# The Evolution of Finger: moving to a component based architecture

## Introduction

This is the fourth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger.

In this section of the tutorial, we'll move our code to a component architecture so that adding new features is trivial.

## Write Maintainable Code

In the last version, the service class was three times longer than any other class, and was hard to understand. This was because it turned out to have multiple responsibilities. It had to know how to access user information, by rereading the file every half minute, but also how to display itself in a myriad of protocols. Here, we used the component-based architecture that Twisted provides to achieve a separation of concerns. All the service is responsible for, now, is supporting getUser/getUsers. It declares its support via the __implements__ keyword. Then, adapters are used to make this service look like an appropriate class for various things: for supplying a finger factory to TCPServer, for supplying a resource to site's constructor, and to provide an IRC client factory for TCPClient. All the adapters use are the methods in FingerService they are declared to use: getUser/getUsers. We could, of course, skip the interfaces and let the configuration code use things like FingerFactoryFromService(f) directly. However, using interfaces provides the same flexibility inheritance gives: future subclasses can override the adapters.

finger19.py

## Advantages of Latest Version

- Readable -- each class is short
- Maintainable -- each class knows only about interfaces
- Dependencies between code parts are minimized
- Example: writing a new IFingerService is easy

finger19a_changes.py

Full source code here: finger19a.py

## Aspect-Oriented Programming

At last, an example of aspect-oriented programming that isn't about logging or timing. This code is actually useful! Watch how aspect-oriented programming helps you write less code and have fewer dependencies!