# Fly.io

`Dotenvy` can be used when deploying an app to [Fly.io](https://fly.io/); other hosting services may require customizations similar to the ones demonstrated here. Following the instructions on the [Phoenix](docs/phoenix.md) page will get you most of the way there, but you also need to modify the `Dockerfile` so it copies over your `envs/` directory: the `.env` files must be available to the `release` command. The default Fly.io Dockerfile needs a small alteration to ensure that your `.env` files are along for the ride.

## Dockerfile

A `Dockerfile` gets generated when you setup your app (it is ultimately generated from `mix phx.gen.release --docker`). The `Dockerfile` is ultimately what is responsible for running `mix release`, so you need to ensure that all of your files and folders are copied into the container.

In the examples provided in this documentation, `.env` files are stored in a directory named `envs/`.  If that's the directory housing your `.env` files, then you will need to ensure that folder gets copied into the Docker container so it can properly build the release.  To do this, you will need to add a line `COPY envs envs` _before_ the `Run mix release` command.

```docker
# ... existing Docker stuff...

# Compile the release
RUN mix compile

# Changes to config/runtime.exs don't require recompiling the code
COPY config/runtime.exs config/

COPY rel rel

# <---- make the directory of envs available to the release!!!
COPY envs envs

RUN mix release

# ... existing Docker stuff cont'd...
```

Once that line is there, then running `fly deploy` and other `flyctl` commands should be able to build the release and include the `.env` files as expected. You can test the standard GitHub workflows by pushing changes to `main`.

## The importance of path names

Because your app executes inside a Docker container, it is really important to use `Path.absname/2` to reference your `.env` files. The reason we mention this _ad nauseam_ in these docs is because it's so critical.  Different release strategies may set the `RELEASE_ROOT` variable with or without a trailing slash, and that can mean that `Dotenvy` can't find your `.env` files.  It can be helpful to specify the `:require_files` option to `Dotenvy.source/2` just to help alert you when the paths may be pointing to the wrong place, e.g.

```elixir
env_dir_prefix = System.get_env("RELEASE_ROOT") || Path.expand("./envs")

source!(
  [
    Path.absname(".env", env_dir_prefix),
    Path.absname(".#{config_env()}.env", env_dir_prefix),
    Path.absname(".#{config_env()}.overrides.env", env_dir_prefix),
    System.get_env()
  ],
  require_files: [Path.absname(".env", env_dir_prefix)]
)
```

## Environment Variables

Fly.io sets a handful of environment variables when it deploys an app (which you can inspect by running `System.get_env()` from an `iex` shell).  The following are the most significant environment variables used in the config files generated by the [Dotenvy Generators](https://hexdocs.pm/dotenvy_generators/):

- `RELEASE_ROOT`
- `DATABASE_URL`
- `PHX_HOST`
- `PORT`
- `PHX_SERVER`
- `DNS_CLUSTER_QUERY`
- `SECRET_KEY_BASE`

See the [Phoenix](docs/phoenix.md) page for getting a Phoenix app to run using `Dotenvy`.