[ogsa-wg] BES query
Dave Berry
daveb at nesc.ac.uk
Wed Aug 31 15:23:26 CDT 2005
Ian,
As always, you raise interesting questions. Some I want to ask you
about further; some I would like to see other people address.
However, I was surprised by your reply about the "three levels" of
naming. While WS-naming doesn't include these concepts, the OGSA v1.0
document does. They also form the basis of the GFS working group work
that led to RNS and WS-Naming, and are important parts of EDG/EGEE, for
example. That doesn't mean that you have to agree with it but I'm
surprised that this issue wasn't resolved earlier.
I'm also surprised that you give name equivalence as the main
characteristic of an abstract name. I thought the main characteristic
was that it was a persistent name that does not specify a particular
location. (Although this could be a property of the WS-Name package
rather than the string - see below).
I'm afraid I don't follow your example of a file replication system.
Clearly we need to name the separate entities involved and there may be
many levels of indirection. I don't see why that is an argument against
the three level approach. We don't need to have one "level" of name for
each "level" of indirection.
Turning to a question I'd like to see other people address:
Ian's point about uniqueness within a particular context makes good
sense to me. If a WS-Name is a combination of an arbitrary string and a
resolver, isn't it the combination of the two that should be globally
unique? E.g. if one WS-Name resolves "foo" to my hard drive and another
resolves "foo" to Ian's, the two WS-Name packages themselves are still
globally unique. I suppose this is why Ian took name equivalence as a
key property. Do we perhaps need a scope notion in WS-Names, so that
they may be valid within a VO, or globally unique, or have some other
scope?
I guess the counter to this is whether it is really such an imposition
to create a globally unique string, given that UUIDs seem to do the job.
Dave.
-----Original Message-----
From: Ian Foster [mailto:foster at mcs.anl.gov]
Sent: 31 August 2005 00:35
To: Dave Berry; Tom Maguire
Cc: Andrew Grimshaw; ogsa-wg at ggf.org
Subject: RE: [ogsa-wg] BES query
Dave:
My "proposal" was just a request that we have the "resolver EPR"
in a separate specification.
However, in a bit more detail, here are my opinions in this
area:
1) The BES specification should just use EPRs, and not mandate
the use of WS-Naming AbstractNames.
Rationale: Such a requirement imposes a cost on BES users that
they may not want to pay--for a benefit that they may not require.
Mandating the use of WS-Naming names imposes an overhead in
terms of both time (the cost of generating the AbstractName) and space
(the storage that it requires).
Yet:
a) I may not need to perform the equivalence test that
AbstractNames are introduced to allow. (E.g., I haven't thought deeply
about this--but I can't recall that we've ever encountered that need.)
b) I may need to perform an equivalence test, but not require
the "globally unique in time and space" constraint that WS-Naming
requires. [This constraint presumably constrains the form of the name
that I can create, preventing me, for example, from using user-supplied
strings.]
c) I may need to perform an equivalence test, but want to do
that at a different level: e.g., I may already maintain my own table of
jobs for some other purpose, so the WS-Naming AbstractName just
replicates unique data that I am already maintaining.
If people WANT to pass around EPRs that have these AbstractNames
in them (or other similar constructs with different
constraints/semantics: e.g., "unique within my VO"), all power to them:
but I don't see a reason to require this in BES or any other
specification.
2) We should have a separate specification for the "resolver
EPR" (and associated Resolver portType) and the "AbstractName" concept.
Rationale: The "resolver" EPR is useful in a wide range of
situations independent of the specific name syntax and semantics
described in WS-Naming.
I can imagine lots of places where I might use a resolver EPR.
E.g., maybe in the BES case my jobs can migrate, and so I want the EPR
that I get back to include a resolver EPR that I can use to determine
its new location. But as noted above, I don't necessarily need an
AbstractName in the BES case.
Similarly for relocatable services: maybe I'll do a metadata
query on a registry to get a service EPR (with a resolver EPR in it),
then if the service is not where the registry says, contact the resolver
to determine its new location. Again, I don't see that I will need
AbstractName semantics here.
Thus, given that we have two concepts that seem logically quite
distinct, and that are certainly distinct in terms of when and how that
will be used, I argue that they should be separated.
3) You also asked about three levels and about "human-oriented"
vs. "abstract". These concepts are not in the WS-Naming spec, but for
what it's worth, I don't see them as useful or relevant here. People use
many sorts of "names" of different sorts, and they don't cleanly
separate into three levels or into "human-oriented" or "abstract". E.g.,
if I'm implementing a replica location service, I often want to map
first from a "logical file name" to a "site name" and then from there to
a "physical location." Furthermore, if you build a system of this sort,
you have to allow people to stick whatever names are convenient for them
in at each level, because often these names come from elsewhere. Some
users may make some of these names "human-oriented", others may not--it
shouldn't matter to the service.
Regards -- Ian.
At 08:58 PM 8/30/2005 +0100, Dave Berry wrote:
Ian,
I wonder whether you or one of your team could write a
note about the
background to your proposals? It would help me to
understand how they
fit into the wider architecture. At present I don't
feel able to make
an informed contribution to the discussion.
The current shape of OGSA (if I can call it "current"
without biasing
the discussion) has three concepts relevant to this
discussion:
1. Three levels of naming (human-oriented, abstract,
and address).
2. An implementation of abstract names as an EPR
containing a string
and a resolver.
3. A service interface for BES that takes abstract
names as
parameters.
Each has associated questions raised by your concerns:
1. Do you want to have fewer levels of naming across
the architecture
or are you happy with the three listed here?
2a. Are you happy that abstract names should be
represented in this
way?
2b. What would a "resolver without an abstract name"
resolve? When do
you need it in addition to or instead of an abstract
name? Does it
fulfill a different function or is there some overlap?
3. What should the BES interface accept instead of or
as well as
abstract names? When do you need this functionality?
How should the
interface cope with the different types of entity?
I can make a guess at part of these answers but I'd
rather read an
explanation from the horse's mouth.
Best wishes,
Dave.
-----Original Message-----
From: owner-ogsa-wg at ggf.org
[mailto:owner-ogsa-wg at ggf.org] On Behalf Of
Ian Foster
Sent: 30 August 2005 04:02
To: Tom Maguire
Cc: Andrew Grimshaw; ogsa-wg at ggf.org
Subject: Re: [ogsa-wg] BES query
I'm interested in that question too. As I've said on
quite a few
occasions,
I like the EPR + ResolverEPR (as in the original
WS-RenewableReferences)
--
that is a nice simple mechanism. I don't understand why
we can't have
this
in a separate specification.
Ian.
At 09:23 PM 8/29/2005 -0400, Tom Maguire wrote:
>So just to get some nomenclature correct. By WS-Names
we mean an EPR
>that
>is profiled to
>contain both a ResolverEPR and an AbstractName. So
what do we call an
EPR
>which just
>contains a ResolverEPR?
>
>Tom
>
>Ian Foster wrote:
>
>>I believe that the opinion was expressed by some at
the San Diego
>>meeting
>>(e.g., by Steve Tuecke) that WS-Names should NOT be
mandated.
>>
>>It certainly defines a nice way of using EPRs that
will be useful in
>>some
>>situations. But it surely can't be the case that we
always want to
>>mandate this particular set of extensions to EPRs.
That requirement
>>certainly doesn't jibe with how we use them in all
cases, for example.
>>
>>Ian.
>>
>>
>>At 09:25 AM 8/29/2005 -0400, Andrew Grimshaw wrote:
>>
>>>All,
>>>
>>>In the BES working group call last week the issue of
naming came up.
>>>The
>>>current DRAFT specification calls for passing
WS-Names in and out of
the
>>>various function calls. There was the question as to
whether EPRs is
all
>>>that should be specified. We thought this is an OGSA
issue: mainly is
>>>OGSA endorsing the use of WS-Names where appropriate.
Clearly I think
we
>>>should. But this should be discussed.
>>>
>>>Andrew
>>
>>_______________________________________________________________
>>Ian Foster www.mcs.anl.gov/~foster
>><http://www.mcs.anl.gov/%7Efoster
<http://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
<http://www.globus.org/> <http://www.globus.org/>
_______________________________________________________________
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
<http://www.globus.org/>
_______________________________________________________________
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 <http://www.globus.org/>
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