# 2.6.6. Writing Change Sources

For some version-control systems, making Buildbot aware of new changes can be a challenge. If the pre-supplied classes in Change Sources and Changes are not sufficient, then you will need to write your own.

There are three approaches, one of which is not even a change source. The first option is to write a change source that exposes some service to which the version control system can “push” changes. This can be more complicated, since it requires implementing a new service, but delivers changes to Buildbot immediately on commit.

The second option is often preferable to the first: implement a notification service in an external process (perhaps one that is started directly by the version control system, or by an email server) and delivers changes to Buildbot via PBChangeSource. This section does not describe this particular approach, since it requires no customization within the buildmaster process.

The third option is to write a change source which polls for changes - repeatedly connecting to an external service to check for new changes. This works well in many cases, but can produce a high load on the version control system if polling is too frequent, and can take too long to notice changes if the polling is not frequent enough.

## 2.6.6.1. Writing a Notification-based Change Source

A custom change source must implement `buildbot.interfaces.IChangeSource`.

The easiest way to do this is to subclass `buildbot.changes.base.ChangeSource`, implementing the `describe` method to describe the instance. `ChangeSource` is a Twisted service, so you will need to implement the `startService` and `stopService` methods to control the means by which your change source receives notifications.

When the class does receive a change, it should call `self.master.data.updates.addChange(..)` to submit it to the buildmaster. This method shares the same parameters as `master.db.changes.addChange`, so consult the API documentation for that function for details on the available arguments.

You will probably also want to set `compare_attrs` to the list of object attributes which Buildbot will use to compare one change source to another when reconfiguring. During reconfiguration, if the new change source is different from the old, then the old will be stopped and the new started.

## 2.6.6.2. Writing a Change Poller

Polling is a very common means of seeking changes, so Buildbot supplies a utility parent class to make it easier. A poller should subclass `buildbot.changes.base.ReconfigurablePollingChangeSource`, which is a subclass of `ChangeSource`. This subclass implements the `Service` methods, and calls the `poll` method according to the `pollInterval` and `pollAtLaunch` options. The `poll` method should return a Deferred to signal its completion.

Aside from the service methods, the other concerns in the previous section apply here, too.