using opaque strings to determine uniqueness

Brian Behlendorf writes:
 > On Tue, 14 Nov 1995, Laurent Demailly wrote:
 > > I agree that there should be some *opaque* string used to select if the
 > > object is the same or not (string which could be for instance a last
 > > modified date, an MD5 digest,... whatever the server wants)
 > While this sounds good in theory, I believe there are situations where 
 > this breaks down. 
 > Time    Action
 > T+0:    client A connects through to a server which 
 >         contains a document which changes hourly, and has a CID of "X".
 > T+1h:   client B connects through to the same server,
 >         and gets the hourly-changing document which now has a CID of "Y".
 > T+1h1m: client B "refreshes" the page by doing an IMS request, but this
 >         time goes through, either because of round-robin
 >         DNS or some sort of client load-balancing[*].
 >         client B says "send me the document, unless it has the CID of 'Y'"
 > sees its CID is "X", not "Y", and sends the OLD

Good point, let's think it through.

The rule for a proxy performing this kind of comparison must be different
from the current rule for GET if-modified-since.

In the case of GET if-modified-since, the proxy is allowed to service
the request out of the cache if the last-modified date of its
(non-expired) copy of the document is after the request's if-modified-since
date, otherwise it has to pass on the request.  It can return 304 if
it holds a non-expired document that was modified <= the request's
if-modified-since time.

In the case of an opaque string match, let's consider two versions of
a document, served by the server at times T and T+n.  At T, the
server provides a document with an opaque string "X".  At time T+n
the server begins serving a new version of that document with opaque
string "Y".

A proxy can only service the request for `unless "X"' out of its cache,
when it contains a copy of the document, under these conditions:

(a) If the proxy holds no document matching "X":

If the proxy has records of having received the document version "X"
and knows that it received its version, "Y", more recently than it
received version "X", it can return "Y".

If it is unaware of version "X", it cannot serve version
"Y" out of its cache, even if it turns out that "Y" is newer than "X".
*** This is the condition under which this scheme differs most from
the if-modified-since scheme ***.

(b) If the proxy contains a cached copy matching "X":

If the proxy has received a version of the document more recently than
it received one with "X", it returns the more recently received one.

If it hasn't received a copy more recently, the proxy can follow the
same rules as GET if-modified-since, using the expiration date to
decide if it must forward the request, or can return a 304.

So using opaque strings results in some more cache misses when proxies
have only a sparse history of the versions of a document.  The
additional "hair" in proxies is that to make this work optimally, they
have to keep a history of the versions of a document they have
received, and when they have received them.

I'm sure if I made an error in this, someone will point it out...


Received on Tuesday, 14 November 1995 12:39:36 UTC