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190 lines
6.9 KiB
XML
190 lines
6.9 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id='chap-distributed-builds'>
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<title>Remote Builds</title>
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<para>Nix supports remote builds, where a local Nix installation can
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forward Nix builds to other machines. This allows multiple builds to
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be performed in parallel and allows Nix to perform multi-platform
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builds in a semi-transparent way. For instance, if you perform a
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build for a <literal>x86_64-darwin</literal> on an
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<literal>i686-linux</literal> machine, Nix can automatically forward
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the build to a <literal>x86_64-darwin</literal> machine, if
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available.</para>
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<para>To forward a build to a remote machine, it’s required that the
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remote machine is accessible via SSH and that it has Nix
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installed. You can test whether connecting to the remote Nix instance
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works, e.g.
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<screen>
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$ nix ping-store --store ssh://mac
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</screen>
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will try to connect to the machine named <literal>mac</literal>. It is
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possible to specify an SSH identity file as part of the remote store
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URI, e.g.
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<screen>
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$ nix ping-store --store ssh://mac?ssh-key=/home/alice/my-key
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</screen>
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Since builds should be non-interactive, the key should not have a
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passphrase. Alternatively, you can load identities ahead of time into
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<command>ssh-agent</command> or <command>gpg-agent</command>.</para>
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<para>If you get the error
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<screen>
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bash: nix-store: command not found
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error: cannot connect to 'mac'
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</screen>
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then you need to ensure that the <literal>PATH</literal> of
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non-interactive login shells contains Nix.</para>
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<warning><para>If you are building via the Nix daemon, it is the Nix
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daemon user account (that is, <literal>root</literal>) that should
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have SSH access to the remote machine. If you can’t or don’t want to
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configure <literal>root</literal> to be able to access to remote
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machine, you can use a private Nix store instead by passing
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e.g. <literal>--store ~/my-nix</literal>.</para></warning>
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<para>The list of remote machines can be specified on the command line
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or in the Nix configuration file. The former is convenient for
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testing. For example, the following command allows you to build a
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derivation for <literal>x86_64-darwin</literal> on a Linux machine:
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<screen>
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$ uname
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Linux
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$ nix build \
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'(with import <nixpkgs> { system = "x86_64-darwin"; }; runCommand "foo" {} "uname > $out")' \
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--builders 'ssh://mac x86_64-darwin'
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[1/0/1 built, 0.0 MiB DL] building foo on ssh://mac
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$ cat ./result
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Darwin
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</screen>
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It is possible to specify multiple builders separated by a semicolon
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or a newline, e.g.
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<screen>
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--builders 'ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd'
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</screen>
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</para>
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<para>Each machine specification consists of the following elements,
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separated by spaces. Only the first element is required.
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To leave a field at its default, set it to <literal>-</literal>.
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<orderedlist>
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<listitem><para>The URI of the remote store in the format
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<literal>ssh://[<emphasis>username</emphasis>@]<emphasis>hostname</emphasis></literal>,
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e.g. <literal>ssh://nix@mac</literal> or
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<literal>ssh://mac</literal>. For backward compatibility,
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<literal>ssh://</literal> may be omitted. The hostname may be an
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alias defined in your
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<filename>~/.ssh/config</filename>.</para></listitem>
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<listitem><para>A comma-separated list of Nix platform type
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identifiers, such as <literal>x86_64-darwin</literal>. It is
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possible for a machine to support multiple platform types, e.g.,
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<literal>i686-linux,x86_64-linux</literal>. If omitted, this
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defaults to the local platform type.</para></listitem>
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<listitem><para>The SSH identity file to be used to log in to the
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remote machine. If omitted, SSH will use its regular
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identities.</para></listitem>
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<listitem><para>The maximum number of builds that Nix will execute
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in parallel on the machine. Typically this should be equal to the
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number of CPU cores. For instance, the machine
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<literal>itchy</literal> in the example will execute up to 8 builds
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in parallel.</para></listitem>
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<listitem><para>The “speed factor”, indicating the relative speed of
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the machine. If there are multiple machines of the right type, Nix
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will prefer the fastest, taking load into account.</para></listitem>
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<listitem><para>A comma-separated list of <emphasis>supported
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features</emphasis>. If a derivation has the
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<varname>requiredSystemFeatures</varname> attribute, then Nix will
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only perform the derivation on a machine that has the specified
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features. For instance, the attribute
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<programlisting>
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requiredSystemFeatures = [ "kvm" ];
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</programlisting>
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will cause the build to be performed on a machine that has the
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<literal>kvm</literal> feature.</para></listitem>
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<listitem><para>A comma-separated list of <emphasis>mandatory
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features</emphasis>. A machine will only be used to build a
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derivation if all of the machine’s mandatory features appear in the
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derivation’s <varname>requiredSystemFeatures</varname>
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attribute..</para></listitem>
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</orderedlist>
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For example, the machine specification
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<programlisting>
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nix@scratchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 8 1 kvm
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nix@itchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 8 2
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nix@poochie.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 1 2 kvm benchmark
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</programlisting>
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specifies several machines that can perform
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<literal>i686-linux</literal> builds. However,
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<literal>poochie</literal> will only do builds that have the attribute
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<programlisting>
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requiredSystemFeatures = [ "benchmark" ];
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</programlisting>
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or
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<programlisting>
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requiredSystemFeatures = [ "benchmark" "kvm" ];
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</programlisting>
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<literal>itchy</literal> cannot do builds that require
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<literal>kvm</literal>, but <literal>scratchy</literal> does support
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such builds. For regular builds, <literal>itchy</literal> will be
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preferred over <literal>scratchy</literal> because it has a higher
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speed factor.</para>
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<para>Remote builders can also be configured in
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<filename>nix.conf</filename>, e.g.
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<programlisting>
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builders = ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd
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</programlisting>
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Finally, remote builders can be configured in a separate configuration
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file included in <option>builders</option> via the syntax
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<literal>@<emphasis>file</emphasis></literal>. For example,
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<programlisting>
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builders = @/etc/nix/machines
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</programlisting>
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causes the list of machines in <filename>/etc/nix/machines</filename>
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to be included. (This is the default.)</para>
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<para>If you want the builders to use caches, you likely want to set
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the option <link linkend='conf-builders-use-substitutes'><literal>builders-use-substitutes</literal></link>
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in your local <filename>nix.conf</filename>.</para>
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<para>To build only on remote builders and disable building on the local machine,
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you can use the option <option>--max-jobs 0</option>.</para>
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</chapter>
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