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From J David <j.david.li...@gmail.com>
Subject Migrating from squid
Date Fri, 27 Feb 2015 05:51:30 GMT
(Not sure if this is best for the user list or the dev list, as it's
development, but not of ATS per se.)

Squid offers a feature called url_rewrite_program that can be used to
change its behavior based on client IP, requested URL, and various
other criteria.  It runs a bunch of copies of an external program and
passes information to and from them over pipes.

Overall, ATS is a much better fit for our environment, but we are
heavily dependent on this feature of Squid.

The short version of what we need to do is this:

Based on a (ClientIP,RequestURL) pair, either choose a backend (by
name or IP) to handle the request *or* issue an arbitrary redirect
*or* return an HTTP error.

According to the "squid config translation" docs, the equivalent for
url_rewrite_program is remap.config.  Since that does not actually
support external programs, it appears the intent is to use the
built-in remapping abilities to replicate the functionality of the
external program.  However, that is not always possible.

The messy part that prevents us from doing that or hacking up a module
is that the external program we use with squid is ridiculously
complicated, pulling together information from multiple sources, all
of them dynamic, and gets linked to all kinds of libraries like MySQL,
memcached, and distributed message passing stuff.

What is the best way to migrate this functionality to ATS?  The whole
big ball of wax seems like a really poor candidate for a built-in ATS
module due to all the external dependencies, many of which run
asynchronously and can block.  It seems really advantageous to seal
that off So what we probably really need is a pretty efficient way to
emulate squid's ability to call out to an external program.

If that's true, has anyone done something similar?  Which ATS module
hook(s) would be the best to use, and how would we handle the need to
yield while we wait for the external program to work?

Thanks for any advice!

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