When scaling from a single web server to multiple web servers, the typical
practice is to put a load-balancing reverse HTTP proxy in front. This is a
web server that forwards incoming HTTP requests to other internal web
servers and thus distributes the load across all the different HTTP servers, allows
for failover, and all sorts of good things.
However, a simple trick I learned early on is that even if you have
only a single web server, a proxy in front can help out performance
significantly. Through the simple expedient of buffering the communication
with slow web clients, your potentially heavyweight (especially when mod_perl meant that each process was dozens
or even a hundred megabytes apiece)
and/or expensive Apache processes don't have to waste time
serving every request for the entire length of time the client is
connected. This allows you to run vastly fewer Apache processes.
In the past, I've used
pound and
perlbal. Pound is fast
and lightweight, and allows routing based on the HTTP query;
for example, everything under /img/ got routed to a high-speed
thttpd
instead of the Apache itself. Perlbal is much more configurable
but slightly harder to get running, and the documentation was sparse.
These days, I'd also
investigate
nginx and
varnish.
Pen, a generalized TCP load-balancer with server affinity (connections will go to servers they've gone to recently in the past) is also quite interesting but will not help with the slow client problem. Finally, a second
set of apache processes, configured to reverse-proxy via
mod_proxy,
will also do the trick. A
