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Re: Interest in standardizing Batch methods?

From: Eric Sedlar <eric.sedlar@oracle.com>
Date: Thu, 17 Jul 2003 15:03:24 -0700
Message-ID: <01a601c34caf$3e82c090$86aa2382@us.oracle.com>
To: "Julian Reschke" <julian.reschke@gmx.de>, "Geoffrey M Clemm" <geoffrey.clemm@us.ibm.com>, "WebDAV" <w3c-dist-auth@w3.org>

I think something along the lines of what NFSv4 supports with the COMPOUND
method would be in order.  The big benefit of doing a COMPOUND/BATCH method
is the atomicity.  Note that the transaction you want to do may operate on
multiple resources, and while LOCK will help, rolling back incomplete
changes is a big pain in the ass for the client to do.  Do you expect the
client to implement a log file (like a database) which will be updated on
each successful response, and then require that the client know how to apply
undo from the log file in case of an error?

Let's look at a use case:  I want to PROPPATCH resources /a1, /a2 ... /a10
with the same dead property called foo to some new value either as a unit or
not at all.  With today's WebDAV, I LOCK all of these resources and start
pipelining the PROPPATCH requests.  As the replies come back in, I log them
to a file on the client that is fsync'ed to the disk (so that I'm sure my
logging makes it to disk in case the client crashes at some point, the way
databases do).  It turns out that there was an error in the PROPPATCH on /a8
(ACL violation) and I want to rollback all of the preceding proppatches in
the client log.  Now the client starts to replay the log (since the last
transaction start), re-PROPPATCHing /a1 through /a7 hoping there is no other
error, and then cleans up the log entries (or marks them as rolled back).
Just in case this work sounds easy for client implementors to do, it's not.
Plus, this strategy would only work if I happened to know the old values of
the foo property so that I could undo them.  PROPPATCH doesn't tell me what
the old values were (you need the equivalent of the SQL statement "UPDATE
foo RETURNING foo INTO :1") to do this.  In general, undoing WebDAV
operations is not easy and not always possible (e.g. some versioning
operations).

Given that NFSv4 could handle COMPOUND, I don't think that this is too hard
or out of scope for WebDAV.  Also, I believe that none of the publically
announced NFSv4 implementations are built on databases, so I don't think
this is too hard for server implementors in general, but we could make BATCH
methods an optional feature to address this concern.

As to Julian's particular concern about transactions started but not
completed, the same solution should be applied as for any method in which
part of the request has arrived but the rest has not after a few
minutes--you timeout the entire request.

--Eric


----- Original Message -----
From: "Julian Reschke" <julian.reschke@gmx.de>
To: "Geoffrey M Clemm" <geoffrey.clemm@us.ibm.com>; "WebDAV"
<w3c-dist-auth@w3.org>
Sent: Thursday, July 17, 2003 2:20 PM
Subject: RE: Interest in standardizing Batch methods?


>
> True, but...
>
> ...speaking from personal experience working on a document management
> system, the temptation "give up" and just state "transactions are going to
> solve this problem in general" is big. However, transactions can be very
> expensive to implement, in particular in a HTTP server scenario. How many
> concurrent transactions are you going to support? What are you going to do
> if a client starts a transaction but doesn't finish it within a few
minutes?
>
> IMHO it makes a lot of sense to try to *avoid* them by
>
> a) defining methods with the "right" level of transactionality (for
> instance, see PROPPATCH or ORDERPATCH) or
>
> b) try to do things using the existing locking and/or versioning
machinery.
>
>
> So at a minimum, I'd expect a working group activity on transactions to
> start with a list of use cases. Then we can take a look at the use cases
and
> decide whether they require a generic transaction mechanism, or whether
> something simpler can do that as well (and please don't say: I want to do
> JDBC over HTTP).
>
>
> Julian
>
> --
> <green/>bytes GmbH -- http://www.greenbytes.de -- tel:+492512807760
>
> > -----Original Message-----
> > From: w3c-dist-auth-request@w3.org
> > [mailto:w3c-dist-auth-request@w3.org]On Behalf Of Geoffrey M Clemm
> > Sent: Thursday, July 17, 2003 11:02 PM
> > To: WebDAV
> > Subject: Re: Interest in standardizing Batch methods?
> >
> >
> >
> > If you are interested in defining a transactioning model, then
> > it would be best to make that explicit, because the term "batch"
> > is used to encompass many things.  To date, the WebDAV approach has
> > been to define specific transacted operations (e.g. PROPPATCH),
> > because that is something that can be implemented on a wide range
> > of repositories, while generic transactioning support is likely to
> > only be feasible on repositories that are actually databases.
> >
> > But enough repositories really are databases that it probably is
> > worthwhile trying to define generic transactioning support for
> > WebDAV.
> >
> > Cheers,
> > Geoff
> >
> > >
> > > I am sure that you guys, who don't agree with standardising Batch
> > > method, have good reasons for that. But shall we be more clear about
> > > those reasons which might not quite clear for all. Of course I know
> > > some of those reasons by chasing the thread with the same subject in
> > > the WebDAV mailing list. But I think the WebDAV protocol is almost
> > > enough mature to interest many organisations and Web users. So that
> > > they would like to apply this protocol rather HTTP protocol, and use
> > > the WebDAV advantages such as metadata, access control,... to author
> > > Web resources.
> > >
> > > Now the question is: if the WebDAV is going to be a substitute for
HTTP
> > > protocol, can it handle authoring all kind of Web resources as it
> > > claims, or still we need other conventions based on the HTTP protocol
> > > in order to support authoring Web resources except file systems.
> > >
> > > So if you believe the WebDAV as a protocol for authoring all Web
> > > resources (and not only the file systems), then how we can handle Web
> > > resources such as emails and databases. In order to implement some
> > > operations on those resources, you might use a sequence of WebDAV
> > > methods which need to be considered as one transaction. In those cases
> > > lock method cannot help. So what is your suggestion to handle those
> > > operations?
> > >
> > > I think a mechanism for supporting transactions is a strong point that
> > > WebDAV needs.
> > >
> > > >
> > > > I agree with Chris.  The easy things aren't worth doing (you get
> > > > virtually
> > > > all of the benefit by streaming your requests and not blocking
> > > > waiting for
> > > > the response to each request), and the hard things are very complex
> > > > and
> > > > any given approach is unlikely to be a good basis for
> > > > interoperation.
> > > >
> > > > Cheers,
> > > > Geoff
> > > >
> > > > Chris wrote on 07/17/2003 02:27:27 PM:
> > > >
> > > > > And boy, the semantics of batch requests are pretty hairy. At the
> > > > end of
> > > >
> > > > > the day is there much benefit? Is each request in a batch atomic
> > > > and/or
> > > > > is the entire batch? (Do you have multiple levels of
transactions?)
> > > > How
> > > > > do you cache a batch? (Boy, there's a long topic...)
> > > > >
> > > > > The HTTP overhead for multiple requests (made over the same TCP
> > > > > connection, mind you) is small (arguably, couldn't be much
smaller)
> > > > and
> > > > > if you want atomic batches, locks do an ok job of it.
> > > > >
> > > > > I've certainly considered something like a PUT/PROPPATCH
> > > > combination
> > > > > (for example if you had "required dead properties") but it's
> > > > probably
> > > > > just easier to do a null lock and the two requests separately.
> > > > >
> > > >
> > >
> >
>
>
Received on Thursday, 17 July 2003 19:22:37 GMT

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