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RE: Submitting lock tokens without a validity check

From: Clemm, Geoff <gclemm@rational.com>
Date: Thu, 21 Nov 2002 13:37:15 -0500
Message-ID: <E4F2D33B98DF7E4880884B9F0E6FDEE25ED569@SUS-MA1IT01>
To: "'Webdav WG'" <w3c-dist-auth@w3c.org>
The semantics of locks guarantee that a lock never "moves",
so the client can simply store and submit the URL and the lock-token
as a pair, so there is no compelling
reason to have a "*" operator for lock token URLs.  In addition,
the uniqueness of an etag is only guaranteed for a particular
URL, so the "*" operator cannot be used for etag clauses of an
If header.

Cheers,
Geoff  

-----Original Message-----
From: Lisa Dusseault [mailto:lisa@xythos.com]
Sent: Thursday, November 21, 2002 9:34 AM
To: 'Clemm, Geoff'; 'Webdav WG'
Subject: RE: Submitting lock tokens without a validity check


If a client uses "If: urlA (tokenA) (Not tokenA)" then the client will
*not* find out that their lock is invalid.   So that's not a real
advantage of the existing header depending on how the client chooses to
use it.  Since the client could still choose to use the If header if a
new header were defined, the two proposals you outline are equivalent in
that regard.

There is, however, another way to solve this that does have the property
of consistently letting the client know when it has bad tokens: extend
the existing If header to use * to indicate "any resource".  This is
consistent with If-Match and If-None-Match (HTTP 1.1) which use * to
indicate "any token".

To provide a token in this manner the client would use the * for the URL
with the lock token they wanted to use:

    If: * (<lock-token-A>)

Or several in one header:
    If: * (<lock-token-A>) * (<lock-token-B>) <urlC> (<lock-token-C>)

The way the server evaluates this: if <lock-token-A> matches any
resource, AND <lock-token-B> matches any resource, AND <lock-token-C>
matches the resource <urlC>, then the request succeeds.

This gives the advantage (not found with the two proposals discussed
previously), of letting the server return an error if the lock token is
invalid.  The client can use the lock token more easily as long as it's
valid, and the client gets its state corrected if its token is invalid.

--- 

Side note on header length: this is an improvement over the If-OR-NOT-If
syntax in length terms (it has one fewer lock token and a
single-character URL).  However, we might still want to add commas to
the If header.

Side note on backward compatibility: Adding commas and a '*' syntax to
the if header (assuming no OPTIONS flag) would cause some
interoperability issues.  However, it shouldn't last too long (I think
servers can pick this up rather quickly in particular) and is fairly
easy to deal with.  If a client sends the */, syntax to a server that
doesn't yet support it, a 400 Bad Request is the most likely response,
so I expect for a few years clients that want to use the */, syntax will
see this and fail back to the older syntax.  Servers should be able to
parse the If header with or without tokens, and the addition of the '*'
URL is simply an extension of the syntax that doesn't affect how the
existing URL stuff works.  (And of course, we could also add a "I
support rfc2518 bis" flag to the OPTIONS response to prevent the error
roundtrip for the case of new client --> old server.)

Side note on how I came up with "four times" larger: Assuming the new
header requires one copy of the token and zero URLs, and the
If-OR-NOT-If solution requires two copies of the token and one URL (per
resource), I should have said "three times" larger.  Just an off by one
error.  But in my experience URLs are twice as long as lock tokens so
maybe in practice it is four times larger :)

Lisa


-----Original Message-----
From: Clemm, Geoff [mailto:gclemm@rational.com] 
Sent: Wednesday, November 20, 2002 7:52 PM
To: 'Webdav WG'
Subject: Submitting lock tokens without a validity check

One of the topics discussed at this weeks WebDAV working group meeting
was how to provide a mechanism that would allow a client to submit a
set of lock tokens without a validity check (i.e. the request could
succeed even if some or all of those lock tokens have expired).
Note that a client needs to submit an If header with etags with such
a request, to avoid lock protection.
There are currently two alternative proposals for this (the semantics
of these two proposals are identical, so this is a marshalling
question):
Proposal One: Extend the If header so that it can take a comma
separated list of arguments (and therefore can be split into multiple
If statements).  To submit a set of lock tokens without a validity
check, the following pattern would be used:
  If: urlA (tokenA [etagA]) (Not tokenA [etagA])
  If: urlB (tokenB [etagA]) (Not tokenB [etagB])
  ...
Proposal Two: Add a new header for a comma separated list of lock
tokens that indicate possession of the lock token but do not cause the
request to fail if they are invalid (I neglected to write down the
proposed name, so I'll just call it New-Header).  Since the etag list
can be long when the client holds a large number of locks, the
extension defined in alternative one is also required, to handle the
possibly large number of etags.  The pattern of usage for this
proposal would be:
  New-Header: tokenA
  If: urlA ([etagA])
  New-Header: tokenB
  If: urlB ([etagB])
  ...

Advantage of proposal 1:
- It does not require defining an extra header.
Advantage of proposal 2:
- It requires 40% fewer strings per resource (3 non-constant strings
instead of 5 non-constant strings).  Lisa: You calculated that
proposal one requires four times as many non-constant strings ... how
did you get that number?

I believe that it is not appropriate to add a new header to the protocol
just to decrease the header length for this particular use case by 40%.
I am particularly disinclined to optimize this kind of request,
because I believe that it is significantly simpler for a client to
use a standard If header, and if locks have expired, the request
fails, the client deletes from its state those expired locks, and then
resubmits the request, replacing the expired locks with etags.  This
allows the client to just issue very simple If header requests,
i.e. if the lock token for urlA is still valid but the lock token for
urlB has expired:
If: urlA (tokenA)
If: urlB ([etagB])
Cheers,
Geoff
Received on Thursday, 21 November 2002 13:38:13 GMT

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