Merge pull request 'Add architecture-docs/NixOps4-based-installation-process.md' (#13) from roberth/meta:nixops4-based-install into main

Reviewed-on: Fediversity/meta#13

architecture-docs/NixOps4-based-installation-process.md

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+status: draft
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+
+# NixOps4-based Installation Process
+
+This documents explains how NixOps4 could be leveraged to provide a smooth installation experience for a simple, monolithic deployment.
+
+It does not go into detail about the way Nix Panel hooks into this, or whether Nix Panel manages the same NixOps4 deployment that also contains the infrastructure; a single NixOps4 deployment could provide a nice simplification in the early stages of the project.
+
+## Components
+
+Required for the setup steps in this document:
+
+- `nixops4`: generic deployment tool using Nix
+- `landscaper.iso`: installation image that bootstraps a completely new and independent Fedi cluster - a NixOS installer with a handful of extra definitions (@roberth: easy)
+- `nixops4-modules-ssh-keypair`: NixOps4 resource provider that generates keys
+- `nixops4-pxe-nbp`: NixOps4 resource provider that inserts an NBP image into a (local) PXE server configuration
+- `nixops4-modules-ssh`: A module that calls `ssh` using `nixops4-resources-local`
+- `nixops4-resources-local`: A module that can create files and call commands locally (prototyped)
+- `nixops4-modules-nixos`: A module that calls `nixos-rebuild switch --target-host` or implements similar functionality
+
+Expected needs:
+
+- `nixops4-resources-proxmox`
+- A resource provider that talks to a DNS server's API
+
+## Steps
+
+### 1. `landscaper` setup
+
+1. connect your first server into the network
+
+2. boot `landscaper.iso` from a usb stick, and install NixOS onto this server we'll call `landscaper`
+
+`landscaper.iso` is  a slightly customized NixOS installer that includes the `landscaper` NixOS module
+     in the default config that it generates
+  - mvp: copy and import the landscaper module by hand into the normal NixOS live system
+
+The `landscaper` module contains 
+    - nixops4 (mvp and initial setup)
+    - a nixops4 service (if/when nix-panel is capable of managing the infra)
+    - a tftp server
+
+The NixOS installer puts the template NixOps4 expression in `/root/fediversity-network`
+
+### 2. Add a proxmox host
+
+1. - add a MAC address and IP address to the `proxmoxMACs` option (or similar)
+    - this automatically declares resources that will set up the new server
+      - `installation_host_key`: a temporary SSH host key pair
+      - `installation_pxe_nbp`: an entry in `landscaper`'s PXE server, referring to/including a kernel and initrd that run disko and performs a NixOS installation on the server disk(s)
+      - `ssh_host_public_key`: a resource that waits for SSH to come up, logs in to replace the host key and sends the returns the new public key as its resource output
+      - `nixos`: a resource that `nix copy`-es a NixOS toplevel, then updates the system profile and activates it
+
+      - unknown: built-in services like Nix-panel, perhaps depending on the number of `proxmoxMACs`
+2. `nixops apply`
+    - this starts to create resources,
+    - up to the point that `ssh_host_public_key` waits for the server to boot
+3. turn on the server
+4. wait for `nixops apply` to complete
+
+## Notes
+
+Unknown: add to step 2 or step 3 (new), depending on the amount of shared infra:
+ - resources for certain objects in the proxmox api
+    - this creates nixpanel, and shared infrastructure
+