design_model.md

Design model

If you are familiar with rack, you know that it models a two-way pipe. In it, each middleware has the ability to:

• During the outbound trip modifying the request as it heads to the actual application.

• During the return trip modifying the response as it gets back from the application.

  --------------------->  request ----------------------->

  Middleware 1          Middleware 2          Application

  <--------------------- response <-----------------------

web_pipe follows a simpler but equally powerful model: a one-way pipe abstracted on top of rack. A struct that contains data from a web request is piped through a stack of operations (functions). Each operation takes as argument an instance of the struct and also returns an instance of it. You can add response data to the struct at any moment in the pipe.

  Operation 1          Operation 2          Operation 3

  --------------------- request/response ---------------->

Additionally, any operation in the stack can halt the propagation of the pipe, leaving downstream operations unexecuted. In this way, the final response is the one contained in the struct at the moment the pipe was halted, or the last one if the pipe wasn't halted.

As you may know, this is the same model used by Elixir's plug, from which web_pipe takes inspiration.