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8.7 Master-Slave API

This section is a (very incomplete) description of the master-slave interface. The interface is based on Twisted's Perspective Broker.


The slave connects to the master, using the parameters supplied to buildslave create-slave. It uses a reconnecting process with an exponential backoff, and will automatically reconnect on disconnection.

Once connected, the slave authenticates with the Twisted Cred (newcred) mechanism, using the username and password supplied to buildslave create-slave. The "mind" is the slave bot instance (class

On the master side, the realm is implemented by buildbot.master.Dispatcher, which examines the username of incoming avatar requests. There are special cases for change, debug, and statusClient, which are not discussed here. For all other usernames, the botmaster is consulted, and if a slave with that name is configured, its buildbot.buildslave.BuildSlave instance is returned as the perspective.

Build Slaves

At this point, the master-side BuildSlave object has a pointer to the remote, slave-side Bot object in self.slave, and the slave-side Bot object has a reference to the master-side BuildSlave object in self.perspective.

Bot methods

The slave-side object has the following remote methods:

Returns a list of (name, version) for all commands the slave recognizes
Given a list of builders and their build directories, ensures that those builders, and only those builders, are running. This can be called after the initial connection is established, with a new list, to add or remove builders.

This method returns a dictionary of SlaveBuilder objects - see below

Adds a message to the slave logfile
Returns the contents of the slave's info/ directory. Also contains the keys
copy of the slaves environment
OS the slave is running (extracted from pythons
base directory where slave is running

Returns the slave's version

BuildSlave methods

The master-side object has the following method:

Does nothing - used to keep traffic flowing over the TCP connection

Slave Builders

Each build slave has a set of builders which can run on it. These are represented by distinct classes on the master and slave, just like the BuildSlave and Bot objects described above.

On the slave side, builders are represented as instances of the class. On the master side, they are represented by the buildbot.process.builder.SlaveBuilder class. The following will refer to these as the slave-side and master-side SlaveBuilder classes. Each object keeps a reference to its opposite in self.remote.

slave-side SlaveBuilder methods

Provides a reference to the master-side SlaveBuilder
Adds a message to the slave logfile; used to check round-trip connectivity
Indicates that a build is about to start, and that any subsequent commands are part of that build
Invokes a command on the slave side
Interrupts the currently-running command
Shuts down the slave cleanly

master-side SlaveBuilder methods

The master side does not have any remotely-callable methods.


After the initial connection and trading of a mind (Bot) for an avatar (BuildSlave), the master calls the Bot's setBuilderList method to set up the proper slave builders on the slave side. This method returns a reference to each of the new slave-side SlaveBuilder objects. Each of these is handed to the corresponding master-side SlaveBuilder object. This immediately calls the remote setMaster method, then the print method.


To ping a remote SlaveBuilder, the master calls the print method.


When a build starts, the msater calls the slave's startBuild method. Each BuildStep instance will subsequently call the startCommand method, passing a reference to itself as the stepRef parameter. The startCommand method returns immediately, and the end of the command is signalled with a call to a method on the master-side BuildStep object.

master-side BuildStep methods

Update information about the running command. See below for the format.
Signal that the command is complete, either successfully or with a Twisted failure.

Updates from the slave are a list of individual update elements. Each update element is, in turn, a list of the form [data, 0] where the 0 is present for historical reasons. The data is a dictionary, with keys describing the contents, e.g., header, stdout, or the name of a logfile. If the key is rc, then the value is the exit status of the command. No further updates should be sent after an rc.