What is middleware when referenced in the context of Ruby on Rails?

Question

I often hear the term 'middleware' in the context of Ruby on Rails. What exactly is it? Can you provide specific examples?

Answer 1

Middleware is related to Rack, the standard Ruby API for web applications. Since Rails applications are Rack applications these days, they apply to both.

Rack middleware is everything between application servers (Webrick, Thin, Unicorn, Passenger, ...) and the actual application, such as your Rails application. It is the pipeline between the web application server and the application itself.

The input to a Rack application is an "environment" which contains all the HTTP request details (and more). The output is a HTTP response. Middleware layers are like filters which can modify the input, the output or both. Rails uses middleware to implement some of its features (query caching, cookie stores, http method expansion), but you can add your own.

Rack middleware is an effective way to reuse simple web-related behavior across web applications that use Rack, regardless of the underlying framework. If a part of your application adds functionality, but is not responsible for a HTTP response, it qualifies as Rack middleware.

Some examples of things you could implement as Rack middleware include:

• HTTP caching (server side and client side)
• Logging
• Authentication
• Monitoring
• HTTP header filtering

See also this SO question.