.. _fiveminutes:

===================================================
Buildbot in 5 minutes - a user-contributed tutorial
===================================================

(Ok, maybe 10.)

Buildbot is really an excellent piece of software, however it can be a bit confusing for a newcomer
(like me when I first started looking at it). Typically, at first sight, it looks like a bunch of
complicated concepts that make no sense and whose relationships with each other are unclear. After
some time and some reread, it all slowly starts to be more and more meaningful, until you finally
say "oh!" and things start to make sense. Once you get there, you realize that the documentation is
great, but only if you already know what it's about.

This is what happened to me, at least. Here, I'm going to (try to) explain things in a way that
would have helped me more as a newcomer. The approach I'm taking is more or less the reverse of
that used by the documentation. That is, I'm going to start from the components that do the actual
work (the builders) and go up the chain to the change sources. I hope purists will forgive this
unorthodoxy. Here I'm trying to only clarify the concepts and not go into the details of each
object or property; the documentation explains those quite well.

Installation
------------

I won't cover the installation; both Buildbot master and worker are available as packages for the
major distributions, and in any case the instructions in the official documentation are fine. This
document will refer to Buildbot 0.8.5 which was current at the time of writing, but hopefully the
concepts are not too different in future versions. All the code shown is of course python code, and
has to be included in the master.cfg configuration file.

We won't cover basic things such as how to define the workers, project names, or other
administrative information that is contained in that file; for that, again the official
documentation is fine.

Builders: the workhorses
------------------------

Since Buildbot is a tool whose goal is the automation of software builds, it makes sense to me to
start from where we tell Buildbot how to build our software: the `builder` (or builders, since
there can be more than one).

Simply put, a builder is an element that is in charge of performing some action or sequence of
actions, normally something related to building software (for example, checking out the source, or
``make all``), but it can also run arbitrary commands.

A builder is configured with a list of workers that it can use to carry out its task. The other
fundamental piece of information that a builder needs is, of course, the list of things it has to
do (which will normally run on the chosen worker). In Buildbot, this list of things is represented
as a ``BuildFactory`` object, which is essentially a sequence of steps, each one defining a certain
operation or command.

Enough talk, let's see an example. For this example, we are going to assume that our super software
project can be built using a simple ``make all``, and there is another target ``make packages``
that creates rpm, deb and tgz packages of the binaries. In the real world things are usually more
complex (for example there may be a ``configure`` step, or multiple targets), but the concepts are
the same; it will just be a matter of adding more steps to a builder, or creating multiple
builders, although sometimes the resulting builders can be quite complex.

So to perform a manual build of our project, we would type the following on the command line
(assuming we are at the root of the local copy of the repository):

.. code-block:: bash

    $ make clean    # clean remnants of previous builds
    ...
    $ svn update
    ...
    $ make all
    ...
    $ make packages
    ...
    # optional but included in the example: copy packages to some central machine
    $ scp packages/*.rpm packages/*.deb packages/*.tgz someuser@somehost:/repository
    ...

Here we're assuming the repository is SVN, but again the concepts are the same with git, mercurial
or any other VCS.

Now, to automate this, we create a builder where each step is one of the commands we typed above. A
step can be a shell command object, or a dedicated object that checks out the source code (there
are various types for different repositories, see the docs for more info), or yet something else::

    from buildbot.plugins import steps, util

    # first, let's create the individual step objects

    # step 1: make clean; this fails if the worker has no local copy, but
    # is harmless and will only happen the first time
    makeclean = steps.ShellCommand(name="make clean",
                                   command=["make", "clean"],
                                   description="make clean")

    # step 2: svn update (here updates trunk, see the docs for more
    # on how to update a branch, or make it more generic).
    checkout = steps.SVN(baseURL='svn://myrepo/projects/coolproject/trunk',
                         mode="update",
                         username="foo",
                         password="bar",
                         haltOnFailure=True)

    # step 3: make all
    makeall = steps.ShellCommand(name="make all",
                                 command=["make", "all"],
                                 haltOnFailure=True,
                                 description="make all")

    # step 4: make packages
    makepackages = steps.ShellCommand(name="make packages",
                                      command=["make", "packages"],
                                      haltOnFailure=True,
                                      description="make packages")

    # step 5: upload packages to central server. This needs passwordless ssh
    # from the worker to the server (set it up in advance as part of the worker setup)
    uploadpackages = steps.ShellCommand(
        name="upload packages",
        description="upload packages",
        command="scp packages/*.rpm packages/*.deb packages/*.tgz someuser@somehost:/repository",
        haltOnFailure=True)

    # create the build factory and add the steps to it
    f_simplebuild = util.BuildFactory()
    f_simplebuild.addStep(makeclean)
    f_simplebuild.addStep(checkout)
    f_simplebuild.addStep(makeall)
    f_simplebuild.addStep(makepackages)
    f_simplebuild.addStep(uploadpackages)

    # finally, declare the list of builders. In this case, we only have one builder
    c['builders'] = [
        util.BuilderConfig(name="simplebuild", workernames=['worker1', 'worker2', 'worker3'],
                           factory=f_simplebuild)
    ]

So our builder is called ``simplebuild`` and can run on either of ``worker1``, ``worker2`` or
``worker3``. If our repository has other branches besides trunk, we could create another one or
more builders to build them; in this example, only the checkout step would be different, in that it
would need to check out the specific branch. Depending on how exactly those branches have to be
built, the shell commands may be recycled, or new ones would have to be created if they are
different in the branch. You get the idea. The important thing is that all the builders be named
differently and all be added to the ``c['builders']`` value (as can be seen above, it is a list of
``BuilderConfig`` objects).

Of course the type and number of steps will vary depending on the goal; for example, to just check
that a commit doesn't break the build, we could include just up to the ``make all`` step. Or we
could have a builder that performs a more thorough test by also doing ``make test`` or other
targets. You get the idea. Note that at each step except the very first we use
``haltOnFailure=True`` because it would not make sense to execute a step if the previous one failed
(ok, it wouldn't be needed for the last step, but it's harmless and protects us if one day we add
another step after it).

Schedulers
----------

Now this is all nice and dandy, but who tells the builder (or builders) to run, and when? This is
the job of the `scheduler` which is a fancy name for an element that waits for some event to
happen, and when it does, based on that information, decides whether and when to run a builder (and
which one or ones). There can be more than one scheduler. I'm being purposely vague here because
the possibilities are almost endless and highly dependent on the actual setup, build purposes,
source repository layout and other elements.

So a scheduler needs to be configured with two main pieces of information: on one hand, which
events to react to, and on the other hand, which builder or builders to trigger when those events
are detected. (It's more complex than that, but if you understand this, you can get the rest of the
details from the docs).

A simple type of scheduler may be a periodic scheduler that runs a certain builder (or builders)
when a configurable amount of time has passed. In our example, that's how we would trigger a build
every hour::

    from buildbot.plugins import schedulers

    # define the periodic scheduler
    hourlyscheduler = schedulers.Periodic(name="hourly",
                                          builderNames=["simplebuild"],
                                          periodicBuildTimer=3600)

    # define the available schedulers
    c['schedulers'] = [hourlyscheduler]

That's it. Every hour this ``hourly`` scheduler will run the ``simplebuild`` builder. If we have
more than one builder that we want to run every hour, we can just add them to the ``builderNames``
list when defining the scheduler. Or since multiple schedulers are allowed, other schedulers can be
defined and added to ``c['schedulers']`` in the same way.

Other types of schedulers exist; in particular, there are schedulers that can be more dynamic than
the periodic one. The typical dynamic scheduler is one that learns about changes in a source
repository (generally because some developer checks in some change) and triggers one or more
builders in response to those changes. Let's assume for now that the scheduler "magically" learns
about changes in the repository (more about this later); here's how we would define it::

    from buildbot.plugins import schedulers

    # define the dynamic scheduler
    trunkchanged = schedulers.SingleBranchScheduler(name="trunkchanged",
                                                    change_filter=util.ChangeFilter(branch=None),
                                                    treeStableTimer=300,
                                                    builderNames=["simplebuild"])

    # define the available schedulers
    c['schedulers'] = [trunkchanged]

This scheduler receives changes happening to the repository, and among all of them, pays attention
to those happening in "trunk" (that's what ``branch=None`` means). In other words, it filters the
changes to react only to those it's interested in. When such changes are detected, and the tree has
been quiet for 5 minutes (300 seconds), it runs the ``simplebuild`` builder. The
``treeStableTimer`` helps in those situations where commits tend to happen in bursts, which would
otherwise result in multiple build requests queuing up.

What if we want to act on two branches (say, trunk and 7.2)? First, we create two builders, one for
each branch, and then we create two dynamic schedulers::

    from buildbot.plugins import schedulers

    # define the dynamic scheduler for trunk
    trunkchanged = schedulers.SingleBranchScheduler(name="trunkchanged",
                                                    change_filter=util.ChangeFilter(branch=None),
                                                    treeStableTimer=300,
                                                    builderNames=["simplebuild-trunk"])

    # define the dynamic scheduler for the 7.2 branch
    branch72changed = schedulers.SingleBranchScheduler(
        name="branch72changed",
        change_filter=util.ChangeFilter(branch='branches/7.2'),
        treeStableTimer=300,
        builderNames=["simplebuild-72"])

    # define the available schedulers
    c['schedulers'] = [trunkchanged, branch72changed]

The syntax of the change filter is VCS-dependent (above is for SVN), but again, once the idea is
clear, the documentation has all the details. Another feature of the scheduler is that it can be
told which changes, within those it's paying attention to, are important and which are not. For
example, there may be a documentation directory in the branch the scheduler is watching, but
changes under that directory should not trigger a build of the binary. This finer filtering is
implemented by means of the ``fileIsImportant`` argument to the scheduler (full details in the docs
and - alas - in the sources).

Change sources
--------------

Earlier, we said that a dynamic scheduler "magically" learns about changes; the final piece of the
puzzle is `change sources`, which are precisely the elements in Buildbot whose task is to detect
changes in a repository and communicate them to the schedulers. Note that periodic schedulers don't
need a change source since they only depend on elapsed time; dynamic schedulers, on the other hand,
do need a change source.

A change source is generally configured with information about a source repository (which is where
changes happen). A change source can watch changes at different levels in the hierarchy of the
repository, so for example, it is possible to watch the whole repository or a subset of it, or just
a single branch. This determines the extent of the information that is passed down to the
schedulers.

There are many ways a change source can learn about changes; it can periodically poll the
repository for changes, or the VCS can be configured (for example through hook scripts triggered by
commits) to push changes into the change source. While these two methods are probably the most
common, they are not the only possibilities. It is possible, for example, to have a change source
detect changes by parsing an email sent to a mailing list when a commit happens. Yet other methods
exist and the manual again has the details.

To complete our example, here's a change source that polls a SVN repository every 2 minutes::

    from buildbot.plugins import changes, util

    svnpoller = changes.SVNPoller(repourl="svn://myrepo/projects/coolproject",
                                  svnuser="foo",
                                  svnpasswd="bar",
                                  pollInterval=120,
                                  split_file=util.svn.split_file_branches)

    c['change_source'] = svnpoller

This poller watches the whole "coolproject" section of the repository, so it will detect changes in
all the branches. We could have said::

    repourl = "svn://myrepo/projects/coolproject/trunk"

or::

    repourl = "svn://myrepo/projects/coolproject/branches/7.2"

to watch only a specific branch.

To watch another project, you need to create another change source, and you need to filter changes
by project. For instance, when you add a change source watching project 'superproject' to the above
example, you need to change the original scheduler from::

    trunkchanged = schedulers.SingleBranchScheduler(
        name="trunkchanged",
        change_filter=filter.ChangeFilter(branch=None),
        # ...
        )

to e.g.::

    trunkchanged = schedulers.SingleBranchScheduler(
        name="trunkchanged",
        change_filter=filter.ChangeFilter(project="coolproject", branch=None),
        # ...
        )

otherwise, coolproject will be built when there's a change in superproject.

Since we're watching more than one branch, we need a method to tell in which branch the change
occurred when we detect one. This is what the ``split_file`` argument does, it takes a callable
that Buildbot will call to do the job. The split_file_branches function, which comes with Buildbot,
is designed for exactly this purpose so that's what the example above uses.

And of course this is all SVN-specific, but there are pollers for all the popular VCSs.

Note that if you have many projects, branches, and builders, it probably pays not to hardcode all
the schedulers and builders in the configuration, but generate them dynamically starting from the
list of all projects, branches, targets, etc, and using loops to generate all possible combinations
(or only the needed ones, depending on the specific setup), as explained in the documentation
chapter about :doc:`../manual/customization/index`.

Reporters
---------

Now that the basics are in place, let's go back to the builders, which is where the real work
happens. `Reporters` are simply the means Buildbot uses to inform the world about what's happening,
that is, how builders are doing. There are many reporters: a mail notifier, an IRC notifier, and
others. They are described fairly well in the manual.

One thing I've found useful is the ability to pass a domain name as the lookup argument to a
``mailNotifier``, which allows you to take an unqualified username as it appears in the SVN change
and create a valid email address by appending the given domain name to it::

    from buildbot.plugins import reporter

    # if jsmith commits a change, an email for the build is sent to jsmith@example.org
    notifier = reporter.MailNotifier(fromaddr="buildbot@example.org",
                                   sendToInterestedUsers=True,
                                   lookup="example.org")
    c['reporters'].append(notifier)

The mail notifier can be customized at will by means of the ``messageFormatter`` argument, which is
a class that Buildbot calls to format the body of the email, and to which it makes available lots
of information about the build. For more details, look into the :ref:`Reporters` section of the
Buildbot manual.

Conclusion
----------

Please note that this article has just scratched the surface; given the complexity of the task of
build automation, the possibilities are almost endless. So there's much much more to say about
Buildbot. Hopefully this has been a gentle introduction before reading the official manual. Had I
found an explanation as the one above when I was approaching Buildbot, I'd have had to read the
manual just once, rather than multiple times. I hope this can help someone else.

(Thanks to Davide Brini for permission to include this tutorial, derived from one he originally posted at http://backreference.org .)
