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Re: Range locking

From: Fabio Vitali <fabio@CS.UniBO.IT>
Date: Fri, 21 Feb 1997 21:12:32 +0000
Message-Id: <v03007803af33b71de800@[130.136.2.202]>
To: w3c-dist-auth@w3.org
I'm slightly amused by this discussion, because it is beginning to prove
what I deeply think (and have never failed to show in this group): that
locking is inherently evil.

Larry Masinter wrote:
>This would justify "section" locking, but not "byte range"
>locking, since in word, powerpoint, HTML, PDF, GIF, JPEG,
>tiff, and most other formats, editing a section of a document
>generally changes all of the byte ranges.

Mark_Day/CAM/Lotus@lotus.com wrote:
>Section locking also has this
>problem, but in a milder form: the problem arises as sections are added and
>removed, which one would expect to be much less common than adding and
>removing bytes.

Yaron Goland wrote:
>It a'int necessarily so. Much like a LOCK is a way of breaking ties
>among users w/equivalent access rights, so range locks help to break
>ties for people trying to edit the same section of a document.

Larry Masinter wrote:
>Document in MS Word, chapter 2 is bytes 30-50 and bytes 80-90.
>
>User A wants to edit chapter 2
>User B wants to edit chapter 1
>User C wants to edit chapter 2
>
>A locks bytes 30-50, bytes 80-90
>B locks bytes 10-20
>B edits chapter 1, adds 20 bytes. Can B check this in?
>Create a new version? Increases the size of the existing
>version?

Byte locking is evil because the byte count may change outside the scope of
the lock, thereby voiding the addresses we hold in the lock. So an
extremely fine-grained lock won't work.

But a middle-grained lock won't work either, as Mark Day hints without
really stressing: we are simply rarefying the troublesome situations, not
removing them: if sections are counted, instead of named, there will be the
situation in which, after I obtained a lock on version 5, someone comes and
creates a new section 3, and my lock is void. BTW, what are sections in
HTML?

So we are coming to the conclusion that locking something based on a
relative address (such as byte count) is bad anyway, because the base on
which addresses are related to may change, thereby voiding the address we
hold.

Relying on names is the only solution we can rely on, because locks on
named entities will exist as long as the *name* is valid, and not the
position. What are the named entities? And how can I be sure that the names
won't be changed while I have a lock on them? Should I provide a different
lock for changing names, which can only be issued when no content lock
exists on the whole entity?

Or should we allow owners of locks on named objects to act on names? And in
this case, how do we handle name clashes? (e.g.: I have a lock on section
3, which is an object named "Section3". Someone comes and adds a new
section between 2 and 3, and would like to call it "Section3" and update
all the names in the following sections. What happens now?)

Middle-grained locking won't work either. Would large-grained lock work?
Not quite. Again, the bad situations rarefy, but never disappear. URLs *can
be* position-dependent, because they most often are placed in a
hierarchical space. With WebDAV we are introducing client-controlled
server-side operations on namespaces, such as MOVE. For instance, what
happens if I have a document lock on http://host.com/dir/file.html, and
someone MOVEs /dir/ to something else?

Should owning a lock on an entity also block all the hierarchically
superior containers? Should a lock on http://host.com/a/b/c/.../z/tiny.html
also imply a lock on a, b, c, ..., and z? And would a lock on /a/ also
block all the hierarchically dependent namespaces b, b1, b2, and c, c1, c2,
etc. ?

The ONLY reliable solution for fine-, middle- or large- grained objects to
work is change-tracking. It must be implemented by the client for the
changes that are performed on the client (e.g. insertions and
modifications), or by the server for the operations that are performed on
the server (e.g. creations, deletions and moves). VTML is an example of
change-tracking language for the client.

At this point keeping locks are just an easy trick not to mess with
multiple change logs and multiple parallel variants of the same entity,
i.e. to avoid having to deal with tree-shaped versions. Which is a
legitimate decision, but adds complexity instead of simplifying the
situation.

Alternatively, you can impose locks only on named entities, and request a
different type of lock for the modification of names, so that changing the
name of a section requires a lock on the entire document, and changing a
directory requires a lock on the entire namespace, and locks can be as
finely-grained as the names you specify.

Fabio




Fabio Vitali                            Tiger got to hunt, bird got to fly,
Dept of Computer Science         Man got to sit and wonder "Why, why, why?'
Univ. of Bologna  ITALY               Tiger got to sleep, bird got to land,
e-mail: fabio@cs.unibo.it            Man got to tell himself he understand.
                                                             Kurt Vonnegut
Received on Friday, 21 February 1997 15:07:05 GMT

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