From: <>
Date: Fri, 17 Jan 97 16:48:18 +0000

> I am confused about the httpd accel-mode that Squid offers. Is this
> mode for caching incoming http requests or outgoing requests? Does
> it enhance the performance of http requests orginating from the site
> or http requests to the site? I only want to set up Squid to cache
> web material fetched from other sites (by outgoing requests).

I have occasionally had trouble explaining accelerators and proxy
caches, usually resulting from mixed up interpretations of "incoming"
and "outgoing". I think in terms of requests; i.e. an outgoing request
is from the local site out to the big bad Internet; the data received
in reply is incoming, of course. Others think in terms of "a request
for incoming data", which is the opposite sense.

An accelerator caches which incoming requests for outgoing data,
i.e. that which you publish to the world. It takes load away from your
HTTP server and internal network. You move the server away from port
80 (or whatever your published port is), and subsitute the
accelerator, which then pulls the the HTTP data from the "real" HTTP
server. The outside world sees no difference (apart from an increase
in speed, with luck).

Quite apart from taking the load of a site's normal web server,
accelerators can also sit outside firewalls or other network
bottlenecks and talk to HTTP servers inside, reducing traffic across
the bottleneck (persistent tcp connections?) and simplifying the
configuration. Two or more accelerators communicating via ICP can
increase the speed and resilience of a web service to any single

The Squid redirector can make one acclerator act as single front end
for multiple servers. If you need to move parts of your filesystem
from one server to another, or if separately administered HTTP servers
should logically appear under a single URL hierarchy, the accelerator
makes the right thing happen. We're in the process of a major
filesystem move, which will be greatly helped by this; we're also
looking at authenticated connections, POSTs and various other changes
to HTTP server functions which can be piloted and developed separately
from the main server by hiding them behind an accelerator.

If you wish only cache the "rest of the world" to improve local users
browsing performance, then accelerator mode is irrelevant. Sites
which own and publish a URL hierarchy use an accelerators to improve
other sites' access to it. Sites wishing to improve their local user's
access to other site's URLs use proxy caches. Many sites, like us, do
both and hence run both.

Peter Lister Email:
Computer Centre, Cranfield University Voice: +44 1234 754200 ext 2828
Cranfield, Bedfordshire MK43 0AL UK Fax: +44 1234 751814
     (1) "Yes" (2) "No" (3) "That would be an ecumenical matter"
Received on Fri Jan 17 1997 - 09:07:46 MST

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