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How to test a NixOS configuration change

TLDR: push a commit to the testing-<hostname> branch (rebased on the main branch) to deploy a change to the machine named <hostname>.

By default, each machine pulls configuration from the branch testing-<hostname>. When this branch is on top of the main branch, comin deploys this configuration by running switch-to-configuration test: the bootloader configuration is not modified.

To test a configuration:

  1. Create a testing-<hostname> branch in your configuration repository on top of the main branch
  2. Add new commits to this branch and push it
  3. comin runs switch-to-configuration test on the configuration: the bootload is not updated

Contrary to the main branch, this branch can be hard reset but always has to be on top of the main branch.

To nixos-rebuild switch to this configuration, the main branch has to be rebased on the testing branch.

Iterate faster with local repository

By default, comin polls remotes every 60 seconds. You could however add a local repository as a comin remote: comin could then poll this branch every second. When you commit to this repository, comin is starting to deploy the new configuration immediately.

However, be careful because this repository could then be used by an attacker to update your machine.

Example of a configuration with a local repository:

services.comin = {
  enable = true;
  remotes = [
    {
      name = "local";
      url = "/your/local/infra/repository";
      poller.period = 2;
    }
  ];
}

How to migrate a configuration from a machine to another one

Suppose you have a running NixOS machine and you want to move this configuration to another machine while preserving the same hostname. If you use a testing branch, both of these machines will be updated when changes are pushed to the testing branch.

To avoid such situation, we could set the option services.comin.machineId. If the machine where comin is running doesn't have this expected machine-id (compared to the content of the /etc/machine-id file), comin won't deploy the configuration.

So, to migrate to another machine, you have to update this option in the testing-<hostname> branch in order to only deploy this configuration to the new machine.