Is there an equivalent to ASP.NET Web API in the Rails world?
So, the answer is Yes to both questions. Rails does have an equivalent and its Rails.
ASP.NET Web API looks like at it's heart is just a RESTful router with type negotiation. I could be totally off base here, but from the tutorials I saw that's what it looked like to me.
So yes, from what I can tell, Rails supports most of the things that the Web API was created for. In fact in Rails most of this stuff is forced onto you until you become informed enough to be able to change it (assuming by that point you know better than to actually do that).
But, as far as Web API functionality. That really comes from the ability to support HTTP verbs (GET, POST, PUT, DELETE) which Rails does.
But a source of confusion might be that in Rails the RESTful API is actually the application itself. Meaning you don't need to implement any other libraries, it's just built that way.
Here's a quick example of that I mean
When you hit /users/1 you will get the data associated with that user depending on the format you requested. So if you request JSON the controller returns JSON, HTML you get HTML, XML you get XML, etc. (As long as said format is implemented for that resource)
A good overview of what I'm talking about are in these two sections:
Rails Guides::Controller: Rendering xml and json data
Rails Guides::Routing: Resources on the Web
So you could build a website, API, or both in a Rails app and they would all start the same way.
But from my limited knowledge on the matter, I would say a ASP.NET MVC with ASP.NET Web API program is actually a lot more like a Rails Program than the regular ASP.NET MVC programs that came before them.
Or it's all just a clever ploy to get as many Capital Letters in a title as humanly possible.
WSO2 looks like a generic web services framework (as opposed to MVC like Rails) I can't vouch for it but it seems to be more a service framework in the style of WCF Web API (service in the generic sense, not just SOAP).