Friday, April 24, 2009

An idea for loading Pylons middleware

Any given web application framework, Python based or otherwise, must support the concept of middleware. Some frameworks have better support for middleware than others but they all support it, even if the support is rigid and implicit. The reason to associate the concept of middleware with web application frameworks is because the middleware is code that runs in between the request and the response. In most cases, this would be an HTTP request-response pair but the same concept applies to other protocols. In the Python world of web applications, there isn't much support for this idea. Most Python web application frameworks support the concept of extension or plugins. These components can achieve the same effect as middleware components but are more geared toward domain-specific problems. Middleware is supposed to be more solution generic in that it will affect the framework as a whole. The best support of this concept is in the Pylons Python web application framework.

The middleware concept plays a big role in Pylons. In fact, several of the core Pylons components act as middleware. For instance, the routing of an HTTP request, the HTTP session, Caching, are all middleware components in Pylons. These components, as well as other middleware components, may affect the state of the core Pylons WSGI application object. Some of these middleware components will surely affect the state of the HTTP response that is sent back to the client. The middleware architecture in web application frameworks is a wise design decision because it helps reduce the complexity of the code found in the core application object. This happens by providing a better distribution of responsibilities. If all this code were located in the core application object, it would be quite messy and hard to maintain. The middleware construct also offers developers more extensibility opportunities. The entire framework functionality may be altered on account of some middleware component. Below is a simple illustration of how the Pylons web application framework interacts with components.

When a new Pylons skeleton project is generated, a middleware Python configuration script is built and placed in the project directory. It is this generated Python script that is responsible for loading the middleware components to be used in the application. This is an important task t perform when the application is started because the core application object needs its' core middleware components. Without them, much crucial functionality will be missing. However, a nice experiment to try out with the Pylons framework would be to alter this generated middleware loading code. Rather than have static Python code which directly instantiates these components, they could be loaded by iterating through a set of entry points. This way, Pylons middleware components could be discovered and loaded as they are installed on the system. This, however, could be a more challenging feat in reality. The default Pylons middleware components would need to be entry points themselves. These components could always have entry points added to them. At the very least, it would be an idea worth exploring.