[mrtron]

My guess is this question is coming from a Ruby developer.

[mrevelle]

No, but the dependency on "horizontal scaling" for medium-traffic web apps is more common due to Rails and one reason I ask.

[mrtron]

I didn't mean it in a negative sense.

Indeed, but horizontal scaling has been common before Rails and will be common after Rails, since most web apps are stateless and transaction based. So, even though Apache and mod_blah only runs as one process, you can still horizontally scale. With rails it is apparently common to have more than one process per server.