Why isn't there a viable mod_ruby for Apache yet?

Question

As popular as Ruby and Rails are, it seems like this problem would already be solved. JRuby and mod_rails are all fine and dandy, but why isn't there an Apache mod for just straight Ruby?

Answer 1

There is Phusion Passenger, a robust Apache module that can run Rack applications with minimum configuration. It's becoming appealing to shared hosts, and turning any program into a Rack application is ridiculously easy:

A Rack application is an Ruby object (not a class) that responds to call. It takes exactly one argument, the environment and returns an Array of exactly three values: The status, the headers, and the body.

Answer 2

The basic problem is this: for a long time, MRI was the only feasible Ruby Implementation. MRI has a number of problems that make it hard to embed it into another application (which is basically what mod_ruby does: it embeds MRI in Apache), especially a multi-threaded one (which Apache is). It is not particularly thread-safe and it has quite a bit of global state.

This global state means for example that if one Rails application modifies some class, then all other Rails applications that run on the same Apache server, will also see this modified class.

Another problem is that the MRI source code is not easily hackable. MRI is now more than 15 years old, and it's starting to show.

As a result of these problems, mod_ruby has never really properly worked, and at some point the maintainers simply gave up.

The C based PHP interpreter, on the other hand, was designed from day one to be run as mod_php inside Apache. Indeed, for the first couple of versions, there wasn't even a commandline version of the interpreter, mod_php was the only way to run PHP.

Phusion Passenger (aka mod_rack aka mod_rails) solves this problem by basically giving up and sidestepping the problem: they simply run a seperate copy of MRI in a seperate process for every application. It works great, and not only for Ruby. It supports WSGI (standard interface for Python Web Frameworks), Rack (standard interface for Ruby Web Frameworks) and direct support for Ruby on Rails.

My hopes are on mod_rubinius, which unfortunately doesn't exist yet. Rubinius was designed from the beginning to be thread-safe, embeddable, free of global state, not use the C stack and so on. It was designed to be able to run multiple Rubinius VMs inside one Rubinius process. This makes mod_rubinius infinitely easier to implement and maintain than mod_ruby. Unfortunately, of course, Rubinius is not released yet, and the real work on mod_rubinius cannot even begin until Rubinius is released. The good news is that mod_rubinius already has more manpower behind it than mod_ruby ever had, because it has paid developers working on it by a Rails hosting company that desperately wants to use it themselves.