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Implementing Caching
It's obvious that the proxy server can check its local cache to see
if a requested object has recently been fetched. It is slightly
more subtle to discover if it's actually the same
object. HTTP/1.1 adds some new response headers to ensure
that caching works correctly:
Expires: Tue, 20 Mar 2001 02:22:52 GMT
Pragma: no-cache- An object can be marked as having a limited lifetime, and once
the specified date/time has passed must be re-fetched from the
originating server. Also, an object can be flagged as
"un-cachable". These were both present in HTTP/1.0.
Etag: "8802-2c72-3ab178fc"
- This is the "Entity Tag", and is used to discover, with
somewhat greater certainty, if the object (or entity) in the local
cache is exactly the same object (eg, isn't
different in any way) as the object stored on the remote server.
The client can use an
If-None-Match: "8802-2c72-3ab178fc"
header with a GET request to specify the version of the object
which it already has. This is a significant improvement over the
HTTP/1.0 "Conditional-GET". Note that HTTP/1.1 has a large number
of other operations which can be used with Entity Tags.
You can discover lots more about HTTP at: http://www.w3.org/pub/WWW/Protocols/Specs.html
The tutorial for this lecture is Tutorial
#07.
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Copyright © 2001 by
Philip Scott, La Trobe
University.
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