me describe the two principal scenarios that we
see for using WS-RM.
(1) Essentially this is about extending reliability to
traditional synchronous web service interactions be they one-way or
request-reply. Here synchronous means that both apps, the client and server,
are running at the same time and interacting with very low latency. (2) In the asynchronous, or queued, case the apps at the
client and server are running independently of each other. In this case the
apps at either side can potentially be subject to high latencies and can
tolerate failure conditions beyond just communication, i.e. the apps may
survive reboots. Applications may use a variety of mechanisms to indicate where
to send reply messages and how or whether those need to be over a reliable
channel. A straightforward implementation of (1) would be one where
the AS and the AD own the RM sequences they use to exchange messages.
Logically, this means the AS hosts its RMS in-proc and the AD hosts it RMD
in-proc. If the WSDL of the service includes two-way operations then Offer
makes establishing the back sequence easier and less-costly. More importantly
when one looks at the case of anonymous clients, particularly on HTTP, there is
one case where you need Offer. In this case if you want to establish a reliable
path back to the client, for acks or responses, the service can not invoke a CS
on the client. Instead the client provides the Offer which is accepted by the
service. Subsequent messages using the Offered sequence are carried on the HTTP
response flow to the anonymous client. I do think that given the above we should not just cut
Offer out of the spec. From our perspective it would compromise our ability to
use the eventual OASIS standard of RM in comparison to how we are using it
today. I’m open to working with others to clarify the above
points. In particular the spec needs to convey the following:
Marc Goodner Technical Diplomat Microsoft Corporation Tel: (425) 703-1903 Blog: http://spaces.msn.com/mrgoodner/ |
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