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RE: questions -- clarifications requested

From: Paul Leach <paulle@microsoft.com>
Date: Tue, 29 Aug 95 15:47:14 PDT
Message-Id: <9508292336.AA15126@netmail2.microsoft.com>
To: sjk@amazon.com
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com

----------
] From: Shel Kaphan  <sjk@amazon.com>
] To: Paul Leach
] Subject: questions -- clarifications requested
] Date: Tuesday, August 29, 1995 10:25AM
]
] Paul Leach writes:
] 	...
]  >
]  > 9. Is the Expires entity-header field ever returned from POST, PUT,
]  > DELETE, LINK or UNLINK methods? (These are never cached, and the
]  > description implies of Expires implies it is only used for caching 
purposes.)
]  >
] 	...
]
] The *results* of these operations can be cached.  The expires header
] is appropriate.  Requests for these methods must go all the way
] through to the origin server, and cannot be served from a cache
] themselves.  The results can be identified with the Location header in
] such a way that a subsequent GET or HEAD can retrieve *its* results
] from the cached resource left in the cache by the POST, PUT, etc.

This wasn't the answer I was expecting, so let me explain why I asked 
the question.

This answer seems inconsistent with the description of the nature of 
the entity-bodies returned from PUT, DELETE, LINK, UNLINK:

(Sec. 6.2.2) "an entity describing the result of the action"

From this I concluded that it *isn't* the content associated with the 
URI, so that a subsequent GET shouldn't return the _entity_ returned 
from these methods. The entity-headers are associated with the resource 
named by the URI, so they could be cached (or the entity-headers 
associated with an already cached copy updated). This won't do much 
good if the entity itself isn't cached, though, as the next GET will 
still miss and force a trip to the origin server.

In the case of DELETE, it would seem logical that any cached info 
should be purged if the request succeeds :-)

In the case of PUT, any previous cached copy would be invalid, so an 
Expires: for the new copy isn't very interesting. And since there's no 
guarantee that a subsequent GET will retrieve what the PUT just wrote 
(at least, I don't recall seeing one on the spec), the entity-body that 
was in the PUT request can't be cached to service subsequent GETs. 
(Adding an indication, in HTTP 1.2, to the response to say that a GET 
of what was just PUT will return the same thing might be a nice 
idea...) So, for now, it seems like a PUT should also cause any cached 
copy to be deleted.

In the case of LINK and UNLINK, if the cache already contained an 
unexpired copy of the entity referred to by the URI, then it is 
plausible it could update the Expires (and other meta-data) on that 
copy if the Last-Modified indicated by the respose hasn't changed.

IMHO, this complexity isn't worth it.  Just say that Expires isn't 
allowed in responses from PUT, DELETE, LINK, and UNLINK.

For POST, the situation is more complex, since it says that the entity 
returned is
	"an entity describing or containing the result of the action"
If there were a way to determine that it contained the result of the 
action, and some guarantee that a subsequent GET (of the URI specified 
by the Location header in the response) would fetch this entity, then 
your argument would hold. However, since neither of the predicates are 
true, it seems that the conclusion doesn't follow.

So, that's why I posited that Expires shouldn't be returned from POST, 
PUT, DELETE, LINK, and UNLINK.

And, having thought about it to respond to Shel, I also conclude that 
POST, PUT, and DELETE should be specified to delete any cached copies 
(if successful), and that the results from all these methods should 
*not* be cached.

Paul
Received on Tuesday, 29 August 1995 15:48:14 EDT

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