Buildbot no longer supports Python 2.7 on the Buildbot master.

2.3. Concepts

This chapter defines some of the basic concepts that the Buildbot uses. You’ll need to understand how the Buildbot sees the world to configure it properly.

2.3.1. Source Stamps

Buildbot uses the concept of source stamp set to identify exact source code that needs to be built for a certain project. A source stamp set is a collection of one or more source stamps.

A source stamp is a collection of information needed to identify a particular version of code on a certain codebase. This information most often is a revision and possibly a branch.

A codebase is a collection of related files and their history tracked as a unit by version control systems. A single codebase may appear in multiple repositories which themselves are identified by URLs. For example, and both contain the Firefox codebase, although not exactly the same code.

A project corresponds to a set of one or more codebases that together may be built and produce some end artifact. For example, a company may build several applications based on the same core library. The “app” codebase and the “core” codebase are in separate repositories, but are compiled together and constitute a single project. Changes to either codebase should cause a rebuild of the application.

A revision is an identifier used by most version control systems to uniquely specify a particular version of the source code. Sometimes in order to do that a revision may make sense only if used in combination with a branch.

To sum up the above, to build a project, Buildbot needs to know exactly which version of each codebase it should build. It uses a source stamp to do so for each codebase, each of which informs Buildbot that it should use a specific revision from that codebase. Collectively these source stamps are called source stamp set for each project.

Source Stamp Sets

2.3.2. Version Control Systems

Buildbot supports a significant number of version control systems, so it treats them abstractly.

For purposes of deciding when to perform builds, Buildbot’s change sources monitor repositories, and represent any updates to those repositories as changes. These change sources fall broadly into two categories: pollers which periodically check the repository for updates; and hooks, where the repository is configured to notify Buildbot whenever an update occurs. For more information see Change Sources and Changes and How Different VC Systems Specify Sources.

When it comes time to actually perform a build, a scheduler prepares a source stamp set, as described above, based on its configuration. When the build begins, one or more source steps use the information in the source stamp set to actually check out the source code, using the normal VCS commands.

2.3.3. Changes

A Change is an abstract way Buildbot uses to represent a single change to the source files performed by a developer. In version control systems that support the notion of atomic check-ins a change represents a changeset or commit.

A Change comprises the following information:

  • the developer that is responsible for the change
  • the list of files that the change added, removed or modified
  • the message of the commit
  • the repository, the codebase and the project that the change corresponds to
  • the revision and the branch of the commit

2.3.4. Scheduling Builds

Each Buildmaster has a set of scheduler objects, each of which gets a copy of every incoming Change. The Schedulers are responsible for deciding when Builds should be run. Some Buildbot installations might have a single scheduler, while others may have several, each for a different purpose.

For example, a quick scheduler might exist to give immediate feedback to developers, hoping to catch obvious problems in the code that can be detected quickly. These typically do not run the full test suite, nor do they run on a wide variety of platforms. They also usually do a VC update rather than performing a brand-new checkout each time.

A separate full scheduler might run more comprehensive tests, to catch more subtle problems. It might be configured to run after the quick scheduler, to give developers time to commit fixes to bugs caught by the quick scheduler before running the comprehensive tests. This scheduler would also feed multiple Builders.

Many schedulers can be configured to wait a while after seeing a source-code change - this is the tree stable timer. The timer allows multiple commits to be “batched” together. This is particularly useful in distributed version control systems, where a developer may push a long sequence of changes all at once. To save resources, it’s often desirable only to test the most recent change.

Schedulers can also filter out the changes they are interested in, based on a number of criteria. For example, a scheduler that only builds documentation might skip any changes that do not affect the documentation. Schedulers can also filter on the branch to which a commit was made.

There is some support for configuring dependencies between builds - for example, you may want to build packages only for revisions which pass all of the unit tests. This support is under active development in Buildbot, and is referred to as “build coordination”.

Periodic builds (those which are run every N seconds rather than after new Changes arrive) are triggered by a special Periodic scheduler.

Each scheduler creates and submits BuildSet objects to the BuildMaster, which is then responsible for making sure the individual BuildRequests are delivered to the target Builders.

Scheduler instances are activated by placing them in the schedulers list in the buildmaster config file. Each scheduler must have a unique name.

2.3.5. Builds

A Build represents a single compile or test run of a particular version of the source code. A build is comprised of a series of steps. The steps may be arbitrary. For example, for compiled software a build generally consists of the checkout, configure, make, and make check sequence. For interpreted projects like Python modules, a build is generally a checkout followed by an invocation of the bundled test suite.

Builds are created by instances of Builder (see below). A BuildFactory (see below) that is attached to the Builder creates a list of the steps for the new build.

2.3.6. BuildSets

A BuildSet represents a set of potentially not yet created Builds that all compile and/or test the same version of the source tree. It tracks whether this set of builds as a whole succeeded or not. The information that is stored in a BuildSet is a set of SourceStamps which define the version of the code to test and a set of Builders which define what builds to create.

2.3.7. BuildRequests

A BuildRequest is a request to start a specific build. A BuildRequest consists of the following information:

  • the name of the Builder (see below) that will start the build.
  • the set of SourceStamps (see above) that specify the version of the source tree to build and/or test.

A BuildRequest may be merged with another BuildRequest if they represent the same version of the source code and the same builder. The user may configure additional restrictions for determining mergeability of build requests.

2.3.8. Builders and Build Factories

A Builder is responsible for creating new builds from BuildRequests. Creating a new build is essentially determining the exact steps and other properties of the build and/or test sequence to execute. This is performed by a BuildFactory that is attached to each Builder.

A Builder will attempt to create a Build from a BuildRequest as soon as it is possible, that is, as soon as the associated worker becomes free. When a worker becomes free, the build master will select the oldest BuildRequest that can run on that worker and notify the corresponding Builder to maybe start a build out of it.

Each Builder by default runs completely independently. This means, that a worker that has N builders attached to it, may potentially attempt to run N builds concurrently. This level of concurrency may be controlled by various kinds of Interlocks.

At a low level, each builder has its own exclusive directory on the build master and one exclusive directory on each of the workers it is attached to. The directory on the master is used for keeping status information. The directories on the workers are used as a location where the actual checkout, compilation and testing steps happen.

2.3.9. Workers

A Worker corresponds to an environment where builds are executed. A single physical machine must run at least one Worker in order for Buildbot to be able to utilize it for running builds. Multiple Workers may run on a single machine to provide different environments that can reuse the same hardware by means of containers or virtual machines.

Each builder is associated with one or more Workers. For example, a builder which is used to perform macOS builds (as opposed to Linux or Windows builds) should naturally be associated with a Mac worker.

If multiple workers are available for any given builder, you will have some measure of redundancy: in case one worker goes offline, the others can still keep the Builder working. In addition, multiple workers will allow multiple simultaneous builds for the same Builder, which might be useful if you have a lot of forced or try builds taking place.

Ideally, each Worker that is configured for a builder should be identical. Otherwise build or test failures will be dependent on which worker the build is ran and this will complicate investigation of failures.

2.3.10. Users

Buildbot has a somewhat limited awareness of users. It assumes the world consists of a set of developers, each of whom can be described by a couple of simple attributes. These developers make changes to the source code, causing builds which may succeed or fail.

Users also may have different levels of authorization when issuing Buildbot commands, such as forcing a build from the web interface or from an IRC channel.

Each developer is primarily known through the source control system. Each Change object that arrives is tagged with a who field that typically gives the account name (on the repository machine) of the user responsible for that change. This string is displayed on the HTML status pages and in each Build’s blamelist.

To do more with the User than just refer to them, this username needs to be mapped into an address of some sort. The responsibility for this mapping is left up to the status module which needs the address. In the future, the responsibility for managing users will be transferred to User Objects.

The who fields in git Changes are used to create User Objects, which allows for more control and flexibility in how Buildbot manages users. User Objects

User Objects allow Buildbot to better manage users throughout its various interactions with users (see Change Sources and Changes and Reporters). The User Objects are stored in the Buildbot database and correlate the various attributes that a user might have: irc, Git, etc.


Incoming Changes all have a who attribute attached to them that specifies which developer is responsible for that Change. When a Change is first rendered, the who attribute is parsed and added to the database if it doesn’t exist or checked against an existing user. The who attribute is formatted in different ways depending on the version control system that the Change came from.

who attributes take the form Full Name <Email>.
who attributes are of the form Username.
who attributes are free-form strings, but usually adhere to similar conventions as git attributes (Full Name <Email>).
who attributes are of the form Username.
who attributes contain an Email and may also include a Full Name like git attributes.
who attributes are free-form strings like hg, and can include a Username, Email, and/or Full Name.


For managing users manually, use the buildbot user command, which allows you to add, remove, update, and show various attributes of users in the Buildbot database (see Command-line Tool).


Correlating the various bits and pieces that Buildbot views as users also means that one attribute of a user can be translated into another. This provides a more complete view of users throughout Buildbot.

One such use is being able to find email addresses based on a set of Builds to notify users through the MailNotifier. This process is explained more clearly in Email Addresses.

Another way to utilize User Objects is through UsersAuth for web authentication. To use UsersAuth, you need to set a bb_username and bb_password via the buildbot user command line tool to check against. The password will be encrypted before storing in the database along with other user attributes. Doing Things With Users

Each change has a single user who is responsible for it. Most builds have a set of changes: the build generally represents the first time these changes have been built and tested by the Buildbot. The build has a blamelist that is the union of the users responsible for all the build’s changes. If the build was created by a Try Schedulers this list will include the submitter of the try job, if known.

The build provides a list of users who are interested in the build – the interested users. Usually this is equal to the blamelist, but may also be expanded, e.g., to include the current build sherrif or a module’s maintainer.

If desired, the buildbot can notify the interested users until the problem is resolved. Email Addresses

The MailNotifier is a status target which can send email about the results of each build. It accepts a static list of email addresses to which each message should be delivered, but it can also be configured to send mail to the Build’s Interested Users. To do this, it needs a way to convert User names into email addresses.

For many VC systems, the User Name is actually an account name on the system which hosts the repository. As such, turning the name into an email address is a simple matter of appending Some projects use other kinds of mappings (for example the preferred email address may be at despite the repository host being named, and some VC systems have full separation between the concept of a user and that of an account on the repository host (like Perforce). Some systems (like Git) put a full contact email address in every change.

To convert these names to addresses, the MailNotifier uses an EmailLookup object. This provides a getAddress method which accepts a name and (eventually) returns an address. The default MailNotifier module provides an EmailLookup which simply appends a static string, configurable when the notifier is created. To create more complex behaviors (perhaps using an LDAP lookup, or using finger on a central host to determine a preferred address for the developer), provide a different object as the lookup argument.

If an EmailLookup object isn’t given to the MailNotifier, the MailNotifier will try to find emails through User Objects. This will work the same as if an EmailLookup object was used if every user in the Build’s Interested Users list has an email in the database for them. If a user whose change led to a Build doesn’t have an email attribute, that user will not receive an email. If extraRecipients is given, those users are still sent mail when the EmailLookup object is not specified.

In the future, when the Problem mechanism has been set up, the Buildbot will need to send mail to arbitrary Users. It will do this by locating a MailNotifier-like object among all the buildmaster’s status targets, and asking it to send messages to various Users. This means the User-to-address mapping only has to be set up once, in your MailNotifier, and every email message the buildbot emits will take advantage of it. IRC Nicknames

Like MailNotifier, the buildbot.status.words.IRC class provides a status target which can announce the results of each build. It also provides an interactive interface by responding to online queries posted in the channel or sent as private messages.

In the future, the buildbot can be configured to map User names to IRC nicknames, to watch for the recent presence of these nicknames, and to deliver build status messages to the interested parties. Like MailNotifier does for email addresses, the IRC object will have an IRCLookup which is responsible for nicknames. The mapping can be set up statically, or it can be updated by online users themselves (by claiming a username with some kind of buildbot: i am user warner commands).

Once the mapping is established, the rest of the buildbot can ask the IRC object to send messages to various users. It can report on the likelihood that the user saw the given message (based upon how long the user has been inactive on the channel), which might prompt the Problem Hassler logic to send them an email message instead.

These operations and authentication of commands issued by particular nicknames will be implemented in User Objects.

2.3.11. Build Properties

Each build has a set of Build Properties, which can be used by its build steps to modify their actions.

The properties are represented as a set of key-value pairs. Effectively, a single property is a variable that, once set, can be used by subsequent steps in a build to modify their behaviour. The value of a property can be a number, a string, a list or a dictionary. Lists and dictionaries can contain other lists or dictionaries. Thus, the value of a property could be arbitrarily complex structure.

Properties work pretty much like variables, so they can be used to implement all manner of functionality.

The following are several examples:

  • By default, the name of the worker that runs the build is set to the workername property. If there are multiple different workers and the actions of the build depend on the exact worker, some users may decide that it’s more convenient to vary the actions depending on the workername property instead of creating separate builders for each worker.
  • In most cases the build does not know the exact code revision that will be tested until it checks out the code. This information is only known after a source step runs. To give this information to the subsequent steps, the source step records the checked out revision into the got_revision property.

2.3.12. Multiple-Codebase Builds

What if an end-product is composed of code from several codebases? Changes may arrive from different repositories within the tree-stable-timer period. Buildbot will not only use the source-trees that contain changes but also needs the remaining source-trees to build the complete product.

For this reason a Scheduler can be configured to base a build on a set of several source-trees that can (partly) be overridden by the information from incoming Changes.

As described above, the source for each codebase is identified by a source stamp, containing its repository, branch and revision. A full build set will specify a source stamp set describing the source to use for each codebase.

Configuring all of this takes a coordinated approach. A complete multiple repository configuration consists of:

a codebase generator

Every relevant change arriving from a VC must contain a codebase. This is done by a codebaseGenerator that is defined in the configuration. Most generators examine the repository of a change to determine its codebase, using project-specific rules.

some schedulers

Each scheduler has to be configured with a set of all required codebases to build a product. These codebases indicate the set of required source-trees. In order for the scheduler to be able to produce a complete set for each build, the configuration can give a default repository, branch, and revision for each codebase. When a scheduler must generate a source stamp for a codebase that has received no changes, it applies these default values.

multiple source steps - one for each codebase

A Builders and Build Factories’s build factory must include a source step for each codebase. Each of the source steps has a codebase attribute which is used to select an appropriate source stamp from the source stamp set for a build. This information comes from the arrived changes or from the scheduler’s configured default values.


Each source step has to have its own workdir set in order for the checkout to be done for each codebase in its own directory.


Ensure you specify the codebase within your source step’s Interpolate() calls (ex. http://.../svn/%(src:codebase:branch)s). See Interpolate for details.


Defining a codebaseGenerator that returns non-empty (not '') codebases will change the behavior of all the schedulers.

2.3.13. Multimaster


Buildbot Multimaster is considered experimental. There are still some companies using it in production. Don’t hesitate to use the mailing lists to share your experience.

blockdiag LoadBalancer LoadBalancerUI Worker1 WorkerN User1 User2 Master1 Master2 MasterUI1 MasterUI2 database

Buildbot supports interconnection of several masters. This has to be done through a multi-master enabled message queue backend. As of now the only one supported is wamp and see wamp

There are then several strategy for introducing multimaster in your buildbot infra. A simple way to say it is by adding the concept of symmetrics and asymmetrics multimaster (like there is SMP and AMP for multi core CPUs)

Symmetric multimaster is when each master share the exact same configuration. They run the same builders, same schedulers, same everything, the only difference is that workers are connected evenly between the masters (by any means (e.g. DNS load balancing, etc)) Symmetric multimaster is good to use to scale buildbot horizontally.

Asymmetric multimaster is when each master have different configuration. Each master may have a specific responsibility (e.g schedulers, set of builder, UI). This was more how you did in 0.8, also because of its own technical limitations. A nice feature of asymmetric multimaster is that you can have the UI only handled by some masters.

Separating the UI from the controlling will greatly help in the performance of the UI, because badly written BuildSteps?? can stall the reactor for several seconds.

The fanciest configuration would probably be a symmetric configuration for everything but the UI. You would scale the number of UI master according to your number of UI users, and scale the number of engine masters to the number of workers.

Depending on your workload and size of master host, it is probably a good idea to start thinking of multimaster starting from a hundred workers connected.

Multimaster can also be used for high availability, and seamless upgrade of configuration code. Complex configuration indeed requires sometimes to restart the master to reload custom steps or code, or just to upgrade the upstream buildbot version.

In this case, you will implement following procedure:

  • Start new master(s) with new code and configuration.
  • Send a graceful shutdown to the old master(s).
  • New master(s) will start taking the new jobs, while old master(s) will just finish managing the running builds.
  • As an old master is finishing the running builds, it will drop the connections from the workers, who will then reconnect automatically, and by the mean of load balancer will get connected to a new master to run new jobs.

As buildbot nine has been designed to allow such procedure, it has not been implemented in production yet as we know. There is probably a new REST api needed in order to graceful shutdown a master, and the details of gracefully dropping the connection to the workers to be sorted out.