Note: This question is heavily affected by the main requirement to the web application that I build: high availability and fault tolerance. All the other requirements (like scalability and number of users) is not in question here.
I have got and advice from one of the members of this community to use an Erlang web-server as a back-end for my web application.
The suggestion was that I could use something like Mochiweb as a backend and Django/Ruby on Rails as a front end using JSON and the Service Oriented Model.
The only obvious advantage of this approach that I can understand is that the development of the front-end part is 'as usual' - regular MVC stuff, Ruby on Rails or any other common framework of someone's choice.
But what about other advantages? Do they actually exist?
Sure, Erlang/OTP adds fault-tolerance to the system in question, but doesn't adding a web front-end layer diminish this fault tolerance level to much lower level?
Don't we introduce a 'single point of failure' by coupling Ruby on Rails with Mochiweb? Of course, Mochiweb can cope with faults, but what if something wrong happens on the front-end side?
Technically the Erlang/OTP platform does not do anything about fault-tolerance and high-availability on it's own. It just allows to easily implement concurrent and distributed software. Erlang web server running on a single machine can fail as all other do - just because hardware fails.
So for HA sites it's more important to have proper redundancy and fallback scenarios for cases of hardware and software failures, rather than using any specific software stack or platform. Probably it will just be slightly easier to implement in Erlang compared to other platforms (if you are familiar with it, of course), but absolutely the same results can be achieved using pure Ruby/Python/Java/C or almost any other.