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Re: pipelining vs. deferred content

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 27 Mar 97 13:57:40 PST
Message-Id: <9703272157.AA29930@acetes.pa.dec.com>
To: http-wg@cuckoo.hpl.hp.com
Regarding the issue of how one might defer the retrieval of
an inlined image that will require a long time to generate
(at the server), Ben Laurie <ben@gonzo.ben.algroup.co.uk> writes:

    Isn't this problem effectively solved by multiplexing, which is on
    the agenda for HTTP-NG?

    I agree that we are unlikely to solve it before DS time! I also
    shudder to think of how we'd have to restructure Apache to handle
    it.

    We are planning changes to Apache to be able to do this kind of
    thing in V2.0, ETA sometime next century at the current rate of
    progress.
    
Given the time already spent on HTTP/1.1, I would also not
want to count on deployment of HTTP-NG before the next century :-).

But this is not really the same as multiplexing, because it's not
an attempt to reorder requests and responses, or to interleave
chunks of different messages.  Multiplexing would solve this problem,
but you don't need anything like the full complexity of multiplexing.

If we ignore the issue of compatibility with HTTP/1.0 clients (just
for the sake of this particular speculation), I don't think the
implementation of my "503 + Retry-After" approach would be so bad.

On the server side, this can be done entirely in CGI.  E.g., suppose
we have a CGI script that generates a perspective view of the
topology of a certain locality.  Suppose that the user can ask
for any variation on perspective (e.g., height, viewing angle,
width of scene, latitude/longitude, and lighting model), so it's
going to take some CPU time to generate the scene.

The first time the CGI script is invoked with a request for the
image, instead of generating the image, it estimates how much time
it would take to generate it.  This is probably not that difficult,
and certainly shouldn't require much computation itself.  Suppose
that this estimate is "10 seconds".  So the CGI script does two
things:
	(1) fires up a background process to generate the image,
	and store it in a private server-side cache.
	(2) returns "503 Service Unavailable" with "Retry-after: 10"
Note that the HTTP server itself is entirely uninvolved in this
decision.

10 seconds later, the client makes the same request, so the server
invokes the same CGI script.  This time, the CGI script checks its
private cache, and finds a fully-computed image that matches the
request.  So it simply spits out "200 OK" and the file.  Again,
no involvement from the HTTP server code.

Unfortunately, if the client doesn't understand "Retry-After",
then it will simply treat the first 503 response as an unrecoverable
error, and will never retry the request.  So I don't think this
exact approach will work in real life, at least not until the
current population of browsers is replaced.

-Jeff
Received on Thursday, 27 March 1997 14:04:20 EST

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