[ogsa-rss-wg] On the Difference between a CSG and an EPS

[Donal K. Fellows]

Hi everyone!

I've been thinking about what constitutes a CSG or an EPS. Given that
the EPS does not make any commitments (i.e. actually reserve anything)
it seems that they are remarkably similar services, in that each takes a
description of something that needs to be done, and returns a set
(ordered by some "goodness" metric) of ways to achieve that goal. Now
I've tried thinking about the difference in terms of the CSG dealing
with atomic jobs and the EPS dealing with composite jobs (i.e. workflows
in some sense) but that's in many ways a false dichotomy since the
degree to which a job is atomic merely depends on how abstract your view
of the system is. But that would mean that it is very difficult to
really distinguish between what we actually mean by by a CSG and an EPS.

Having thought about this for a while, it strikes me that the key
difference between a CSG and an EPS is that a CSG can be used as part of
doing things other than execution planning (I believe there's an OGSA
use-case for patch management, and the OGSA-D people seem to want a
service that sounds very much like a CSG but which is specialized to
their needs; there are probably other cases I've forgotten). But if an
EPS is like a CSG but a CSG is more general, then that must mean that an
EPS is a _specialization_ of a CSG! This would mean that the definition
of a CSG encompasses the protocol used (i.e. that it is a "task
description to ordered-set-of-ways-of-doing-the-task mapping service",
and the ways of handling the efficient transfer of such potentially
large sets and embedding in things WSRF, WS-Transfer, REST, etc) and
that when it comes to the details of an EPS, we are instead talking
about how do we describe the task (is pure JSDL enough?) and the
candidates (maybe a WS-Ag template wrapped around something, but we'd
need to specify what goes inside).

I'm still thinking through the meaning of all this (it is after all
quite a change from how we've regarded the CSG/EPS dichotomy in the
past) but my hunch is that this will allow us to move forward much
faster to the description of useful service specifications (in part
because it allows us to ignore most of the workflow difficulties). But
I'd *really* welcome feedback on these ideas now. In particular, am I
going off in entirely the wrong direction? :-)

Donal.