Skalierbarkeit und "Ruby on Rails"

Twitter ist eine stark wachsende Web 2.0-Anwendung, die mit Rails entwickelt wurde.

Ich möchte mich hier gar nicht mit dem Sinn und Unsinn von Twitter beschäftigen, sondern ich möchte mich auf ein aufschlussreiches Interview mit dem Twitter-Entwickler Alex Payne verweisen in dem es auch um die Skalierbarkeit von Rails geht.
In dem Interview heisst es:

Twitter is the biggest Rails site on the net right now. Running on Rails has forced us to deal with scaling issues - issues that any growing site eventually contends with - far sooner than I think we would on another framework.

The common wisdom in the Rails community at this time is that scaling Rails is a matter of cost: just throw more CPUs at it. The problem is that more instances of Rails (running as part of a Mongrel cluster, in our case) means more requests to your database. At this point in time there’s no facility in Rails to talk to more than one database at a time.

The solutions to this are caching the hell out of everything and setting up multiple read-only slave databases, neither of which are quick fixes to implement. So it’s not just cost, it’s time, and time is that much more precious when people can[’t] reach your site.

None of these scaling approaches are as fun and easy as developing for Rails. All the convenience methods and syntactical sugar that makes Rails such a pleasure for coders ends up being absolutely punishing, performance-wise. Once you hit a certain threshold of traffic, either you need to strip out all the costly neat stuff that Rails does for you (RJS, ActiveRecord, ActiveSupport, etc.) or move the slow parts of your application out of Rails, or both.

Dies hat mit der (Grundsatz-)Entscheidung des Ruby Entwicklers David Heinemeier zu tun, sich um Skalierbarkeit und Performance wenig Gedanken zu machen. Er gibt Entwicklern den Rat "Don’t scale".

Ein (in der extremen Form wie ihn David Heinemeier gibt) riskanter Rat wie Greg Linden in diesem Posting von 2005 argumentiert.