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Re: Caching multipart data

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Fri, 25 Oct 96 15:03:39 MDT
Message-Id: <9610252203.AA24974@acetes.pa.dec.com>
To: "Gregory J. Woodhouse" <gjw@wnetc.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
    It occurs to me that under many circumstances the entity carried by
    an HTTP message could consist of a mixture of very static
    information and highly volatile data. For example, if HTTP is used
    to retrieve a database record, many fields included in the reply
    will be very stable (e.g., name, address, hair color, etc.) and
    other fields will be quite volatile, possibly changing daily or
    hourly. It makes sense to bundle such a message in MIME format with
    the stable fields grouped together (albeit with a strong validator)
    and the less stable fields in related groups. The idea is that a
    cache that could handle the parts of a multipart MIME message
    separately would be able to validate a message with considerably
    less overhead. In fact, it would be possible to have data that
    should not be cached at all (such as a stock price, humidity, heart
    rate, etc.) retrieved from the origin server with each request, but
    information that can be cached can simply be revalidated.

This is a good point.  The current HTTP design does not have a way
to provide meta-data (i.e., caching-related headers) for units
with finer grain than an entire HTTP response message, and so it
might be quite difficult to create a compatible extension that
allowed cache validation of individual pieces of a Multipart response.

However, if you view the problem more abstractly, what you are really
asking for is a kind of data-compression mechanism.  I.e., your actual
goal is not really individual validation of the pieces, but rather to
prevent the unnecessary transfer of the stable pieces when the unstable
pieces change.

When one takes that (more abstract) view, it can be applied to all
sorts of resources, not just multipart ones.  For example, imagine
a one-piece HTML file showing a lot of information about a company,
including its current stock price (which changes frequently).  If
we could somehow arrange to transfer just the stock price info
on an update, and rely on the cache for the rest, then we could
save a lot of bits.

This is basically "delta-encoding": saving time by transmitting
the "delta" (difference) between two successive data elements,
rather than transmitting each in its entirety.  And, in fact,
several research projects have already been looking at this
possibility.  For example, there was a brief mention buried in
        Removal Policies in Network Caches for World-Wide Web Documents,
        S. Williams, M. Abrams, C.R. Standridge, G. Abdulla, and E.A. Fox
	(Virginia Tech) Proc. SIGCOMM '96 (August, 1996)
and there will be a paper with a related approach in the forthcoming
USENIX conference (Jan. 1997):
	Optimistic Deltas for WWW Latency Reduction
	Gaurav Banga, Fred Douglis, and Michael Rabinovich, AT&T Research 

There's also a paper at Mobicom that (from its title) might be
related, but I haven't seen a copy yet:
  WebExpress: A System for Optimizing Web Browsing in a Wireless Environment
  B.C. House and D.B. Lindquist, IBM Corporation

Although the basic concept is pretty simple, there are a lot of
really hard research problems to solve.  For example, what is
the best way to compute and encode the difference between two
instances of a resource?  This probably varies based on Content-type!
And how many different "base versions" should the servers and proxy
caches keep around, and for how long?  And how do the client and servers
communicate the necessary meta-information?

And, finally, how well would this actually work in practice?  Or
do most changeable documents change enough that delta-encoding doesn't
save anything?

I've been talking with the AT&T folks and the Virginia Tech folks
about capturing a day or so's worth of the content that flows through
Digital's proxy server (we're up to about 1.5 million requests on
a good day), and then trying to compute deltas based on various
algorithms.  But this is going to require some hacking on our proxy
code, and a fair amount of disk space, so I haven't had a chance
to get started on it.

-Jeff
Received on Friday, 25 October 1996 15:14:51 EDT

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