Add a note file for Datomic

hassan/010-datomic-for-geolog.md

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+## Datomic as a Backend for Geolog
+
+### Short Answer
+
+Datomic could plausibly serve as the **persistence layer** for Geolog, but not as the full **logic/runtime backend**.
+
+It matches Geolog's desire for:
+
+* **Persistent, append-only storage** with history
+* **Interactive mutation** over evolving instances
+* **Datalog-style querying** over stored facts
+
+It does **not** by itself provide the core Geolog semantics described elsewhere in these notes:
+
+* **Existential witness generation** during the chase
+* **Disjunctive branching** into multiple candidate models
+* **Equality saturation / term merging**
+* **Typed theories, dependent theories, and nested instances** as first-class language concepts
+
+So the right mental model is:
+
+* **Datomic stores Geolog state**
+* **Geolog implements its own chase/type/equality engine above Datomic**
+
+---
+
+### Why It Fits Part of the Problem
+
+The notes describe Geolog as infrastructure with **persistent, append-only storage** and an **interactive REPL**, not just as a one-shot solver. Datomic's immutable fact model lines up well with that storage story.
+
+Good candidates to store in Datomic:
+
+* theory definitions
+* schemas for sorts, functions, and relations
+* user-authored instance data
+* derived facts and provenance
+* transaction history
+* saved snapshots / named worlds
+
+This is especially attractive if Geolog wants:
+
+* time travel / "as of" inspection
+* auditability of derivations
+* incremental recomputation from prior states
+* a durable log of REPL activity
+
+---
+
+### Where Datomic Stops Being Enough
+
+The Geolog notes are clear that the system goes beyond plain Datalog.
+
+Geolog needs chase behavior that can:
+
+* invent fresh witnesses for existential conclusions
+* branch when a rule concludes a disjunction
+* merge terms when equations force equality
+
+Those are central to the chase-based model described in the notes, and they are not features Datomic gives you as a database. Datomic gives you a temporal fact store plus Datalog query tools; it does not give you a geometric-logic engine.
+
+That means a Datomic-based Geolog would still need its own:
+
+* rule compiler / evaluator
+* model-branch management
+* equality class machinery
+* fixpoint / chase scheduler
+* termination controls and bounds
+
+Inference: Datomic is a reasonable substrate for storing models and derivation history, but it is not a substitute for the core Geolog runtime.
+
+---
+
+### Licensing and Stack Implications
+
+As of **March 26, 2026**, Datomic's official docs say that **all editions are free** and that the **binaries are licensed under Apache 2.0**, while Datomic's own FAQ says it is **not open source** because the source is not published.
+
+Official sources:
+
+* [Datomic Overview](https://docs.datomic.com/datomic-overview.html)
+* [Datomic Pro Releases](https://docs.datomic.com/releases-pro.html)
+* [Datomic is Free](https://blog.datomic.com/2023/04/datomic-is-free.html)
+* [Datomic Local is Released](https://blog.datomic.com/2023/08/datomic-local-is-released.html)
+
+Practically, that means:
+
+* You **can** use Datomic inside a stack whose other components are open source.
+* You **can** redistribute Datomic binaries where the Apache-licensed binary terms allow it.
+* Datomic does **not** force the rest of the stack to become closed-source.
+* But you are still depending on a **source-unavailable vendor component**.
+
+That source-unavailable dependency has real engineering consequences:
+
+* you cannot inspect or modify Datomic internals
+* you cannot self-maintain a fork if Nubank stops investing in it
+* debugging deep engine behavior depends on vendor docs and black-box observation
+* fully reproducible "build everything from source" workflows become weaker
+* some contributors will reject the stack on principle because it is not fully open
+
+So the licensing issue is not "can it legally coexist with open parts of the stack?" The answer there is mostly **yes**. The more important question is strategic:
+
+* does Geolog want a **fully open, self-hostable, inspectable foundation**
+* or is it acceptable to place the storage layer on a **free but closed-source binary**
+
+If Geolog wants the strongest possible open-source story, Datomic is a poor fit. If Geolog mainly wants a strong immutable fact store and accepts vendor dependence, Datomic remains viable.
+
+---
+
+### Bottom Line
+
+For Geolog as described in these notes:
+
+* **Yes** for storage, history, and query support
+* **No** for the core reasoning engine
+* **Legally workable** in a mixed stack
+* **Strategically questionable** if full openness is a project goal
+
+## Changelog
+
+* **Mar 26, 2026** -- Added note on whether Datomic could serve as a backend for Geolog, including licensing implications.