From dda6ce376fe253e9968b1db53eff2f002fc061c4 Mon Sep 17 00:00:00 2001
From: Antonio Gurgel <antonio@goorzhel.com>
Date: Sun, 3 Dec 2023 13:50:44 -0800
Subject: [PATCH] Write ADR 2

---
 adr/1.md | 19 +++++----------
 adr/2.md | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+), 13 deletions(-)
 create mode 100644 adr/2.md

diff --git a/adr/1.md b/adr/1.md
index 2d0cabb..c3754dc 100644
--- a/adr/1.md
+++ b/adr/1.md
@@ -51,24 +51,17 @@ services = flakeBuilders.services self.serviceData {
 I do instead:
 
 ```nix
-stages = [
-  services/0-istio
-  services/1-kyverno
-  services/2-longhorn
-  services/3-registry-cache
-  services/4-metallb
-  <...>
-];
+stages = (ls ./services);
 
-serviceData = rake.leaves stages;
+serviceData = rake.stages stages;
 # This should return a list of unbuilt service trees.
 ```
 
 I'm not sure what to do past this point but I know what I want:
 
-1. `services/0-istio` is built by flakeBuilders.services with `apiVersions = []`.
-2. `services/1-kyverno` is built by flakeBuilders.services with `apiVersions = [<Istio APIs>]`.
-3. `services/2-longhorn` is built by flakeBuilders.services with `apiVersions = [<Istio APIs> <Kyverno APIs>]`.
+1. `services/00-istio` is built by flakeBuilders.services with `apiVersions = []`.
+2. `services/01-kyverno` is built by flakeBuilders.services with `apiVersions = [<Istio APIs>]`.
+3. `services/02-longhorn` is built by flakeBuilders.services with `apiVersions = [<Istio APIs> <Kyverno APIs>]`.
 4. And so on, until `services/XX-services` is built with all APIs accumulated from previous stages.
 
-It looks like a `pkgs.lib.lists.foldl`. Also, the stages can be gathered with `ls ./services | filter(<starts with number>)`.
+It looks like a `pkgs.lib.lists.foldl`.
diff --git a/adr/2.md b/adr/2.md
new file mode 100644
index 0000000..dc159bd
--- /dev/null
+++ b/adr/2.md
@@ -0,0 +1,73 @@
+# ADR 2: Using directory trees as data
+
+In `a2abba2` or sometime around then, I borrowed Nixhelm's [leaf-raking paradigm](https://github.com/farcaller/nixhelm/blob/e0383f7c11000f8c141a7d26197ee402424b017d/flake.nix#L9-L23) (and named it after [Digga's](https://github.com/divnix/digga/blob/baa54f8641ee9128cdda8b508553284c331fc9f1/src/importers.nix#L61-L114)...although this is the first time I'm looking directly at it, and now I'm realizing I could have just borrowed that very code.)
+
+Example input and output, using `kubernetes@2de43d7`:
+
+```
+services
+├── cert-manager
+│   └── cert-manager
+│       ├── default.nix
+│       └── prometheusrule.yaml
+├── cnpg-system
+│   └── cloudnative-pg
+│       └── default.nix
+...
+system
+├── argo
+│   └── workflows
+│       └── default.nix
+├── cert-manager
+│   └── cert-manager
+│       ├── default.nix
+│       └── k8s-intermediate.sops.yaml
+├── cnpg-system
+│   └── cloudnative-pg
+│       └── default.nix
+```
+
+```nix
+systemServiceData = rake.leaves ./system;
+serviceData = rake.leaves ./services;
+```
+
+```nix
+serviceData = {
+  # These two derivations consist only of resources
+  # (`ClusterIssuer`, CNPG `Cluster`); the applications themselves
+  # are installed in `system`.
+  cert-manager.cert-manager = {lib, ...} -> {builder, args};
+  cnpg-system.cloudnative-pg = {lib, ...} -> {builder, args};
+  ...
+};
+systemServiceData = {
+  argo.workflows = {charts, lib, user, ...} -> {builder, args, extraObjects};
+  cert-manager.cert-manager = {charts, lib, user, ...} -> {builder, args, extraObjects};
+  cnpg-system.cloudnative-pg = {charts, lib, ...} -> {builder, args};
+  ...
+};
+```
+
+Raking a service tree like this reduces the need to litter the source tree with
+`default.nix` files (as seen in, e.g., [nixos-hardware](https://github.com/NixOS/nixos-hardware/tree/a89745edd5f657e2e5be5ed1bea86725ca78d92e)).
+However, it imposes assumptions about the source tree that ADR 1 rendered invalid.
+
+## Considered Options
+
+### Adapt existing code
+
+The most work.
+
+### Borrow Digga's code
+
+It's MIT-licensed, hence compatible with this project's Apache 2.
+
+### Use Haumea
+
+In reading about Digga's deprecation I was tipped off to Haumea.
+It does exactly what I want: map a filesystem tree to an attrset.
+
+## Decision
+
+Use Haumea.