# 2.5.1. Configuring Buildbot

Buildbot’s behavior is defined by the config file, which normally lives in the `master.cfg` file in the buildmaster’s base directory (but this can be changed with an option to the buildbot create-master command). This file completely specifies which `Builder`s are to be run, which workers they should use, how `Change`s should be tracked, and where the status information is to be sent. The buildmaster’s `buildbot.tac` file names the base directory; everything else comes from the config file.

A sample config file was installed for you when you created the buildmaster, but you will need to edit it before your Buildbot will do anything useful.

This chapter gives an overview of the format of this file and the various sections in it. You will need to read the later chapters to understand how to fill in each section properly.

## 2.5.1.1. Config File Format

The config file is, fundamentally, just a piece of Python code which defines a dictionary named `BuildmasterConfig`, with a number of keys that are treated specially. You don’t need to know Python to do the basic configuration, though; you can just copy the sample file’s syntax. If you are comfortable writing Python code, however, you can use all the power of a full programming language to build more complicated configurations.

The `BuildmasterConfig` name is the only one which matters: all other names defined during the execution of the file are discarded. When parsing the config file, the Buildmaster generally compares the old configuration with the new one and performs the minimum set of actions necessary to bring Buildbot up to date: `Builder`s which are not changed are left untouched, and `Builder`s which are modified get to keep their old event history.

The beginning of the `master.cfg` file typically starts with something like:

    BuildmasterConfig = c = {}

Therefore a config key like `change_source` will usually appear in `master.cfg` as `c['change_source']`.

See Buildmaster Configuration Index for a full list of `BuildMasterConfig` keys.

### Basic Python Syntax

The master configuration file is interpreted as Python, allowing the full flexibility of the language. For the configurations described in this section, a detailed knowledge of Python is not required, but the basic syntax is easily described.

Python comments start with a hash character `#`, tuples are defined with `(parenthesis, pairs)`, and lists (arrays) are defined with `[square, brackets]`. Tuples and lists are mostly interchangeable. Dictionaries (data structures which map keys to values) are defined with curly braces: `{'key1': value1, 'key2': value2}`. Function calls (and object instantiations) can use named parameters, like `steps.ShellCommand(command=["trial", "hello"])`.

The config file starts with a series of `import` statements, which make various kinds of `Step`s and `Status` targets available for later use. The main `BuildmasterConfig` dictionary is created, and then it is populated with a variety of keys, described section-by-section in the subsequent chapters.

## 2.5.1.2. Predefined Config File Symbols

The following symbols are automatically available for use in the configuration file.

`basedir`

  the base directory for the buildmaster. This string has not been expanded, so it may start with a tilde. It needs to be expanded before use. The config file is located in:

      os.path.expanduser(os.path.join(basedir, 'master.cfg'))

`__file__`

  the absolute path of the config file. The config file’s directory is located in `os.path.dirname(__file__)`.

## 2.5.1.3. Testing the Config File

To verify that the config file is well-formed and contains no deprecated or invalid elements, use the `checkconfig` command, passing it either a master directory or a config file.

    % buildbot checkconfig master.cfg
    Config file is good!
    # or
    % buildbot checkconfig /tmp/masterdir
    Config file is good!

If the config file has deprecated features (perhaps because you’ve upgraded the buildmaster and need to update the config file to match), they will be announced by checkconfig. In this case, the config file will work, but you should really remove the deprecated items and use the recommended replacements instead:

    % buildbot checkconfig master.cfg
    /usr/lib/python2.4/site-packages/buildbot/master.py:559: DeprecationWarning: c['sources'] is
    deprecated as of 0.7.6 and will be removed by 0.8.0 . Please use c['change_source'] instead.
    Config file is good!

If you have errors in your configuration file, checkconfig will let you know:

    % buildbot checkconfig master.cfg
    Configuration Errors:
    c['workers'] must be a list of Worker instances
    no workers are configured
    builder 'smoketest' uses unknown workers 'linux-002'

If the config file is simply broken, that will be caught too:

    % buildbot checkconfig master.cfg
    error while parsing config file:
    Traceback (most recent call last):
    File "/home/buildbot/master/bin/buildbot", line 4, in <module>
        runner.run()
    File "/home/buildbot/master/buildbot/scripts/runner.py", line 1358, in run
        if not doCheckConfig(so):
    File "/home/buildbot/master/buildbot/scripts/runner.py", line 1079, in doCheckConfig
        return cl.load(quiet=quiet)
    File "/home/buildbot/master/buildbot/scripts/checkconfig.py", line 29, in load
        self.basedir, self.configFileName)
    --- <exception caught here> ---
    File "/home/buildbot/master/buildbot/config.py", line 147, in loadConfig
        exec f in localDict
    exceptions.SyntaxError: invalid syntax (master.cfg, line 52)
    Configuration Errors:
    error while parsing config file: invalid syntax (master.cfg, line 52) (traceback in logfile)

## 2.5.1.4. Loading the Config File

The config file is only read at specific points in time. It is first read when the buildmaster is launched.

Note

If the configuration is invalid, the master will display the errors in the console output, but will not exit.

### Reloading the Config File (reconfig)

If you are on the system hosting the buildmaster, you can send a `SIGHUP` signal to it: the buildbot tool has a shortcut for this:

    buildbot reconfig BASEDIR

This command will show you all of the lines from `twistd.log` that relate to the reconfiguration. If there are any problems during the config-file reload, they will be displayed in the output.

When reloading the config file, the buildmaster will endeavor to change as little as possible about the running system. For example, although old status targets may be shut down and new ones started up, any status targets that were not changed since the last time the config file was read will be left running and untouched. Likewise any `Builder`s which have not been changed will be left running. If a `Builder` is modified (say, the build command is changed), this change will apply only for new `Build`s. Any existing build that is currently running or was already queued will be allowed to finish using the old configuration.

Note that if any lock is renamed, old and new instances of the lock will be completely unrelated in the eyes of the buildmaster. This means that buildmaster will be able to start new builds that would otherwise have waited for the old lock to be released.

Warning

Buildbot’s reconfiguration system is fragile for a few difficult-to-fix reasons:

- Any modules imported by the configuration file are not automatically reloaded. Python modules such as https://docs.python.org/3/library/importlib.html and importlib.reload() may help here, but reloading modules is fraught with subtleties and difficult-to-decipher failure cases.

- During the reconfiguration, active internal objects are divorced from the service hierarchy, leading to tracebacks in the web interface and other components. These are ordinarily transient, but with HTTP connection caching (either by the browser or an intervening proxy) they can last for a long time.

- If the new configuration file is invalid, it is possible for Buildbot’s internal state to be corrupted, leading to undefined results. When this occurs, it is best to restart the master.

- For more advanced configurations, it is impossible for Buildbot to tell if the configuration for a `Builder` or `Scheduler` has changed, and thus the `Builder` or `Scheduler` will always be reloaded. This occurs most commonly when a callable is passed as a configuration parameter.

The bbproto project (at https://github.com/dabrahams/bbproto) may help to construct large (multi-file) configurations which can be effectively reloaded and reconfigured.