Deploying via SSH

By running either from packages or through systemd services microvm.nix tries to support a wholesome Nix workflow: develop and test on your local laptop, then deploy to staging and later to production.

Let's explore alternative ways before detailing our elaboration:

• You could build .#nixosConfiguration.my-microvm.config.microvm.declaredRunner locally, then nix copy it to the target host for installation. This comes at the expense of your laptop's battery time and it can also become quite network-heavy.
• You may transfer each change to the remote host to build entirely remote. There you're going to have a repository state that is going to confuse fellow operators. Also, your local --override-input parameters will become meaningless on the remote filesystem.

microvm.deploy.rebuild

The easy interface that is named after nixos-rebuild combines the two scripts that are described below:

• First, we evaluate locally and build remotely with microvm.deploy.installOnHost
• Depending on whether the host's /nix/store is mounted and SSH is running in the MicroVM:
  • We either run microvm.deploy.sshSwitch as described below
  • Alternatively, we restart the MicroVM's systemd service on the host

Because it needs to know about both the host and the MicroVM, these ssh addresses must come before the actual switch argument:

nix run .#nixosConfigurations.my-microvm.config.microvm.deploy.rebuild root@example.com root@my-microvm.example.com switch

microvm.deploy.installOnHost

This script will evaluate only the system's derivations locally. It then transfers these and their dependencies to the remote system so the actual build can be performed there.

Just like the microvm command, it then installs the MicroVM under /var/lib/microvms/$NAME so that the systemd services of the host module can pick it up.

It is irrelevant whether you create a new MicroVM or update an existing one.

microvm.deploy.sshSwitch

Once the host has an updated MicroVM in its /nix/store (see above) the new system must be activated. For a proper state, this script does a bit more in the MicroVM than just switch-to-configuration:

• First, the config.networking.hostName is compared to the running system for safety reasons.
• The Nix database registration will be imported which is important if you build packages into a microvm.writableStoreOverlay.
• The new system is installed into /nix/var/nix/profiles/system which is optional but expected by some Nix tooling.
• Finally, run switch-to-configuration with the provided parameter (eg. switch).