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RE: Modified: Auto update of VCR when checking an associated work ing resource

From: Clemm, Geoff <gclemm@rational.com>
Date: Sat, 14 Jul 2001 10:28:00 -0400
Message-ID: <3906C56A7BD1F54593344C05BD1374B103A385E6@SUS-MA1IT01>
To: ietf-dav-versioning@w3.org
   From: Lisa Dusseault [mailto:lisa@xythos.com]

   I'm in favour of this.

   One addendum:  if the server does NOT support the UPDATE method, then the
   client MUST include the DAV:auto-update property in the CHECKIN request.

The DAV:auto-update property is set at CHECKOUT time, not at CHECKIN
time.  (I assume when you say "include the DAV:auto-update property",
you meant "include a value for the DAV:auto-update property").

   From: John Hall [mailto:johnhall@evergo.net]

   One addendum: Explicitly point out that if the CHECKIN on the VCR
   fails, the client would have to do a CHECKIN of the working
   resource followed by a MERGE or UPDATE to retry the operation.

I assume you meant "if the CHECKIN of a working resource with
a DAV:auto-update property fails ...".  The server must fail
a CHECKIN operation if the DAV:auto-update cannot be performed,
so MERGE and UPDATE are not relevant here.

   From: Lisa Dusseault [mailto:lisa@xythos.com]

   > From: Tim Ellison
   > (The DAV:auto-update property is on the working resource).  I
   > don't want to make it mandatory since there are cases where
   > creating a version that does not update the version-controlled
   > resource is desirable.  Without UPDATE it would only be
   > 'referenceable'(ahem) by the version URL.

   So are we back to saying that supporting Working Resource feature
   requires also supporting UPDATE?  That's what I'm trying to avoid.
   Is there a better way for these two features to not depend on each
   other?

Tim is talking about the case where the CHECKOUT was applied to a
version URL, and therefore there is no DAV:auto-update property set on
the working resource.  This doesn't imply that the working resource
feature requires supporting UPDATE.  

   Furthermore, it's surprising to me, and perhaps to clients as well,
   that working resource checkins and in-place checkins work so
   differently: one ends up with the VCR pointing to the latest
   version at the end, and the other doesn't (by default).  I would
   expect the default behaviour of both of these to be the same.

One of the primary use cases for a working resource is to allow you
to create a "hidden version", i.e. a version that is not exposed at
a VCR.  This is the natural semantics for checking out a version,
since there is no VCR to be updated.  So I believe the protocol
currently provides the "hidden version" functionality in a natural
way.

   Perhaps Jim's proposal could instead be worded so that auto-update
   was the default, and "do not update" is the exceptional behaviour
   that the client must request.

Auto-update is the default (and only behavior) when you create a
working resource by checking out a VCR, and do-not-update is the
default (and only behavior) when you create a working resource by
checking out a version.

   From: Lisa Dusseault [mailto:lisa@xythos.com]
   >
   > I have an application in mind that creates versions and doesn't
   > update any version-controlled resource.  It always references the
   > versions by their version URL.  In this case there is no need for
   > DAV:auto-update or an UPDATE method, so I would object to making
   > them required.

Note: Tim is saying that he would object to making DAV:auto-update
required when a version is checked out, and that UPDATE is not required
since these are "hidden" versions (i.e. versions that will not appear
in VCR space).  DAV:auto-update is of course required when you are
creating a working resource by checking out a VCR.

   That really badly serves the purposes of non-versioning clients.
   The server can't interact with those clients in your scenario,
   because those clients would always just GET the VCR, and end up
   with the root version instead of the latest versions.

If you want to expose the new states to a non-versioning client, you
would of course need to update the VCR, but Tim's scenario did not
involve exposing the new states to non-versioning client.  He's just
saying that creating a new version without updating the VCR needs to
be possible.

   Let me phrase the problem in these terms, I must find a solution to: How
do
   I write a server implementation that
    - supports Working Resources
OK.
    - supports non-versioning clients
OK.
   Some of the assumptions I've made in framing this problem are that
    - UPDATE is not supported
OK.
    - I may not be able to rely on versioning-aware clients to use the
   auto-update flag in CHECKIN.  I don't know how to send an error if they
   don't use this flag.

This is an unreasonable assumption.  The protocol provides well
defined responses to well defined requests.  If a client choses
not to use the defined protocol, then they should not be surprised
if they do not get the results defined by the protocol.  If they
want the functionality provided by the DAV:apply-to-version flag,
they need to use the DAV:apply-to-version flag.  A CHECKIN applied
to a VCR without the DAV:apply-to-version flag produces
an in-place checkout.  In-place checkout is the default behavior,
because that is required to support auto-versioning for 
non-versioning clients.

    - It's the wrong thing to always show non-versioning clients the root
   version.  They will never have the opportunity to see the latest version.
That of course depends on what your client wants to show non-versioning
clients, but I would agree that a client sometimes wants to show
non-versioning clients the state of a non-root version.

   From: John Hall [mailto:johnhall@evergo.net]

   The modifications are intended to:
   1) Allow a client to recover if the auto-update fails.  By clearing the
   auto-update value, a second CHECKIN would work just like it does now,
   and the client would have to followup with a UPDATE or MERGE.

A client can do this by creating a non-auto-update working resource by
checking out the appropriate version, copying the auto-update working
resource to the new working resource, and then checking in the
non-auto-update working resource.  I believe keeping the auto-update
property protected (i.e. not modifiable by a client) is more important
than saving a round trip in this recovery use case.

   2) My server would never fail the auto-update, because I don't allow
   forking.

That is of course fine.

   3) The new precondition is designed to let a client know that a server
   ONLY supports WORKING-RESOURCES with the auto-update feature.

This can already be made clear by having CHECKOUT not appear in the
DAV:supported-method-set of a version resource, and having a 405
response if CHECKOUT is applied to a version resource.

Cheers,
Geoff
Received on Saturday, 14 July 2001 10:20:36 GMT

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