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doc: update mkBinaryCache section with admonitions and conventions

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The mkBinaryCache section now follows the convention of using one
line per sentence, as well as using admonitions for notes and
examples.

The example syntax was updated to define a fully-working nix package.
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doc/build-helpers/images/binarycache.section.md
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# pkgs.mkBinaryCache {#sec-pkgs-binary-cache}
`pkgs.mkBinaryCache` is a function for creating Nix flat-file binary caches. Such a cache exists as a directory on disk, and can be used as a Nix substituter by passing `--substituter file:///path/to/cache` to Nix commands.
`pkgs.mkBinaryCache` is a function for creating Nix flat-file binary caches.
Such a cache exists as a directory on disk, and can be used as a Nix substituter by passing `--substituter file:///path/to/cache` to Nix commands.
Nix packages are most commonly shared between machines using [HTTP, SSH, or S3](https://nixos.org/manual/nix/stable/package-management/sharing-packages.html), but a flat-file binary cache can still be useful in some situations. For example, you can copy it directly to another machine, or make it available on a network file system. It can also be a convenient way to make some Nix packages available inside a container via bind-mounting.
Nix packages are most commonly shared between machines using [HTTP, SSH, or S3](https://nixos.org/manual/nix/stable/package-management/sharing-packages.html), but a flat-file binary cache can still be useful in some situations.
For example, you can copy it directly to another machine, or make it available on a network file system.
It can also be a convenient way to make some Nix packages available inside a container via bind-mounting.
Note that this function is meant for advanced use-cases. The more idiomatic way to work with flat-file binary caches is via the [nix-copy-closure](https://nixos.org/manual/nix/stable/command-ref/nix-copy-closure.html) command. You may also want to consider [dockerTools](#sec-pkgs-dockerTools) for your containerization needs.
`mkBinaryCache` expects an argument with the `rootPaths` attribute.
`rootPaths` must be a list of derivations.
The transitive closure of these derivations' outputs will be copied into the cache.
## Example {#sec-pkgs-binary-cache-example}
::: {.note}
This function is meant for advanced use cases.
The more idiomatic way to work with flat-file binary caches is via the [nix-copy-closure](https://nixos.org/manual/nix/stable/command-ref/nix-copy-closure.html) command.
You may also want to consider [dockerTools](#sec-pkgs-dockerTools) for your containerization needs.
:::
[]{#sec-pkgs-binary-cache-example}
:::{.example #ex-mkbinarycache-copying-package-closure}
# Copying a package and its closure to another machine with `mkBinaryCache`
The following derivation will construct a flat-file binary cache containing the closure of `hello`.
```nix
{ mkBinaryCache, hello }:
mkBinaryCache {
rootPaths = [hello];
}
```
- `rootPaths` specifies a list of root derivations. The transitive closure of these derivations' outputs will be copied into the cache.
Here's an example of building and using the cache.
Build the cache on one machine, `host1`:
Build the cache on a machine.
Note that the command still builds the exact nix package above, but adds some boilerplate to build it directly from an expression.
```shellSession
nix-build -E 'with import <nixpkgs> {}; mkBinaryCache { rootPaths = [hello]; }'
$ nix-build -E 'let pkgs = import <nixpkgs> {}; in pkgs.callPackage ({ mkBinaryCache, hello }: mkBinaryCache { rootPaths = [hello]; }) {}'
/nix/store/azf7xay5xxdnia4h9fyjiv59wsjdxl0g-binary-cache
```
Copy the resulting directory to another machine, which we'll call `host2`:
```shellSession
/nix/store/cc0562q828rnjqjyfj23d5q162gb424g-binary-cache
$ scp result host2:/tmp/hello-cache
```
Copy the resulting directory to the other machine, `host2`:
At this point, the cache can be used as a substituter when building derivations on `host2`:
```shellSession
scp result host2:/tmp/hello-cache
```
Substitute the derivation using the flat-file binary cache on the other machine, `host2`:
```shellSession
nix-build -A hello '<nixpkgs>' \
$ nix-build -A hello '<nixpkgs>' \
--option require-sigs false \
--option trusted-substituters file:///tmp/hello-cache \
--option substituters file:///tmp/hello-cache
/nix/store/zhl06z4lrfrkw5rp0hnjjfrgsclzvxpm-hello-2.12.1
```
```shellSession
/nix/store/gl5a41azbpsadfkfmbilh9yk40dh5dl0-hello-2.12.1
```
:::