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Re: Working collections

From: Greg Stein <gstein@lyra.org>
Date: Sat, 18 Nov 2000 15:52:27 -0800
To: "Geoffrey M. Clemm" <geoffrey.clemm@rational.com>
Cc: ietf-dav-versioning@w3.org
Message-ID: <20001118155227.C21426@lyra.org>
On Sat, Nov 18, 2000 at 04:47:56PM -0500, Geoffrey M. Clemm wrote:
>    From: Greg Stein <gstein@lyra.org>
> 
>    We need to be able to do a CHECKOUT on a collection version so that we can
>    perform operations within the collection and on the collection itself.
>    In Subversion, I've been planning to do a CHECKOUT on a parent collection
>    version resource, which will return a location such as:
>        http://host.name/repo/$svn/wrk/100.3
> 
> We should first confirm that we agree on the semantics of a collection
> version and a collection version selector, namely, that each member of
> a collection version is a version history,  while each members of a
> collection version selector is either an unversioned resource or a
> version selector.

Sounds fine to me.

>...
> By analogy with collection versions, the members could be version
> histories.  By analogy with collection version selectors, the members
> could be unversioned resources or other working resources.

... or version selectors.

Let's call the first one, Model A. And the second one Model B.

> Or we
> could make up completely different semantics for a working collection
> that are distinct from those of either collection versions or
> collection version selectors.  I'd like to avoid this third alternative,
> since it is likely to cause additional complexity in what is already
> the most complex part of the protocol.

Okay. I can tentatively agree with that. (read: avoid Model C)

>    I can then do the following:
>        PUT http://host.name/repo/$svn/wrk/100.3/newfile.html
>    or even:
>        COPY http://host.name/repo/somedir/foo.c
>        Destination: http://host.name/repo/$svn/wrk/100.3/foo.c
> 
> If working collections were like version selectors, then this
> would be fine since then working collections could have 
> unversioned members.
> 
> If working collections were like collection versions, then this
> would be illegal since only version histories can be members.

Agreed on both.

>        (or a MOVE if the source's parent is checked out)
> 
> A MOVE is fine, if the MOVE was from another working collection,
> but if the MOVE was from a non-versioned collection, then you
> run into the same problems as PUT.

If the MOVE source is:

1) an unversioned resource: no problem under Model B. Illegal under Model A
   since the source is not a version history.

2) a version selector: same as (1), however the parent collection (if
   versioned) must be checked out or auto-versioned.

3) a working resource: same as (2)

4) a version history: allowed under Model A, assuming the parent collection
   is checked out or auto-versioned. Not allowed under Model B.


Of these, I'd rather take the flexibility of Model B. I don't have to track
down the darn version history just to satisfy a MOVE. I also find it a bit
weird to move a *history*. I'm really trying to move a version selector for
that history. Not to mention the fact that, typically, a version history is
located in some server-controlled namespace; the semantics of that namespace
would probably disallow a version history to be moved from it, or for a
version history to exist outside of it (e.g. under the working resource).

Concretely: Subversion has several server-defined namespaces. One each for
activities, working resources, versions, and histories. The history
resources cannot be moved out of their space. And since a checked-out
collection (whether a checked-out version selector, or a working resource
corresponding to a checked-out selector or version) exists *outside* of the
history namespace, then it is illegal to move a version history.

If any server uses server-defined namespaces for histories, then a MOVE
would be impossible if you tried to enforce "working collections must
contain version histories."

>    The working resource for the collection is also handy for deleting or for
>    replacing the collection:
> 
>        DELETE http://host.name/repo/$svn/wrk/100.3
>        (note that this request also requires a checked-out parent)
> 
> This is fine.

Note that Tim also pointed out that we probably would not delete the working
collection itself, but delete the member from the parent collection. The
above request is ambiguous with "do an UNCHECKOUT".

I'm still a bit woozy on deletion of resources, both in WebDAV and DeltaV.
In WebDAV, you lock and delete the target. In DeltaV, do we checkout the
target to make sure we are allowed to delete it? (e.g. watch out for
reserved checkouts and stuff)

I'm tempted to just say "check out both of them, then delete relative to the
parent collection".  (sigh... but checking out the target is probably
redundant, then) ... Ideas, anyone?

>        COPY http://host.name/repo/somedir/
>        Destination: http://host.name/repo/$svn/wrk/100.3
>        Overwrite: T
> 
> This wouldn't work, because deleting a working resource (as is done by
> an Overwrite:T) cancels the checkout.  You would need to use the new
> "update" value for the Overwrite header.

Nits. You know what I meant, and the server really should, too. This is
where people read the letter of the law too strictly and screw up clients
because the client wasn't as anal as the server.

But for discussion. Sure... let's assume that I used "update" in there.

The point is that I need to check out the collection because I'm about to
replace its contents.

>    DELETE of a member is a CHECKOUT of two items: the thing to DELETE and the
>    parent collection.
> 
> Only the parent collection needs to be checked-out in order to
> DELETE a member of that collection.

Yah, this was my rambling above. What about reserved checkouts and stuff for
the member? e.g. is it possible to just delete the thing without having it
checked out?

>    Without being able to do a CHECKOUT on a collection, there wouldn't be a way
>    to do any of the above.
> 
> You could just use a VERSION-CONTROL request to create a new version
> selector whose target is the desired collection version.  Then you can
> do all your operations on that version selector.  It is one extra round
> trip, but if that really matters, we could easily add a "DAV:keep-checked-out"
> option to the VERSION-CONTROL request.

That feels *way* bogus. Use VERSION-CONTRL? And just how do I get that to
participate within an activity? Or to MERGE it? etc

> This would allow us to avoid
> all the extra complexity that would result from having two ways to manipulate
> versioned collections.

I'm not seeing the complexity. Check out a collection version; you get a
working collection; modify it at will; check it in.

The working collection operates just the same as a checked-out collection
version selector. And this is right: they're both representative of a
checked-out [collection] resource. Seemed pretty obvious to me.

Where's the problem? :-)

Cheers,
-g

-- 
Greg Stein, http://www.lyra.org/
Received on Saturday, 18 November 2000 18:51:22 GMT

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