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extract reference documentation on remote builds
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- move all reference documentation to the `builders` configuration setting
- reword documentation on machine specification, add examples
- disable showing the default value, as it rendered as `@/dummy/machines`, which is wrong
- highlight the examples
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@Janik-Haag @fricklerhandwerk
Janik-Haag authored and fricklerhandwerk committed Dec 2, 2023
1 parent 84fe429 commit b1cf58a
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100 changes: 8 additions & 92 deletions doc/manual/src/advanced-topics/distributed-builds.md
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Expand Up @@ -36,16 +36,9 @@ error: cannot connect to 'mac'
then you need to ensure that the `PATH` of non-interactive login shells
contains Nix.

> **Warning**
>
> If you are building via the Nix daemon, it is the Nix daemon user account (that is, `root`) that should have SSH access to a user (not necessarily `root`) on the remote machine.
>
> If you can’t or don’t want to configure `root` to be able to access the remote machine, you can use a private Nix store instead by passing e.g. `--store ~/my-nix` when running a Nix command from the local machine.
The list of remote machines can be specified on the command line or in
the Nix configuration file. The former is convenient for testing. For
example, the following command allows you to build a derivation for
`x86_64-darwin` on a Linux machine:
The [list of remote machines](@docroot@/command-ref/conf-file.md#conf-builders) can be specified on the command line or in the Nix configuration file.
The former is convenient for testing.
For example, the following command allows you to build a derivation for `x86_64-darwin` on a Linux machine:

```console
$ uname
Expand All @@ -60,97 +53,20 @@ $ cat ./result
Darwin
```

It is possible to specify multiple builders separated by a semicolon or
a newline, e.g.
It is possible to specify multiple builders separated by a semicolon or a newline, e.g.

```console
--builders 'ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd'
```

Each machine specification consists of the following elements, separated
by spaces. Only the first element is required. To leave a field at its
default, set it to `-`.

1. The URI of the remote store in the format
`ssh://[username@]hostname`, e.g. `ssh://nix@mac` or `ssh://mac`.
For backward compatibility, `ssh://` may be omitted. The hostname
may be an alias defined in your `~/.ssh/config`.

2. A comma-separated list of Nix platform type identifiers, such as
`x86_64-darwin`. It is possible for a machine to support multiple
platform types, e.g., `i686-linux,x86_64-linux`. If omitted, this
defaults to the local platform type.

3. The SSH identity file to be used to log in to the remote machine. If
omitted, SSH will use its regular identities.

4. The maximum number of builds that Nix will execute in parallel on
the machine. Typically this should be equal to the number of CPU
cores. For instance, the machine `itchy` in the example will execute
up to 8 builds in parallel.

5. The “speed factor”, indicating the relative speed of the machine. If
there are multiple machines of the right type, Nix will prefer the
fastest, taking load into account.

6. A comma-separated list of *supported features*. If a derivation has
the `requiredSystemFeatures` attribute, then Nix will only perform
the derivation on a machine that has the specified features. For
instance, the attribute

```nix
requiredSystemFeatures = [ "kvm" ];
```
will cause the build to be performed on a machine that has the `kvm`
feature.
7. A comma-separated list of *mandatory features*. A machine will only
be used to build a derivation if all of the machine’s mandatory
features appear in the derivation’s `requiredSystemFeatures`
attribute.
8. The (base64-encoded) public host key of the remote machine. If omitted, SSH
will use its regular known-hosts file. Specifically, the field is calculated
via `base64 -w0 /etc/ssh/ssh_host_ed25519_key.pub`.
For example, the machine specification
nix@scratchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 8 1 kvm
nix@itchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 8 2
nix@poochie.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy_auto 1 2 kvm benchmark
specifies several machines that can perform `i686-linux` builds.
However, `poochie` will only do builds that have the attribute
```nix
requiredSystemFeatures = [ "benchmark" ];
```

or

```nix
requiredSystemFeatures = [ "benchmark" "kvm" ];
```

`itchy` cannot do builds that require `kvm`, but `scratchy` does support
such builds. For regular builds, `itchy` will be preferred over
`scratchy` because it has a higher speed factor.

Remote builders can also be configured in `nix.conf`, e.g.
Remote builders can also be configured in [`nix.conf`](@docroot@/command-ref/conf-file.md), e.g.

builders = ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd

Finally, remote builders can be configured in a separate configuration
file included in `builders` via the syntax `@file`. For example,
file included in `builders` via the syntax `@/path/to/file`. For example,

builders = @/etc/nix/machines

causes the list of machines in `/etc/nix/machines` to be included. (This
is the default.)

If you want the builders to use caches, you likely want to set the
option `builders-use-substitutes` in your local `nix.conf`.

To build only on remote builders and disable building on the local
machine, you can use the option `--max-jobs 0`.
causes the list of machines in `/etc/nix/machines` to be included.
(This is the default.)
118 changes: 115 additions & 3 deletions src/libstore/globals.hh
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Expand Up @@ -261,9 +261,121 @@ public:
Setting<std::string> builders{
this, "@" + nixConfDir + "/machines", "builders",
R"(
A semicolon-separated list of build machines.
For the exact format and examples, see [the manual chapter on remote builds](../advanced-topics/distributed-builds.md)
)"};
A semicolon- or newline-separated list of build machines.
In addition to the [usual ways of setting configuration options](@docroot@/command-ref/conf-file.md), the value can be read from a file by prefixing its absolute path with `@`.
> **Example**
>
> This is the default setting:
>
> ```
> builders = @/etc/nix/machines
> ```
Each machine specification consists of the following elements, separated by spaces.
Only the first element is required.
To leave a field at its default, set it to `-`.
1. The URI of the remote store in the format `ssh://[username@]hostname`.
> **Example**
>
> `ssh://nix@mac`
For backward compatibility, `ssh://` may be omitted.
The hostname may be an alias defined in `~/.ssh/config`.
2. A comma-separated list of [Nix system types](@docroot@/contributing/hacking.md#system-type).
If omitted, this defaults to the local platform type.
> **Example**
>
> `aarch64-darwin`
It is possible for a machine to support multiple platform types.
> **Example**
>
> `i686-linux,x86_64-linux`
3. The SSH identity file to be used to log in to the remote machine.
If omitted, SSH will use its regular identities.
> **Example**
>
> `/home/user/.ssh/id_mac`
4. The maximum number of builds that Nix will execute in parallel on the machine.
Typically this should be equal to the number of CPU cores.
5. The “speed factor”, indicating the relative speed of the machine as a positive integer.
If there are multiple machines of the right type, Nix will prefer the fastest, taking load into account.
6. A comma-separated list of supported [system features](#conf-system-features).
A machine will only be used to build a derivation if all the features in the derivation's [`requiredSystemFeatures`](@docroot@/language/advanced-attributes.html#adv-attr-requiredSystemFeatures) attribute are supported by that machine.
7. A comma-separated list of required [system features](#conf-system-features).
A machine will only be used to build a derivation if all of the machine’s required features appear in the derivation’s [`requiredSystemFeatures`](@docroot@/language/advanced-attributes.html#adv-attr-requiredSystemFeatures) attribute.
8. The (base64-encoded) public host key of the remote machine.
If omitted, SSH will use its regular `known_hosts` file.
The value for this field can be obtained via `base64 -w0`.
> **Example**
>
> Multiple builders specified on the command line:
>
> ```console
> --builders 'ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd'
> ```
> **Example**
>
> This specifies several machines that can perform `i686-linux` builds:
>
> ```
> nix@scratchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy 8 1 kvm
> nix@itchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy 8 2
> nix@poochie.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy 1 2 kvm benchmark
> ```
>
> However, `poochie` will only build derivations that have the attribute
>
> ```nix
> requiredSystemFeatures = [ "benchmark" ];
> ```
>
> or
>
> ```nix
> requiredSystemFeatures = [ "benchmark" "kvm" ];
> ```
>
> `itchy` cannot do builds that require `kvm`, but `scratchy` does support such builds.
> For regular builds, `itchy` will be preferred over `scratchy` because it has a higher speed factor.
For Nix to use substituters, the calling user must be in the [`trusted-users`](#conf-trusted-users) list.
> **Note**
>
> A build machine must be accessible via SSH and have Nix installed.
> `nix` must be available in `$PATH` for the user connecting over SSH.
> **Warning**
>
> If you are building via the Nix daemon (default), the Nix daemon user account on the local machine (that is, `root`) requires access to a user account on the remote machine (not necessarily `root`).
>
> If you can’t or don’t want to configure `root` to be able to access the remote machine, set [`store`](#conf-store) to any [local store](store/types/local-store.html), e.g. by passing `--store /tmp` to the command on the local machine.
To build only on remote machines and disable local builds, set [`max-jobs`](#conf-max-jobs) to 0.
If you want the remote machines to use substituters, set [`builders-use-substitutes`](#conf-builders-use-substituters) to `true`.
)",
{}, false};

Setting<bool> alwaysAllowSubstitutes{
this, false, "always-allow-substitutes",
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