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RE: how to perform undeletion

From: Clemm, Geoff <gclemm@rational.com>
Date: Mon, 18 Mar 2002 13:02:06 -0500
Message-ID: <3906C56A7BD1F54593344C05BD1374B103F8B07E@SUS-MA1IT01>
To: "Ietf-Dav-Versioning@W3. Org" <ietf-dav-versioning@w3.org>
   From: Sohn, Matthias [mailto:matthias.sohn@sap.com]

	 I think when the resource version to be revived in order to
	 undelete the resource has been found the undelete operation can be
	 done in the following way :

	 - check out the collection which shall contain the resource
	 to be undeleted, this yields a working collection which is
	 bound to an activity (since I want to track all changes in
	 activities)

	 - issue VERSION-CONTROL on the resource version contained in
	 the old collection version before the deletion of the
	 resource took place

      A collection version does not contain resource versions, it
      contains a DAV:version-controlled-binding-set property that
      contains a set of name/version-history pairs.

   Sorry it seems that my sentence was too sloppy. 
   - I meant that by using some report provided by my server I can
   find out to which version of the deleted resource the VCR has
   pointed to just before the resource has been deleted.

Yes, you could define such a custom report.

   - Since the resource has been deleted before the undeletion is done
   there is no VCR for the version history of the deleted resource in 
   my workspace. 

Agreed.

   By issueing VERSION-CONTROL on the version I want to put back to
   life.  I thought I can create a new VCR in the checked out
   collection (which belongs to my workspace) which will then point
   again to the same version the old VCR pointed to before it has been
   deleted.

If your server supports workspaces (with checked-out
version-controlled collections), then yes, that is exactly what you
would do.  But if you are using working collections, you need to use a
different set of requests, because working collections do not contain
version-controlled resources, and therefore cannot have members added
to them with the VERSION-CONTROl request.

      Also, you don't issue VERSION-CONTROL requests to members of a
      working collection, because a working collection does not
      contain version-controlled resources (and VERSION-CONTROL
      creates a version-controlled resource).  The VERSION-CONTROL
      request is used to restore resources to checked-out collections
      (i.e. to collections that can be checked-out in place), not to
      working collections.

   I don't want to issue VERSION-CONTROL to a member of the working
   collection but to the resource version I want to revive in order to
   create a new VCR for it.  This version I found using a report.

This VERSION-CONTROL request takes two parameters: the request-URL
(indicating where the new version-controlled resource should be
created), and the version (indicating what should be the
DAV:checked-in version of the new VCR).  So yes, you do indicate
the version, but you also have to indicate where the new VCR should
be located.  And it can't be located in a working collection,
because working collections do not contain VCRs.

   I only want to checkout the target collection in order to allow the
   recreation of the resource (which shall become a member of 
   the checked out collection).

A VCR can be a member of a checked-out version-controlled collection,
but it cannot be a member of a working collection.  So the requests
you need to issue to manipulate working collections will be different
from the requests you need to issue to manipulate checked-out
version-controlled collections.

      ... with working resources, you have to CHECKOUT a collection
      version that identifies the version history of the desired
      version in its DAV:version-controlled-binding-set.  Then you can
      MOVE that version history from that working collection into the
      destination working collection, DELETE the source working
      collection, and then CHECKIN the destination working collection.

   Huh, I feel this partial move is ugly. 

   First this needs 5 requests to do a simple undeletion:
   - two CHECKOUTs (of the source and target collection versions) 
   - (partial) MOVE 
This is a regular MOVE, not a "partial" MOVE.
   - DELETE (or UNCHECKOUT) on the source working collection 
   - and a CHECKIN on the target collection. 

The working group did not believe that this "restore of a deleted
resource" is sufficiently common to merit defining a special request
for the purpose.

   In addition this would require that MOVE is not an atomic
   operation (either done completely or not at all).

There are no "partial" or "non-atomic" MOVEs here.  The only sense
in which the MOVE is partial is that the working collection that is
the source of the MOVE is deleted following the MOVE. 

   If MOVE is
   not atomic I expect that the probability of inconsistencies 
   on the server will increase if some client issues a similar 
   partial MOVE for a different purpose. Since we use a stateless
   protocol it's no option for the server to wait for the DELETE
   and CHECKIN until the transaction doing the MOVE is committed.
   Without waiting for DELETE and CHECKIN the server has no chance
   to find out that this request sequence is an undelete operation 
   (which should be atomic as well).

I see no reason for the "undelete" operation to be atomic.  To the
contrary, a user is likely to have several adjustments they want to
make to the collection before committing the changes via a "CHECKIN".

   I would prefer if there is some solution which only needs CHECKOUT 
   and CHECKIN of the target collection (this seems to be necessary 
   with versioned collections) and only one atomic command (e.g. UNDELETE 
   or VERSION-CONTROL with additional semantics) which does recreate 
   the deleted VCR and ensures that the same version is revived which 
   has been in the workspace just before the resource has been deleted. 

The version that appeared in the workspace at the time the VCR was
deleted is not necessarily an especially interesting version, since
someone in a different workspace (that hasn't seen the deletion yet)
might have produced a variety of more interesting successors to that
version.  We discussed whether or not to require that a particular
version be restored, and the consensus was that this should be left
up to the server.  So your server is welcome to restore the version
that was visible before it was deleted, but clients cannot assume
that to be the case (nor can the assume that a server will keep track
of that version).

   This would mean 3 requests instead of 5,

This was not considered a sufficiently common use case to warrant
concern over an additional couple of round trips.

   less chance for inconsistencies (since MOVE is atomic)

Only normal (atomic) MOVE semantics are used here.

   and clearer semantics for the sake of an additional method (or
   additional semantics for VERSION-CONTROL ?).

It is highly desireable for a particular request to have
consistent semantics.  In the case of VERSION-CONTROL, one of the
postconditions is that a version-controlled resource exist at the
request URL when the request succeeds.  We could have made that
postcondition context sensitive (i.e. creates a VCR normally, but
creates a version history in a working collection), but this use case
was not considered sufficiently common to merit introducing a new
method or increasing the complexity of VERSION-CONTROL, just to avoid
a couple of round trips for an infrequent use case.

Cheers,
Geoff
Received on Monday, 18 March 2002 13:02:40 GMT

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