Extend NixOS: Difference between revisions

The simplest way to implement this is to add a simple snippet of code to the <code>/etc/nixos/configuration.nix</code> file:

  {pkgs, ...}:

   

     description = "Start the irc client of username.";

     startOn = "started network-interfaces";

   };

   

   # ... usual configuration ...

  }

What does this do? The <code>jobs.ircSession</code> bit is an option which adds a new system service. This option is defined elsewhere in the NixOS configuration files. This is one of many options; you can see a list of all NixOS configuration options in the [https://nixos.org/nixos/manual/options.html NixOS Manual: List of Options] (or use https://nixos.org/nixos/options.html to search for options). We add attributes to this option to configure our new service. As you can see, we configure it to start when the network connects, and to execute a shell command.

NixOS is now using systemd by default, and thus the way to define nix modules has changed.  Here's the commented equivalent of the snippet above:

   {

     systemd.services.ircSession = {

         Type = "forking";

         User = "username";

       };

     };

   }

== Conditional Implementation ==

We can use the <code>mkIf</code> function in the <code>configuration.nix</code> file to add conditional behavior. Here's the new implementation:

  {config, pkgs, ...}:

   

       description = "Start the irc client of username.";

       startOn = "started network-interfaces";

     };

   };

   # ... usual configuration ...

  }

This works, but if we use too many conditionals, our code will become difficult to read and modify. For example, what do we do when we want to change the hostname?

If we move the IRC stuff into the <code>irc-client.nix</code> file, we change the <code>configuration.nix</code> file like this:

  {

   imports = [

   # ... usual configuration ...

  }

The <code>irc-client.nix</code> file will, of course, look like this:

  {config, pkgs, ...}:

   

     description = "Start the irc client of username.";

     startOn = "started network-interfaces";

   };

   

   security.sudo.enable = true;

  }

If we organize our configuration like this, sharing it across machines is easier. In addition, our IRC client can be consistent across machines that choose to use it.

NixOS supports this idea, but it is called "options". We can add options to our module for both the condition and the username. Here is what a <code>irc-client.nix</code> module with parameters/options looks like:

  {config, pkgs, ...}:

   

   };

  }

This module is now independent of the system. Now, we must update our <code>configuration.nix</code> file to pass our condition and hostname into our new module.

{config, ...}:

   # ... usual configuration ...

  }

== Multiple Daemons ==

Now, suppose your server has multiple people as users, and each wants the same IRC client running in background. One possibility is to replace the user name with a list of user names.  A better solution, for this tutorial, is to '''extend''' the <code>jobs</code> option. This will allow us to add "IRC jobs", each of which is for a specific user. This should create configuration options like this:

The following code will add more options inside the <code>jobs.<name></code> option. This code is quite advanced, using many concepts from functional language paradigm.

  {config, pkgs, ...}:

   

   };

  }

= Testing Configuration Changes in a VM =

To see how this works, create a file like this:

  {config, pkgs, ...}:

  {

   };

  }

Then, we build the new configuration inside a VM. If we named the above file <code>my-new-service.nix</code>, we can use these commands:

  $ NIXOS_CONFIG=`pwd`/vmtest.nix nixos-rebuild  -I nixos=/path/to/nixos/ build-vm

  $ ./result/bin/run-vmhost-vm

= What Next? =