In the post titled HttpKernel middlewares I brought up the idea of bringing rack middlewares to Symfony2.
In order to solidify this effort I would like to announce a new project: Stack.
Stack is a convention for composing HttpKernelInterface middlewares.
The name and logo are based on the idea of building a stack of middleware layers, each of which handles a particular piece of logic.
The main goal of stack is to give the idea of "HttpKernel middlewares" a name, so that we can more easily talk about it.
It aims to make the conventions explicit and clearly define what a middleware should do and what it should look like. Following them allows for better interoperability and consistency between middlewares.
Check the stack website for the actual conventions.
The stackphp organisation on GitHub contains a very small set of basic tools make working with stack middlewares easier. However, none of those tools are mandatory.
You can create and use stack middlewares without using any of the provided tools, as long as you follow the conventions.
The most prominent tool is the builder. Stack\Builder is an evolution of the original Stack object from the HttpKernel middlewares blog post. This ultimately makes stack usable, and provides a basis for configuration.
Why HttpKernelInterface? Why limit ourselves to Symfony2? Why not target all of PHP?
First of all, Symfony provides a nice sandbox to experiment in. The basic primitives are there: An HTTP request/response abstraction, a widely deployed interface for sending those requests. Lots of popular frameworks are adopting the HttpKernelInterface, which provides a large user base.
Of course, PHP itself also already has such an abstraction. It's called SAPI. It would be interesting to extend the scope of middlewares and target all of PHP. But in order to do that properly, PHP core would have to be modified to make SAPIs extensible.
If we can prove that the concept works for HttpKernel, the broader PHP community might be interested in bringing that capability to PHP core. That's a long journey however, we'll have to take it one step at a time.