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Re: 'PUT' transaction reconsidered (was Re: two-phase send concerns )

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Mon, 11 Dec 95 10:33:15 PST
Message-Id: <9512111833.AA14231@acetes.pa.dec.com>
To: Larry Masinter <masinter@parc.xerox.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
    I'm becoming more and more convenced that single-exchange PUT is
    unworkable. That is, PUT might have to happen in two steps:
    
    1. client: wanna PUT this file of this size of this media type 
	       in this location with these PERMS with
	       this validator 
    2. server: OK, send file (or permission denied)
    3. client: OK, here's the file.
    4. server: OK, got the file

Two questions:

(1) What is the purpose of the validator in this example?  If you
mean "cache validator", then presumably this is a token that the
client has obtained from the server which can be used to decide
if the client's cached version of the file is still valid.  This
might be useful if we want to ensure that some other client doesn't
come along midway through this exchange (i.e., between steps 2 and
3) and update the same file.  But if we want to make this work,
then we need to be honest about supporting atomic transactions.

For example, in this case if the server "locks" the file against
updates from other clients, what happens if the client then crashes
before step 3?  How long does the lock persist?

If we are going to introduce transaction semantics, let's do that
directly, not with some half-way approach.

(2) Someone else (sorry, I forgot who) has already pointed out
that this adds at least one RTT of latency.  As with my comments
on the two-phase send mechanism, I'd ask: what is the common
case, and can we optimize for it?

I would assume that the common case is that the server accepts
the PUT (otherwise, people would soon stop doing it).  So to me
this implies that the protocol should avoid extra RTTs if the
PUT succeeds, even if that means a certain amount of messy
state reconstruction if the PUT fails.  I think this is another
argument in favor of the "optimistic two-phase" approach that
I outlined last week: try doing the PUT in a single step, and
then if it fails for any reason (e.g., the server closes the
connection prematurely), try again with the cautious two-phase
method.

Of course, a client that manages to read the server's error
code before the first connection is aborted can optimize
out the second attempt, if the error code is definitive
(e.g., permission denied).

-Jeff
Received on Monday, 11 December 1995 10:44:01 EST

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