Start on the derivations section

doc/manual/src/SUMMARY.md.in

@@ -26,7 +26,7 @@
       - [Input-Addressing](design/store/drvs/ia.md)
       - [Content-Addressing (Experimental)](design/store/drvs/ca.md)
     - [Building](design/store/building.md)
-    - [Advanced Topic: store dir relocatability](design/store/relocatability.md)
+    - [Advanced Topic: store entry relocatability](design/store/relocatability.md)
 - [Package Management](package-management/package-management.md)
   - [Basic Package Management](package-management/basic-package-mgmt.md)
   - [Profiles](package-management/profiles.md)

doc/manual/src/design/store/drvs/drvs.md

@@ -0,0 +1,59 @@
+# Derivations
+
+Derivations are recipes to create store entries.
+
+Derivations are the heart of Nix.
+Other system (like Git or IPFS) also store and transfer immutable data, but they don't concern themselves with *how* that data was created.
+This is where Nix comes in.
+
+Derivations produce data by running arbitrary commands, like Make or Ninja rules.
+Unlike those systems, derivations do not produce arbitrary files, but only specific store entries.
+They cannot modify the store in any way, other than creating those store entries.
+This rigid specification of what they do is what allows Nix's caching to be so simple and yet robust.
+
+Based on the above, we can conceptually break derivations down into 3 parts:
+
+1. What command will be run?
+
+2. What existing store entries are needed as inputs?
+
+3. What store entries will be produced as outputs?
+
+## What command will be run?
+
+The original core of Nix was very simple about this, in the mold of traditional Unix.
+Commands consist of 3 parts:
+
+1. Path to executable
+
+2. Arguments (Excecpt for `argv[0]`, which is taken from the path in the usual way)
+
+3. Environment variables.
+
+## What existing store entries are needed as inputs?
+
+The previous sub-section begs the question "how can we be sure the path to the executable points to what we think it does?"
+It's a good questions!
+
+## What store entries will be produced as outputs?
+
+## Extra extensions
+
+### `__structuredAttrs`
+
+Historically speaking, most users of Nix made GNU Bash with a script the command run, regardless of what they were doing.
+Bash variable are automatically created from env vars, but bash also supports array and string-keyed map variables in addition to string variables.
+People also usually create derivations using language which also support these richer data types.
+It was thus desired a way to get this data from the language "planning" the derivation to language to bash, the language evaluated at "run time".
+
+`__structuredAttrs` does this by smuggling inside the core derivation format a map of named richer data.
+At run time, this becomes two things:
+
+1. A JSON file containing that map.
+2. A bash script setting those variables.
+
+The bash command can be passed a script which will "source" that Nix-created bash script, setting those variables with the richer data.
+The outer script can then do whatever it likes with those richer variables as input.
+
+However, since derivations can already contain arbitary input sources, the vast majority of `__structuredAttrs` can be handled by upper layers.
+We might consider implementing `__structuredAttrs` in higher layers in the future, and simplifying the store layer.