@cindex introduction

The BuildBot is a system to automate the compile/test cycle required by most
software projects to validate code changes. By automatically rebuilding and
testing the tree each time something has changed, build problems are
pinpointed quickly, before other developers are inconvenienced by the
failure. The guilty developer can be identified and harassed without human
intervention. By running the builds on a variety of platforms, developers
who do not have the facilities to test their changes everywhere before
checkin will at least know shortly afterwards whether they have broken the
build or not. Warning counts, lint checks, image size, compile time, and
other build parameters can be tracked over time, are more visible, and
are therefore easier to improve.

The overall goal is to reduce tree breakage and provide a platform to
run tests or code-quality checks that are too annoying or pedantic for
any human to waste their time with. Developers get immediate (and
potentially public) feedback about their changes, encouraging them to
be more careful about testing before checkin.

Features:

@itemize @bullet
@item
run builds on a variety of slave platforms
@item
arbitrary build process: handles projects using C, Python, whatever
@item
minimal host requirements: python and Twisted
@item
slaves can be behind a firewall if they can still do checkout
@item
status delivery through web page, email, IRC, other protocols
@item
track builds in progress, provide estimated completion time
@item
flexible configuration by subclassing generic build process classes
@item
debug tools to force a new build, submit fake Changes, query slave status
@item
released under the GPL
@end itemize

@menu
* History and Philosophy::
* System Architecture::
* Control Flow::
@end menu


@node History and Philosophy
@section History and Philosophy

@cindex Philosophy of operation

The Buildbot was inspired by a similar project built for a development
team writing a cross-platform embedded system. The various components
of the project were supposed to compile and run on several flavors of
unix (linux, solaris, BSD), but individual developers had their own
preferences and tended to stick to a single platform. From time to
time, incompatibilities would sneak in (some unix platforms want to
use @code{string.h}, some prefer @code{strings.h}), and then the tree
would compile for some developers but not others. The buildbot was
written to automate the human process of walking into the office,
updating a tree, compiling (and discovering the breakage), finding the
developer at fault, and complaining to them about the problem they had
introduced. With multiple platforms it was difficult for developers to
do the right thing (compile their potential change on all platforms);
the buildbot offered a way to help.

Another problem was when programmers would change the behavior of a
library without warning its users, or change internal aspects that
other code was (unfortunately) depending upon. Adding unit tests to
the codebase helps here: if an application's unit tests pass despite
changes in the libraries it uses, you can have more confidence that
the library changes haven't broken anything. Many developers
complained that the unit tests were inconvenient or took too long to
run: having the buildbot run them reduces the developer's workload to
a minimum.

In general, having more visibility into the project is always good,
and automation makes it easier for developers to do the right thing.
When everyone can see the status of the project, developers are
encouraged to keep the tree in good working order. Unit tests that
aren't run on a regular basis tend to suffer from bitrot just like
code does: exercising them on a regular basis helps to keep them
functioning and useful.

The current version of the Buildbot is additionally targeted at
distributed free-software projects, where resources and platforms are
only available when provided by interested volunteers. The buildslaves
are designed to require an absolute minimum of configuration, reducing
the effort a potential volunteer needs to expend to be able to
contribute a new test environment to the project. The goal is for
anyone who wishes that a given project would run on their favorite
platform should be able to offer that project a buildslave, running on
that platform, where they can verify that their portability code
works, and keeps working.

@node System Architecture
@comment  node-name,  next,  previous,  up
@section System Architecture

The Buildbot consists of a single @code{buildmaster} and one or more
@code{buildslaves}, connected in a star topology. The buildmaster
makes all decisions about what, when, and how to build. It sends
commands to be run on the build slaves, which simply execute the
commands and return the results. (certain steps involve more local
decision making, where the overhead of sending a lot of commands back
and forth would be inappropriate, but in general the buildmaster is
responsible for everything).

The buildmaster is usually fed @code{Changes} by some sort of version control
system (@pxref{Change Sources}), which may cause builds to be run. As the
builds are performed, various status messages are produced, which are then sent
to any registered Status Targets (@pxref{Status Targets}).

@c @image{FILENAME, WIDTH, HEIGHT, ALTTEXT, EXTENSION}
@image{images/overview,,,Overview Diagram,}

The buildmaster is configured and maintained by the ``buildmaster
admin'', who is generally the project team member responsible for
build process issues. Each buildslave is maintained by a ``buildslave
admin'', who do not need to be quite as involved. Generally slaves are
run by anyone who has an interest in seeing the project work well on
their favorite platform.

@menu
* BuildSlave Connections::
* Buildmaster Architecture::
* Status Delivery Architecture::
@end menu

@node BuildSlave Connections
@subsection BuildSlave Connections

The buildslaves are typically run on a variety of separate machines,
at least one per platform of interest. These machines connect to the
buildmaster over a TCP connection to a publically-visible port. As a
result, the buildslaves can live behind a NAT box or similar
firewalls, as long as they can get to buildmaster. The TCP connections
are initiated by the buildslave and accepted by the buildmaster, but
commands and results travel both ways within this connection. The
buildmaster is always in charge, so all commands travel exclusively
from the buildmaster to the buildslave.

To perform builds, the buildslaves must typically obtain source code
from a CVS/SVN/etc repository. Therefore they must also be able to
reach the repository. The buildmaster provides instructions for
performing builds, but does not provide the source code itself.

@image{images/slaves,,,BuildSlave Connections,}

@node Buildmaster Architecture
@subsection Buildmaster Architecture

The Buildmaster consists of several pieces:

@image{images/master,,,BuildMaster Architecture,}

@itemize @bullet

@item
Change Sources, which create a Change object each time something is
modified in the VC repository. Most ChangeSources listen for messages
from a hook script of some sort. Some sources actively poll the
repository on a regular basis. All Changes are fed to the Schedulers.

@item
Schedulers, which decide when builds should be performed. They collect
Changes into BuildRequests, which are then queued for delivery to
Builders until a buildslave is available.

@item
Builders, which control exactly @emph{how} each build is performed
(with a series of BuildSteps, configured in a BuildFactory). Each
Build is run on a single buildslave.

@item
Status plugins, which deliver information about the build results
through protocols like HTTP, mail, and IRC.

@end itemize

Each Builder is configured with a list of BuildSlaves that it will use
for its builds. These buildslaves are expected to behave identically:
the only reason to use multiple BuildSlaves for a single Builder is to
provide a measure of load-balancing.

Within a single BuildSlave, each Builder creates its own SlaveBuilder
instance. These SlaveBuilders operate independently from each other.
Each gets its own base directory to work in. It is quite common to
have many Builders sharing the same buildslave. For example, there
might be two buildslaves: one for i386, and a second for PowerPC.
There may then be a pair of Builders that do a full compile/test run,
one for each architecture, and a lone Builder that creates snapshot
source tarballs if the full builders complete successfully. The full
builders would each run on a single buildslave, whereas the tarball
creation step might run on either buildslave (since the platform
doesn't matter when creating source tarballs). In this case, the
mapping would look like:

@example
Builder(full-i386)  ->  BuildSlaves(slave-i386)
Builder(full-ppc)   ->  BuildSlaves(slave-ppc)
Builder(source-tarball) -> BuildSlaves(slave-i386, slave-ppc)
@end example

and each BuildSlave would have two SlaveBuilders inside it, one for a
full builder, and a second for the source-tarball builder.

Once a SlaveBuilder is available, the Builder pulls one or more
BuildRequests off its incoming queue. (It may pull more than one if it
determines that it can merge the requests together; for example, there
may be multiple requests to build the current HEAD revision). These
requests are merged into a single Build instance, which includes the
SourceStamp that describes what exact version of the source code
should be used for the build. The Build is then randomly assigned to a
free SlaveBuilder and the build begins.

The behaviour when BuildRequests are merged can be customized, @pxref{Merging
Build Requests}.

@node Status Delivery Architecture
@subsection Status Delivery Architecture

The buildmaster maintains a central Status object, to which various
status plugins are connected. Through this Status object, a full
hierarchy of build status objects can be obtained.

@image{images/status,,,Status Delivery,}

The configuration file controls which status plugins are active. Each
status plugin gets a reference to the top-level Status object. From
there they can request information on each Builder, Build, Step, and
LogFile. This query-on-demand interface is used by the html.Waterfall
plugin to create the main status page each time a web browser hits the
main URL.

The status plugins can also subscribe to hear about new Builds as they
occur: this is used by the MailNotifier to create new email messages
for each recently-completed Build.

The Status object records the status of old builds on disk in the
buildmaster's base directory. This allows it to return information
about historical builds.

There are also status objects that correspond to Schedulers and
BuildSlaves. These allow status plugins to report information about
upcoming builds, and the online/offline status of each buildslave.


@node Control Flow
@comment  node-name,  next,  previous,  up
@section Control Flow

A day in the life of the buildbot:

@itemize @bullet

@item
A developer commits some source code changes to the repository. A hook
script or commit trigger of some sort sends information about this
change to the buildmaster through one of its configured Change
Sources. This notification might arrive via email, or over a network
connection (either initiated by the buildmaster as it ``subscribes''
to changes, or by the commit trigger as it pushes Changes towards the
buildmaster). The Change contains information about who made the
change, what files were modified, which revision contains the change,
and any checkin comments.

@item
The buildmaster distributes this change to all of its configured
Schedulers. Any ``important'' changes cause the ``tree-stable-timer''
to be started, and the Change is added to a list of those that will go
into a new Build. When the timer expires, a Build is started on each
of a set of configured Builders, all compiling/testing the same source
code. Unless configured otherwise, all Builds run in parallel on the
various buildslaves.

@item
The Build consists of a series of Steps. Each Step causes some number
of commands to be invoked on the remote buildslave associated with
that Builder. The first step is almost always to perform a checkout of
the appropriate revision from the same VC system that produced the
Change. The rest generally perform a compile and run unit tests. As
each Step runs, the buildslave reports back command output and return
status to the buildmaster.

@item
As the Build runs, status messages like ``Build Started'', ``Step
Started'', ``Build Finished'', etc, are published to a collection of
Status Targets. One of these targets is usually the HTML ``Waterfall''
display, which shows a chronological list of events, and summarizes
the results of the most recent build at the top of each column.
Developers can periodically check this page to see how their changes
have fared. If they see red, they know that they've made a mistake and
need to fix it. If they see green, they know that they've done their
duty and don't need to worry about their change breaking anything.

@item
If a MailNotifier status target is active, the completion of a build
will cause email to be sent to any developers whose Changes were
incorporated into this Build. The MailNotifier can be configured to
only send mail upon failing builds, or for builds which have just
transitioned from passing to failing. Other status targets can provide
similar real-time notification via different communication channels,
like IRC.

@end itemize



