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Re: Fewer new versioning methods, please!

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Wed, 4 Oct 2000 23:37:53 -0400 (EDT)
Message-Id: <200010050337.XAA08773@tantalum.atria.com>
To: ietf-dav-versioning@w3.org

Note:  I'll omit issues that have already been addressed in previous
postings.


   From: "Lisa Dusseault" <lisa@xythos.com>

   General principles:
    - New methods require new code.  Old methods may allow reuse of more,
   perhaps all code, either on the client or the server or both.

New methods do not necessarily require much new code.  They just
require new entry points that are free to call existing code whenever
appropriate.

    - New methods require new specification.  This seems easy, but I find the
   cost to be actually high.  Don't forget, not only must the draft-writers
   define the new method in all its combinatorial glory, but also every
   implementor has to fill in the holes which necessarily exist. 

A new method does require a new specification, but this can be based
on inheritance or delegation (i.e. this is the same as foo but adds
the following postconditions).  This the approach taken by the
advanced versioning extensions to the core versioning methods.

Whether or not the extension has a new method name or not shouldn't
affect this much one way or the other.

    - New methods may require firewall or proxy attention.  What if the
   adminstrator forgets to add MKACTIVITY to the list of methods that their
   firewall allows?

If you care about firewall penetration, then you'll just use CONNECT,
because you can't be sure that even PROPPATCH will make it through.

   The specific commands I'd like to see NOT invented:

   SET-TARGET
   ----------

   Here is a case where I strongly feel that too much extra implementation (and
   specification, and test) effort is required to implement this functionality
   with new methods.  Isn't this an easy one to replace with PROPPATCH?

Not on the server side, because the content and all the dead
properties have to be updated as well, and there is nothing in
existing PROPPATCH that will do this.

   The
   only reason I can imagine for having a new method is so that I can name a
   version-name or label in SET-TARGET, and the server can then turn around and
   put the full version URL in the DAV:target property.  But it doesn't seem
   that versions have URLs any more, or at least I'm confused about whether you
   have version URLs any more.

Yes, versions have URL's.  We just don't use a special name like
"version URL's" for them anymore.  (After all, there had to be
something in all those properties containing href's that refer to versions :-).

   LABEL
   -----

   I would even prefer a new method called "ADDPROPVALUE" or "MVPROPPATCH" or
   "ADDVALUE" that serves to add a new value to any multi-value prop, rather
   than this extremely specialized LABEL method.

Clients need a reliable way of saying "fail if this label is already
set on another version".  They also need a way of saying "remove this
label from another version if its already on that other version".
There's no way of making this important distinction with a simple
PROPPATCH.

   VERSION-CONTROL
   ---------------

   Obviously, the server needs to have special code to deal with the case where
   "isversioned" is changed with a PROPPPATCH: it needs to set a few other
   properties and create a version-history.  The server would have to have
   special code to deal with VERSION-CONTROL, too.

And a new initial version ... this seems like a lot of side effects for
setting a boolean property to "T".

   I'd like to note, however, that you shouldn't require a version-history to
   be created when a resource is made versioned.  There's only one version at
   the time, the "current" one.  The server can easily go create the
   version-history when the second version is created.  This improvement, which
   I recommend anyway, makes the "isversioned" property even easier to
   PROPPATCH.

This does make the model significantly less regular, for little
obvious benefit.

   I know this one is a little difficult, but I have a strong reason to ask for
   it:  current state of DAV client APIs.   I'd like to be able to use an
   existing DAV client library and start do do versioning without a lot of
   extra trouble.  This is mostly possible:  with auto-versioning, I don't need
   to do CHECKOUT and CHECKIN; mostly RFC2518 actions can be used to interact
   with versioned resources.   The more behaviour we allow with PROPPATCH
   instead of SET-TARGET and VERSION-CONTROL, the closer we come to quick and
   easy client implementation.  I may even be able to use a completely
   un-altered DAV client, as long as it allows me to enter in values to set for
   random properties: just turn versioning ON with PROPPATCH, and I've now got
   a version history,  Although it may be difficult for me to navigate to old
   versions immediatley, at least I know they're there.

It doesn't seem likely that a generic PROPPATCH client will make a
very effective versioning client, so it doesn't seem like a
compelling reason to force the protocol through the property mechanism.

   UNCHECKOUT
   ----------

   This is a minor one, but I thought that UNCHECKOUT might as well be modeled
   as a CHECKIN with "cancel-checkout" in its body.  It seems much simpler to
   state requirements then -- "A CHECKOUT must be followed by a number of write
   operations, followed by a CHECKIN".

Actually, I proposed this a while back, and it didn't get a warm
reception.  I didn't really care that much one way or another, so I
just let it drop.


   BASELINE-CONTROL
   ----------------

   Like VERSION-CONTROL, I believe this can be done with PROPPATCH.  First, it
   sets the "DAV:baseline-selector" on the collection.  That's easily done with
   PROPPATCH.

   The other is more complicated, and would require a little rethinking but the
   result is I think an easier model all around.  It requires not having a
   resource that is a "baseline selector".  What is the point of this resource?
   Why isn't the information contained in this resource instead part of the
   property set of the baseline collection itself?  Does a baseline selector
   have a URL?  Can it be deleted?  Who is the owner?  What properties does it
   have?   Where is it located?  How do I delete a baseline -- do I have to
   delete the baseline selector resource AND proppatch the
   "DAV:baseline-selector" property on the collection?  Can the two be
   inconsistent?  What about the period of time inbetween the two actions?

A baseline selector is just a version selector, where the versions happen
to be baselines.  So a baseline selector just has all the properties and
methods of a version selector.  This makes it very simple and regular to model.

   I do feel that MERGE is a completely appropriate new method, doing something
   quite unlike any previous method. Just to prove I know some moderation.  :)

OK, we'll keep MERGE then (:-).

The most vocal advocate of new methods for everything was Chris Kaler
(he even wanted separate methods for querying version information,
and not even use properties for that).  So we seem to have ended up
in a compromise between your position and Chris's position ... new methods
for operations that have side effects beyond setting the value of a
single property, but using properties to determine the current versioning
state.

Cheers,
Geoff
Received on Wednesday, 4 October 2000 23:38:34 GMT

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