wantedBy = [ "multi-user.target" ];
    after = [
      "network.target"
      "postgresql.target"
    ];
    # note that if you are connecting to a postgres instance on a different host
    # postgresql.target should not be included in the requires.
    requires = [
      "network-online.target"
      "postgresql.target"
    ];
    description = "my app";
    environment = {
      # RELEASE_TMP is used to write the state of the
      # VM configuration when the system is running
      # it needs to be a writable directory
      RELEASE_TMP = working_directory;
      # can be generated in an elixir console with
      # Base.encode32(:crypto.strong_rand_bytes(32))
      RELEASE_COOKIE = "my_cookie";
      MY_VAR = "my_var";
    };
    serviceConfig = {
      Type = "exec";
      DynamicUser = true;
      WorkingDirectory = working_directory;
      # Implied by DynamicUser, but just to emphasize due to RELEASE_TMP
      PrivateTmp = true;
      ExecStart = ''
        ${release}/bin/${release_name} start
      '';
      ExecStop = ''
        ${release}/bin/${release_name} stop
      '';
      ExecReload = ''
        ${release}/bin/${release_name} restart
      '';
      Restart = "on-failure";
      RestartSec = 5;
    };
    unitConfig = {
      StartLimitBurst = 3;
      StartLimitInterval = 10;
    };
    # disksup requires bash
    path = [ pkgs.bash ];
  };

  # in case you have migration scripts or you want to use a remote shell
  environment.systemPackages = [ release ];
}
```

## How to Develop {#how-to-develop}

### Creating a Shell {#creating-a-shell}

Usually, we need to create a `shell.nix` file and do our development inside the environment specified therein. Just install your version of Erlang and any other interpreters, and then use your normal build tools. As an example, with Elixir:

```nix
{
  pkgs ? import <nixpkgs> { },
}:

with pkgs;
let
  elixir = beam.packages.erlang_27.elixir_1_18;
in
mkShell { buildInputs = [ elixir ]; }
```

### Using an overlay {#beam-using-overlays}

If you need to use an overlay to change some attributes of a derivation, e.g. if you need a bugfix from a version that is not yet available in Nixpkgs, you can override attributes such as `version` (and the corresponding `hash`) and then use this overlay in your development environment:

#### `shell.nix` {#beam-using-overlays-shell.nix}