doc: add ssh-deploy

Author: astro

doc/src/SUMMARY.md

@@ -14,6 +14,7 @@
   - [Advanced network setup](./advanced-network.md)
   - [Host systemd services](./host-systemd.md)
 - [Declarative MicroVMs](./declarative.md)
+  - [Deploy via SSH](./ssh-deploy.md)
 - [Imperative MicroVM management](./microvm-command.md)
 - [Conventions](./conventions.md)
 - [Frequently Asked Questions](./faq.md)

doc/src/ssh-deploy.md

@@ -0,0 +1,66 @@
+# 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 [email protected] [email protected] 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](microvm-command.md), 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 MicrVOM 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`).