# The Evolution of Finger: adding features to the finger service

## Introduction

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

In this section of the tutorial, our finger server will continue to sprout features: the ability for users to set finger announces, and using our finger service to send those announcements on the web, on IRC and over XML-RPC.

## Setting Message By Local Users

Now that port 1079 is free, maybe we can run on it a different server, one which will let people set their messages. It does no access control, so anyone who can login to the machine can set any message. We assume this is the desired behavior in our case. Testing it can be done by simply:

    % nc localhost 1079   # or telnet localhost 1079
    moshez
    Giving a tutorial now, sorry!
    ^D

finger12.py

## Use Services to Make Dependencies Sane

The previous version had the setter poke at the innards of the finger factory. It's usually not a good idea: this version makes both factories symmetric by making them both look at a single object. Services are useful for when an object is needed which is not related to a specific network server. Here, we moved all responsibility for manufacturing factories into the service. Note that we stopped subclassing: the service simply puts useful methods and attributes inside the factories. We are getting better at protocol design: none of our protocol classes had to be changed, and neither will have to change until the end of the tutorial.

finger13.py

## Read Status File

This version shows how, instead of just letting users set their messages, we can read those from a centrally managed file. We cache results, and every 30 seconds we refresh it. Services are useful for such scheduled tasks.

sample /etc/users file finger14.py

## Announce on Web, Too

The same kind of service can also produce things useful for other protocols. For example, in twisted.web, the factory itself (the site) is almost never subclassed -- instead, it is given a resource, which represents the tree of resources available via URLs. That hierarchy is navigated by site, and overriding it dynamically is possible with getChild.

finger15.py

## Announce on IRC, Too

This is the first time there is client code. IRC clients often act a lot like servers: responding to events from the network. The reconnecting client factory will make sure that severed links will get re-established, with intelligent tweaked exponential back-off algorithms. The IRC client itself is simple: the only real hack is getting the nickname from the factory in connectionMade.

finger16.py

## Add XML-RPC Support

In Twisted, XML-RPC support is handled just as though it was another resource. That resource will still support GET calls normally through render(), but that is usually left unimplemented. Note that it is possible to return deferreds from XML-RPC methods. The client, of course, will not get the answer until the deferred is triggered.

finger17.py

A simple client to test the XMLRPC finger:

fingerXRclient.py