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multi-host virtual sites for HTTP 1.2

From: Paul Leach <paulle@microsoft.com>
Date: Tue, 9 Jul 1996 14:55:36 -0700
Message-Id: <c=US%a=_%p=msft%l=RED-77-MSG-960709215536Z-17122@abash1.microsoft.com>
To: "'http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com'" <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>
With the Host: header in HTTP/1.1 it is possible to efficiently and
easily make a single host act as many "virtual" Web sites.

What I'd like to see in HTTP/1.2 is the ability to easily and
efficiently make multiple hosts act like a single virtual Web site, so
that one can build huge web sites.

For example,
	http://www.megasoft.com/
could be the root of the whole, HUGE, publically accessible corporate
web of the MegaSoft Corp. The mapping of this corporate web onto
physical Web Servers should be flixible and mutable over time -- as the
site grows, more servers need to be added; and as the content changes,
the load on various pieces will change too, necessitating rebalancing
the content across the servers, even if more aren't needed.

It is _almost_ possible to do this in HTTP/1.1 today. The 302 (Moved
Temporarily) response can be used to automatically redirect someone who
did a GET on
	http://www.megasoft.com/very/long/resource/path/because/its/a/big/site.
html
to
	http://server42.megasoft.com/its/a/big/site.html
by including a Location: header with that URL, where, say, server42 is
set up to handle all requests that start with
	http://www.megasoft.com/very/long/path/because/
(said setup being a server implementation issue, not a protocol issue)
so that the GET is transparently redirected to server42.

I said "almost" above, because the 302 is only allowed by the 1.1 spec
to work automatically for GET and HEAD requests "since this might change
the conditions under which the request was issued". For other requests,
the user has to be prompted first. In order to make a multi-host virtual
site, this needs to be automatic for all requests.

Another factor that makes it an "almost" is that even if 302 was alow to
work automatically for all requests, it wouldn't be efficient enough to
scale well -- every request would come in to www.megasoft.com's server
to be redirected. This would both load www.megasoft.com, and cause the
client to suffer an extra round trip. (The 302s can have an Expires: to
allow them to be cached, and that would help, but there would be an
awful lot of them.) The way to make it efficient is to somehow note that
the 302 applies to _all_ the resources with a particular prefix, and
only cache this one thing.

A third factor is that, in order to scale, it is sometimes necessary to
replicate the content on multiple servers. However, 302 responses can
only contain one URI (and hence server) in the Location: header returned
to specify where the redirect is to go. (Arguably, there is a way to do
this with the DNS, so I'm flexible on this.)

I would like to add an option response header that says that it is OK to
redirect all requests with a certain prefix of the Request-URI to a list
of other places. (It's a new response header instead of a new status
code for backwards compatibility. Old clients will just ignore it and
prompt on methods other than GET and HEAD).

A possible syntax:
	Referral	= "Referral" ":" prefix 1#referral-URI
	prefix  	= absoluteURI
	referral	= absoluteURI

The prefix has to be a prefix of the original Request-URI -- this
prevents spoofing.
The response to this is to take everything after the prefix in the
original requestURI, append it to one of the referrals to get a new URI,
and retry the request using the new URI. If there is a Cache-Control or
Expires header, then it can be cached, and used for subsequent requests
for resources that have the same prefix -- thus getting the client
directly to the correct server without an extra round trip.

This proposal is nothing more than the application to HTTP of the tried
and true referral mechanisms of distributed directory and file systems.

Comments?
----------------------------------------------------
Paul J. Leach            Email: paulle@microsoft.com
Microsoft                Phone: 1-206-882-8080
1 Microsoft Way          Fax:   1-206-936-7329
Redmond, WA 98052
Received on Tuesday, 9 July 1996 15:36:34 EDT

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