Tony:
I think your message captures nicely (although perhaps inadvertently!)
the way in which people are talking past each other in this
discussion.
I would never say that the "messages to single resources
approach" is the "the foundation for all operations on all
services." I understand that some people in this strange religious
debate that we've fallen into have characterized things that way, but
that's far from the truth.
From my perspective, WSRF was motivated by our experiences building
"service oriented infrastructure", and seeing that the same
patterns were occuring repeatedly in different places as we built systems
to manage Grid systems. The codification of those patterns in standard
(and WS-I+-compliant, I like to emphasize) WSDL has allowed us to
simplify many aspects of both service implementation and client tools.
Others report the same positive experiences. The introduction of
WS-Transfer, which provides similar functionality and seems to be
intended for similar purposes, suggests that there is broad recognition
of the importance of these patterns. However, the fact that these
patterns are useful in building certain classes of management
applications (a primary focus of OGSA, by the way) certainly doesn't mean
that they are appropriate everywhere.
I'd also like to suggest that when considering the assertion that
"sending messages to single resources makes systems fragile",
it is useful to recognize that the messages sent over the wire when using
an EPR to a WS-Resource (the WSRF approach) vs. an EPR plus a context id
(e.g., as in the eCommerce systems that are often mentioned) are close to
identical. In fact, the only difference is really just the location of
the "context id": in the EPR vs. in the body of the message! I
don't see how the choice of one placement vs. the other can render a
service "robust and scalable" vs. "fragile and
nonscalable"--especially as the service itself can be implemented in
an essentially identical manner in the two cases.
My preceding paragraph suggest that there are opportunities for common
ground, and I suspect that is the case. However, to find that common
ground we need to identify clearly just what it is we are trying to do
and then address different issues separately. I believe that there are
far too many different issues being mixed together at present for useful
progress to occur. Unfortunately, I'm not sure how to proceed to separate
out the different issues.
Ian.
At 11:37 AM 3/3/2005 +0000, Tony Hey wrote:
The point is not about how well the
WS-RF and WS-Transfer stacks compare
but rather whether it is always appropriate to use the "messages
are
directed at single resources" approach? Many people, including
people
whose technical judgement I respect such as Tony Storey, Ian
Foster,
Dave Snelling and others, apparently believe that the answer to
this
question is "everywhere: it is the foundation for all operations on
all
services". It is therefore not surprising that this group do not see
the
need to worry about the question "is it a good idea to build
architecture around the idea of sending messages to single
resources?"
_______________________________________________________________
Ian
Foster
www.mcs.anl.gov/~foster
Math & Computer Science Div. Dept of Computer Science
Argonne National Laboratory The University of
Chicago
Argonne, IL 60439, U.S.A. Chicago, IL 60637,
U.S.A.
Tel: 630 252
4619
Fax: 630 252 1997
Globus Alliance,
www.globus.org