docs: add nixd worker model (#282)

nixd/docs/dev.md

@@ -3,6 +3,47 @@
 
 ### Design
 
+#### Memory model about nix language & nixd workers
+
+**TLDR: Evaluation is not memory-safe and must be performed in a separeted address space.**
+
+In the context of the Nix language, laziness refers to the evaluation strategy where expressions are not immediately evaluated when they are encountered, but rather when their values are actually needed.
+This means that the evaluation of an expression is deferred until it is required to produce a result.
+
+One consequence of lazy evaluation is the potential for circular references or circular dependencies.
+
+
+Our upstream [C++ nix](https://github.com/NixOS/nix) uses a garbage collector and never actively free used memories.
+Thus all evaluators should be used in "worker" processes.
+
+<!--
+digraph {
+    Controller -> Worker [ label = "JSON RPC" dir = "both" ]
+    Worker -> "Workspace Files"
+    Controller -> "Workspace Files"
+}
+-->
+
+```
+
+┌─────────────────┐
+│   Controller    │ ─┐
+└─────────────────┘  │
+  ▲                  │
+  │                  │
+  │ JSON RPC         │
+  ▼                  │
+┌─────────────────┐  │
+│     Worker      │  │
+└─────────────────┘  │
+  │                  │
+  │                  │
+  ▼                  │
+┌─────────────────┐  │
+│ Workspace Files │ ◀┘
+└─────────────────┘
+```
+
 #### How does language information being collected?
 
 Nix expressions are evaluated using the `nix::Expr::eval` virtual method, with dynamic name bindings and lookups performed in `Env`s.