== 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>>

== 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
{ ... }: {
  environment.systemPackages = [ (import (builtins.fetchTarball https://github.com/hercules-ci/arion/tarball/master) {}).arion ];
  virtualisation.docker.enable = true;
  users.extraUsers.myuser.extraGroups = ["docker"];
}
```

////

== 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, it's contents can be as simple as:

```nix
import <nixpkgs> {}
```

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
{ pkgs, ... }:
{
  config.docker-compose.services = {

    webserver = {
      service.useHostStore = true;
      service.command = [ "sh" "-c" ''
                  cd "$$WEB_ROOT"
                  ${pkgs.python3}/bin/python -m http.server
                '' ];
      service.ports = [
        "8000:8000" # host:container
      ];
      service.environment.WEB_ROOT = "${pkgs.nix.doc}/share/doc/nix/manual";
    };
  };
}


```

==== NixOS: run only one systemd service

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

```nix

{
  docker-compose.services.webserver = { config, pkgs, ... }: {

    nixos.configuration = {config, pkgs, ...}: {
      boot.isContainer = true;
      services.nginx.enable = true;
      services.nginx.virtualHosts.localhost.root = "${pkgs.nix.doc}/share/doc/nix/manual";
      system.build.run-nginx = pkgs.writeScript "run-nginx" ''
        #!${pkgs.bash}/bin/bash
        PATH='${config.systemd.services.nginx.environment.PATH}'
        echo nginx:x:${toString config.users.users.nginx.uid}:${toString config.users.groups.nginx.gid}:nginx web server user:/var/empty:/bin/sh >>/etc/passwd
        echo nginx:x:${toString config.users.groups.nginx.gid}:nginx >>/etc/group
        ${config.systemd.services.nginx.runner}
      '';
    };
    service.command = [ config.nixos.build.run-nginx ];
    service.useHostStore = true;
    service.ports = [
      "8000:80" # host:container
    ];
  };
}

```

==== NixOS: run full OS

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

```nix
{
  docker-compose.services.webserver = { pkgs, ... }: {
    nixos.useSystemd = true;
    nixos.configuration.boot.tmpOnTmpfs = true;
    nixos.configuration.services.nginx.enable = true;
    nixos.configuration.services.nginx.virtualHosts.localhost.root = "${pkgs.nix.doc}/share/doc/nix/manual";
    service.useHostStore = true;
    service.ports = [
      "8000:80" # host:container
    ];
  };
}
```

==== Docker image from DockerHub

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

=== 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.

== A full featured Nix command example

To see how Arion can be used in a project, have a look at
https://github.com/nix-community/todomvc-nix/tree/master/deploy/arion[todomvc-nix].

```bash
$ git clone https://github.com/nix-community/todomvc-nix
$ cd todomvc-nix/deploy/arion
$ arion up
```

== 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.)

If you do want to use Arion for production environments, you’ll probably
want to either build normal container images or manage garbage
collection roots if you control the deployment host. Neither scenario is
made easier by arion at this time.

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
link: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.

=== What about garbage collection?

Arion removes the need for garbage collecting docker images, delegating
this task to Nix.

Arion creates a garbage collection root and cleans it up after
completing the command. This means that `arion up` without `-d` is safe
with respect to garbage collection. A deployment that is more serious
than local development must leave a GC root on the deployment host. This
use case is not supported as of now.

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

Restart it with `arion restart <name>` or if you've changed the image rebuild
them using `arion up -d --always-recreate-deps <name>`.

=== 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~ 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 feel morally obliged to name our stuff after Greek mythology)