2020-07-23 00:17:48 +03:00
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# Remote Builds
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Nix supports remote builds, where a local Nix installation can forward
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Nix builds to other machines. This allows multiple builds to be
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performed in parallel and allows Nix to perform multi-platform builds in
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a semi-transparent way. For instance, if you perform a build for a
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`x86_64-darwin` on an `i686-linux` machine, Nix can automatically
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forward the build to a `x86_64-darwin` machine, if available.
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To forward a build to a remote machine, it’s required that the remote
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machine is accessible via SSH and that it has Nix installed. You can
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test whether connecting to the remote Nix instance works, e.g.
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$ nix ping-store --store ssh://mac
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will try to connect to the machine named `mac`. It is possible to
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specify an SSH identity file as part of the remote store URI, e.g.
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$ nix ping-store --store ssh://mac?ssh-key=/home/alice/my-key
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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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`ssh-agent` or `gpg-agent`.
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If you get the error
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bash: nix-store: command not found
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error: cannot connect to 'mac'
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2020-07-23 11:44:54 +03:00
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then you need to ensure that the `PATH` of non-interactive login shells
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2020-07-23 00:17:48 +03:00
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contains Nix.
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> **Warning**
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>
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> If you are building via the Nix daemon, it is the Nix daemon user
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> account (that is, `root`) that should have SSH access to the remote
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> machine. If you can’t or don’t want to configure `root` to be able to
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> access to remote machine, you can use a private Nix store instead by
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> passing e.g. `--store ~/my-nix`.
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The list of remote machines can be specified on the command line or in
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the Nix configuration file. The former is convenient for testing. For
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example, the following command allows you to build a derivation for
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`x86_64-darwin` on a Linux machine:
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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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It is possible to specify multiple builders separated by a semicolon or
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a newline, e.g.
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```
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--builders 'ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd'
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```
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Each machine specification consists of the following elements, separated
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by spaces. Only the first element is required. To leave a field at its
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default, set it to `-`.
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1. The URI of the remote store in the format
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`ssh://[username@]hostname`, e.g. `ssh://nix@mac` or `ssh://mac`.
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For backward compatibility, `ssh://` may be omitted. The hostname
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may be an alias defined in your `~/.ssh/config`.
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2. A comma-separated list of Nix platform type identifiers, such as
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`x86_64-darwin`. It is possible for a machine to support multiple
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platform types, e.g., `i686-linux,x86_64-linux`. If omitted, this
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defaults to the local platform type.
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3. The SSH identity file to be used to log in to the remote machine. If
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omitted, SSH will use its regular identities.
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4. The maximum number of builds that Nix will execute in parallel on
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the machine. Typically this should be equal to the number of CPU
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cores. For instance, the machine `itchy` in the example will execute
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up to 8 builds in parallel.
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5. The “speed factor”, indicating the relative speed of the machine. If
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there are multiple machines of the right type, Nix will prefer the
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fastest, taking load into account.
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6. A comma-separated list of *supported features*. If a derivation has
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the `requiredSystemFeatures` attribute, then Nix will only perform
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the derivation on a machine that has the specified features. For
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instance, the attribute
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requiredSystemFeatures = [ "kvm" ];
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will cause the build to be performed on a machine that has the `kvm`
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feature.
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7. A comma-separated list of *mandatory features*. A machine will only
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be used to build a derivation if all of the machine’s mandatory
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features appear in the derivation’s `requiredSystemFeatures`
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attribute..
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For example, the machine specification
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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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specifies several machines that can perform `i686-linux` builds.
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However, `poochie` will only do builds that have the attribute
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requiredSystemFeatures = [ "benchmark" ];
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or
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requiredSystemFeatures = [ "benchmark" "kvm" ];
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`itchy` cannot do builds that require `kvm`, but `scratchy` does support
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such builds. For regular builds, `itchy` will be preferred over
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`scratchy` because it has a higher speed factor.
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Remote builders can also be configured in `nix.conf`, e.g.
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builders = ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd
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Finally, remote builders can be configured in a separate configuration
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file included in `builders` via the syntax `@file`. For example,
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builders = @/etc/nix/machines
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causes the list of machines in `/etc/nix/machines` to be included. (This
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is the default.)
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If you want the builders to use caches, you likely want to set the
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option [`builders-use-substitutes`](#conf-builders-use-substitutes) in
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your local `nix.conf`.
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To build only on remote builders and disable building on the local
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machine, you can use the option `--max-jobs 0`.
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