4.6.2 Multi-master mode

Normally buildbot operates using a single master process that uses the configured database to save state.

It is possible to configure buildbot to have multiple master processes that share state in the same database. This has been well tested using a MySQL database. There are several benefits of Multi-master mode:

• You can have large numbers of build slaves handling the same queue of build requests. A single master can only handle so many slaves (the number is based on a number of factors including type of builds, number of builds, and master and slave IO and CPU capacity - there is no fixed formula). By adding another master which shares the queue of build requests, you can attach more slaves to this additional master, and increase your build throughput.
• You can shut one master down to do maintenance, and other masters will continue to do builds.

State that is shared in the database includes:

• List of changes
• Scheduler names and internal state
• Build requests, including the builder name

Because of this shared state, you are strongly encouraged to:

• Ensure that each named scheduler runs on only one master. If the same scheduler runs on multiple masters, it will trigger duplicate builds and may produce other undesirable behaviors.
• Ensure builder names are unique for a given build factory implementation. You can have the same builder name configured on many masters, but if the build factories differ, you will get different results depending on which master claims the build.

One suggested configuration is to have one buildbot master configured with just the scheduler and change sources; and then other masters configured with just the builders.

To enable multi-master mode in this configuration, you will need to set the multiMaster option so that buildbot doesn't warn about missing schedulers or builders. You will also need to set db_poll_interval to the masters with only builders check the database for new build requests at the configured interval.

     # Enable multiMaster mode; disables warnings about unknown builders and
     # schedulers
     c['multiMaster'] = True
     # Check for new build requests every 60 seconds
     c['db_poll_interval'] = 60