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<chapter id='chap-build-farm'><title>Setting up a Build Farm</title>
<para>This chapter provides some sketchy information on how to set up
a Nix-based build farm. Nix is particularly suited as a basis for a
build farm, since:
<itemizedlist>
<listitem><para>Nix supports distributed builds: a local Nix
installation can forward Nix builds to other machines over the
network. This allows multiple builds to be performed in parallel
(thus improving performance), but more in importantly, it allows Nix
to perform multi-platform builds in a semi-transparent way. For
instance, if you perform a build for a
<literal>powerpc-darwin</literal> on an
<literal>i686-linux</literal> machine, Nix can automatically forward
to build to a <literal>powerpc-darwin</literal> machine, if
available.</para></listitem>
<listitem><para>The Nix expression language is ideal for providing
build jobs, plus all their dependencies. For instance, if your
package has some dependency, you don't have to manually install it
on all the machines in the build farm; they will be built
automatically.</para></listitem>
<listitem><para>Proper release management requires that builds (if
deployed) are traceable: it should be possible to figure out from
exactly what sources they were built, in what configuration, etc.;
and it should be possible to reproduce the build, if necessary.
Nix's hashing scheme uniquely identifies builds, and Nix expressions
are self-contained.</para></listitem>
<listitem><para>Nix will only rebuild things that have actually
changed. For instance, if the sources of a component haven't
changed between runs of the build farm, the component won't be
rebuild (unless it was garbage-collected). Also, dependencies
typically don't change very often, so they only need to be built
once.</para></listitem>
<listitem><para>The results of a Nix build farm can be made
available through a channel, so successful builds can be deployed to
users immediately.</para></listitem>
</itemizedlist>
</para>
<section><title>Overview</title>
<para>TODO</para>
<para>The sources of the Nix build farm are at <ulink
url='https://svn.cs.uu.nl:12443/repos/trace/release/trunk' />.</para>
</section>
<section><title>Setting up distributed builds</title>
<para>You can enable distributed builds by setting the environment
variable <envar>NIX_BUILD_HOOK</envar> to point to a program that Nix
will call whenever it wants to build a derivation. The build hook
(typically a shell or Perl script) can decline the build, in which Nix
will perform it in the usual way if possible, or it can accept it, in
which case it is responsible for somehow getting the inputs of the
build to another machine, doing the build there, and getting the
results back.</para>
<example id='ex-remote-systems'><title>Remote machine configuration:
<filename>remote-systems.conf</filename></title>
<programlisting>
nix@mcflurry.labs.cs.uu.nl powerpc-darwin /home/nix/.ssh/id_quarterpounder_auto 2
nix@scratchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 1
</programlisting>
</example>
<para>An example build hook can be found in the Nix build farm
sources: <ulink
url='https://svn.cs.uu.nl:12443/repos/trace/release/trunk/common/distributed/build-remote.pl'
/>. It should be suitable for most purposes, with maybe some minor
adjustments. It uses <command>ssh</command> and
<command>rsync</command> to copy the build inputs and outputs and
perform the remote build. You should define a list of available build
machines and set the environment variable
<envar>REMOTE_SYSTEMS</envar> to point to it. An example
configuration is shown in <xref linkend='ex-remote-systems' />. Each
line in the file specifies a machine, with the following bits of
information:
<orderedlist>
<listitem><para>The name of the remote machine, with optionally the
user under which the remote build should be performed. This is
actually passed as an argument to <command>ssh</command>, so it can
be an alias defined in your
<filename>~/.ssh/config</filename>.</para></listitem>
<listitem><para>The Nix platform type identifier, such as
<literal>powerpc-darwin</literal>.</para></listitem>
<listitem><para>The SSH private key to be used to log in to the
remote machine. Since builds should be non-interactive, this key
should not have a passphrase!</para></listitem>
<listitem><para>The maximum <quote>load</quote> of the remote
machine. This is just the maximum number of jobs that
<filename>build-remote.pl</filename> will execute in parallel on the
machine. Typically this should be equal to the number of
CPUs.</para></listitem>
</orderedlist>
You should also set up the environment variable
<envar>CURRENT_LOAD</envar> to point at a file that
<filename>build-remote.pl</filename> uses to remember how many jobs it
is currently executing remotely. It doesn't look at the actual load
on the remote machine, so if you have multiple instances of Nix
running, they should use the same <envar>CURRENT_LOAD</envar>
file<footnote><para>Although there are probably some race conditions
in the script right now.</para></footnote>. Maybe in the future
<filename>build-remote.pl</filename> will look at the actual remote
load. The load file should exist, so you should just create it as an
empty file initially.</para>
</section>
</chapter>
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