What is Rack middleware?

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Rack as Design

Rack middleware is more than “a way to filter a request and response” – it’s an implementation of the pipeline design pattern for web servers using Rack.

It very cleanly separates out the different stages of processing a request – separation of concerns being a key goal of all well designed software products.

For example with Rack I can have separate stages of the pipeline doing:

  • Authentication: when the request arrives, are the users logon details correct? How do I validate this OAuth, HTTP Basic Authentication, name/password?

  • Authorization: “is the user authorised to perform this particular task?”, i.e. role-based security.

  • Caching: have I processed this request already, can I return a cached result?

  • Decoration: how can I enhance the request to make downstream processing better?

  • Performance & Usage Monitoring: what stats can I get from the request and response?

  • Execution: actually handle the request and provide a response.

Being able to separate the different stages (and optionally include them) is a great help in developing well structured applications.

Community

There’s also a great eco-system developing around Rack Middleware – you should be able to find pre-built rack components to do all of the steps above and more. See the Rack GitHub wiki for a list of middleware.

What’s Middleware?

Middleware is a dreadful term which refers to any software component/library which assists with but is not directly involved in the execution of some task. Very common examples are logging, authentication and the other common, horizontal processing components. These tend to be the things that everyone needs across multiple applications but not too many people are interested (or should be) in building themselves.

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