= Welcome to Arion documentation

== Introduction

Arion is a tool for building and running applications that
consist of multiple docker containers using NixOS modules.
It has special support for docker images that are built with Nix,
for a smooth development experience and improved performance.

It is built on top of https://docs.docker.com/compose/overview/[Docker
Compose], which implements the container orchestration functionality.

Instead of configuring the compositions in YAML files like
`docker-compose.yaml`, Arion uses the https://nixos.org/nix/[Nix]
language to declare the compositions. Because of this, Arion gives you
the ability to declare your deployments, configuration and packaging
in the same language. By replacing multiple tools with a single
language, you decrease your mental load and you can more easily
refactor and maintain your configurations.

Although Arion can be used as a Docker Compose with an improved
configuration front end, there is more to be gained from integrating
with Nix. In particular, the more structured approach of Nix compared
to Dockerfiles allows the following:

 * Build components of your image in *parallel, automatically*
 * *Share packages between images*, regardless of the order they were
   added
 * Improve performance by *skipping container
   image creation*
 * Work with *structured data instead of strings*,
   templates and a multitude of expression languages
 * Refactor across deployments, configuration and packaging

Arion allows to compose containers with different granularity:

  * <<Minimal: Plain command using nixpkgs>>
  * <<NixOS: run only one systemd service>>
  * <<NixOS: run full OS>>
  * <<Docker image from DockerHub>>

Full NixOS is supported on

 * docker-compose + podman with docker socket (NixOS >= 21.05)
 * docker-compose + docker, before cgroupsv2 (NixOS < 21.05)

`podman-compose` support is currently WIP on a separate branch.

== Installation

=== Nix

```bash
$ nix-env -iA arion -f https://github.com/hercules-ci/arion/tarball/master
```

=== NixOS

Add this module to your NixOS configuration:

```nix
{ pkgs, ... }: {
  environment.systemPackages = [
    pkgs.arion

     # Do install the docker CLI to talk to podman.
     # Not needed when virtualisation.docker.enable = true;
    pkgs.docker-client
  ];

  # Arion works with Docker, but for NixOS-based containers, you need Podman
  # since NixOS 21.05.
  virtualisation.docker.enable = false;
  virtualisation.podman.enable = true;
  virtualisation.podman.dockerSocket.enable = true;
  virtualisation.podman.defaultNetwork.dnsname.enable = true;

  # Use your username instead of `myuser`
  users.extraUsers.myuser.extraGroups = ["podman"];
}
```

////

== Not installing: use it in a project

TODO: describe: using nix-shell or in a script, building images as
      part of nix-build, pinning, see also todomvc-nix.

TODO: exposed Nix functions: arion.build, arion.eval (a bit of IFD)


////


== Usage

Arion is configured declaratively with two files:

=== arion-pkgs.nix

Arion needs `arion-pkgs.nix` to import nixpkgs, for example:

```nix
import <nixpkgs> { system = "x86_64-linux"; }
```

or more sophisticated (recommended) setup with https://github.com/nmattia/niv[Niv].

=== arion-compose.nix

Describe containers using NixOS-style modules. There are a few options:

==== Minimal: Plain command using nixpkgs

`examples/minimal/arion-compose.nix`
[,nix]
----
include::arion:ROOT:example$minimal/arion-compose.nix[]
----

==== NixOS: run full OS

`examples/full-nixos/arion-compose.nix`:

[,nix]
----
include::arion:ROOT:example$full-nixos/arion-compose.nix[]
----

==== Docker image from DockerHub

```nix
{
  services.postgres = {
    service.image = "postgres:10";
    service.volumes = [ "${toString ./.}/postgres-data:/var/lib/postgresql/data" ];
    service.environment.POSTGRES_PASSWORD = "mydefaultpass";
  };
}
```

==== NixOS: run only one systemd service

Running individual units from NixOS is possible using an experimental script.
See `examples/nixos-unit/arion-compose.nix`.

=== Run

Start containers and watch their logs:

```bash
$ arion up -d
$ arion logs -f
```

You can go to `examples/*/` and run these commands to give it a quick try.

=== Inspect the config

While developing an arion project, you can make use of `arion repl`, which launches
a `nix repl` on the project configuration.

```
$ arion repl
Launching a repl for you. To get started:

To see deployment-wide configuration
  type config. and use tab completion
To bring the top-level Nixpkgs attributes into scope
  type :a (config._module.args.pkgs) // { inherit config; }

Welcome to Nix. Type :? for help.

Loading '../../src/nix/eval-composition.nix'...
Added 5 variables.

nix-repl> config.services.webserver.service.command
[ "sh" "-c" "cd \"$$WEB_ROOT\"\n/nix/store/66fbv9mmx1j4hrn9y06kcp73c3yb196r-python3-3.8.9/bin/python -m http.server\n" ]

nix-repl>

```

== Build with Nix

You can build a project with `nix-build` using an expression like

```nix
arion.build { modules = [ ./arion-compose.nix ]; pkgs = import ./arion-pkgs.nix; }
```

If you deploy with xref:hercules-ci-effects:ROOT:reference/nix-functions/runArion.adoc[runArion],
and your `pkgs` variable is equivalent to `import ./arion-pkgs.nix`, you can use:

```nix
let
  deployment = pkgs.effects.runArion { /* ... */ });
in deployment.prebuilt
```

== Project Status

This project was born out of a process supervision need for local
development environments while working on
https://www.hercules-ci.com[Hercules CI]. (It was also born out of
ancient Greek deities disguised as horses. More on that later.)

Arion can be used for simple single host deployments, using Docker's TLS
client verification, or https://search.nixos.org/options?channel=unstable&show=virtualisation.podman.networkSocket.enable&query=virtualisation.podman[`virtualisation.podman.networkSocket` options].
Remote deployments do not support `useHostStore`, although an SSH-based deployment method could support this.
Docker Swarm is not currently supported.

Arion has run successfully on Linux distributions other than NixOS, but we only perform CI for Arion on NixOS.


== How it works

Arion is essentially a thin wrapper around Nix and docker-compose. When
it runs, it does the following:

* Evaluate the configuration using Nix, producing a
`docker-compose.yaml` and a garbage collection root
* Invoke `docker-compose`
* Clean up the garbage collection root

Most of the interesting stuff happens in Arion’s Nix expressions, where
it runs the module system (known from NixOS) and provides the
configuration that makes the Docker Compose file do the things it needs
to do.

One of the more interesting built-in modules is the
https://github.com/hercules-ci/arion/blob/master/src/nix/modules/service/host-store.nix[host-store.nix module] which
performs the bind mounts to make the host Nix store available in the
container.

== FAQ

=== Do I need to use Hercules CI?

Nope, it’s just Nix and Docker Compose under the hood.

It does xref:hercules-ci-effects:ROOT:reference/nix-functions/runArion.adoc[integrate] nicely though.

=== What about garbage collection?

Arion removes the need for garbage collecting docker images, delegating
this task to Nix when using `service.useHostStore`.

Arion creates a garbage collection root that it cleans up after completing
the command. This means that `arion up -d` should not be used with `useHostStore`
in production. Instead, disable `useHostStore`, which will use `dockerTools` to
generate images that can be used in production.

=== Why is my container not running latest code?

Rebuild the image using `arion up -d --always-recreate-deps <name>` or simply `arion up -d`.

Like `docker-compose restart`, `arion restart` does not update the image before starting.

=== What is messing with my environment variables?

Docker Compose performs its own environment variable substitution. This
can be a little annoying in `services.command` for example. Either
reference a script from `pkgs.writeScript` or escape the dollar sign as
`$$`.

=== Why name it ``Arion``?

Arion comes from Greek mythology. Poseidon, the god of Docker -- I mean the seas --
had his eye on Demeter. Demeter tried to trick him by disguising as a
horse, but Poseidon saw through the deception and they had Arion.

So Arion is a super fast divine horse; the result of some weird mixing.
Also it talks.

(And we felt morally obliged to name our stuff after Greek mythology)
