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Re: Variant IDs

From: Paul Leach <paulle@microsoft.com>
Date: Mon, 11 Mar 96 11:07:01 PST
To: fielding@avron.ICS.UCI.EDU
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: red-16-msg960311190017MTP[01.52.00]000000b1-139684

----------
] From: "Roy T. Fielding"  <fielding@avron.ICS.UCI.EDU>
] Date: Sunday, March 10, 1996 8:51PM
]
] Given that a Variant ID also needs a Validator in order to be useful,
] wouldn't it make more sense just to require that the validator contain
] the equivalent of a Variant ID when variants are present?
]
] Combined with my other suggestion (to make validators a change indicator),
] we could considerably reduce the number of additional header fields
] sent on these requests and responses.
]
] In other words, this is the same as my IF-ID/Unless-ID/Content-ID
] proposal but without the "world uniqueness" and usage of the <mailbox>
] format.

Good point. The one thing that I think still differs is that lost is 
the ability for the cache to know that several things with the same URI 
are variants of one another, as opposed to just a single instance that 
has changed over time. With IF-ID/Unless-ID/Content-ID:
	Client			Server
	GET /foo.txt HTTP/1.1
				HTTP/1.0 200 OK
				Content-ID: "asdasfa"
	GET /foo.txt HTTP/1.1
				HTTP/1.0 200 OK
				Content-ID: "xxxxx"

How does the cache know if the two "foo.txt" are variants of one 
another, or if the one-and-only "foo.txt" has changed? With Variant-ID, 
it would be clear:
	Client			Server
	GET /foo.txt HTTP/1.1
				HTTP/1.0 200 OK
				Cache-Validator: "asdasfa"
				Variant-ID: 1
	GET /foo.txt HTTP/1.1
				HTTP/1.0 200 OK
				Cache-Validator: "xxxx"
				Variant-ID: 2

I think that you're right that we can condense the number of headers. 
One suggestion: add an "id=" parameter to Content-ID, and allow 
multiple validator/id pairs on IF-ID and Unless-ID. Then the above would be:
	Client			Server
	GET /foo.txt HTTP/1.1
				HTTP/1.0 200 OK
				Content-ID:	"asdasfa";id=1
	GET /foo.txt HTTP/1.1
				HTTP/1.0 200 OK
				Content-ID:	"xxxxx";id=2
	GET /foo.txt HTTP/1.1
	IF-ID: "asdasfa";id=1,
	 "xxxxx";id=2

I thought that Content-ID was aligned with some other MIME standard 
header, though, so this syntax may not be acceptable for that reason -- 
but any other spelling would be OK.

Paul
Received on Monday, 11 March 1996 11:06:07 EST

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