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Re: Submitting lock tokens without a validity check

From: Stefan Eissing <stefan.eissing@greenbytes.de>
Date: Thu, 21 Nov 2002 16:49:15 +0100
Cc: "'Clemm, Geoff'" <gclemm@rational.com>, "'Webdav WG'" <w3c-dist-auth@w3c.org>
To: "Lisa Dusseault" <lisa@xythos.com>
Message-Id: <C993ADA4-FD68-11D6-BA1D-00039384827E@greenbytes.de>

Lisa,

the use case which started the whole thread is "the client wants the
request to succeed irregardless of wether the lock is still there or 
not".
The addition of "*" to the If header does not help to solve this case.

The introduction of "*" addresses the problem where the client is not
sure for exactly which URIs it must provide lock-tokens in the IF 
header.

That is also a problem worth addressing. Jason mentioned some weeks
ago that 2518 does not clearly define when a lock-token is "submitted"
and how the methods should check for submitted tokens.

As I see it, the IF header is used for two purposes:
P1) resource state checking
P2) submitting "known" lock tokens

P1 is, IMHO, pretty good defined. A client puts the URI of the resource
to check in the IF header. If the URI is omitted, the request URI is 
silently
substituted.
All mentioned resource states are then verified by the server. Should
a state not match (e.g. a lock token no longer there), the server will
fail the request immediatly.

For P2 we should define that server implementations should regard all
lock-tokens supplied in the IF header as "submitted tokens".
When verifying locks on resources affected by methods (such as COPY
or MOVE), servers should for locked resources look into the set of
"submitted tokens". If the lock is there, the request can proceed.
Otherwise it will fail with a 423.

Defining this behaviour will increase interoperability and ease life for
clients. Clients just memorize the lock token together with the URI
the lock was created on. They can then simply place those tuples
in the IF headers of requests and everything should work fine.

More optimizing clients might more carefully chose the locks to
submit, but that is left up to their implementation.

//Stefan

Am Donnerstag, 21.11.02, um 15:33 Uhr (Europe/Berlin) schrieb Lisa 
Dusseault:

>
> 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 10:49:35 GMT

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