# relations-on-sx: Cross-domain relationship graph on Datalog

rose-ash's internal `relations` service tracks parent/child and peer relationships
*across* domains — a blog post's comments, a thread's replies, a product's
variants, an order's line items, a resource's containment tree, a federated
content's origin. The questions are graph questions: who are X's children? its
ancestors? is A reachable from B? what's the chain that connects them? is there a
cycle?

That is recursive Datalog in one rule — the same bottom-up reachability `acl-on-sx`
uses for group/resource inheritance. Decisions come with a **trace**: not just
"yes, related," but the path that proves it. relations is an **internal-only**
service (no public URL); other domains call it to resolve hierarchy and linkage.

End-state: a Datalog-on-SX layer for typed relationship facts, with reachability,
path explanation, cycle detection, and a federation extension for cross-instance
links. Reuses `lib/datalog/` — does not reimplement the engine.

## Status (rolling)

`bash lib/relations/conformance.sh` → **0/0** (not yet started)

## Ground rules

- **Scope:** only `lib/relations/**` and `plans/relations-on-sx.md`. Do **not** edit
  `spec/`, `hosts/`, `shared/`, `lib/datalog/**`, or other `lib/<lang>/`. You may
  **import** from `lib/datalog/` (public API in `lib/datalog/datalog.sx`); do not
  copy or modify Datalog.
- **Shared-file issues** → "Blockers" with a minimal repro; do not fix here.
- **SX files:** `sx-tree` MCP tools only; `sx_validate` after every edit.
- **Architecture:** relationships are `rel(Src, Dst, Kind)` Datalog facts;
  reachability/ancestry are recursive rules; the proof tree is the connecting path;
  the lifecycle (assert/retract) is an SX layer over the db. Keep relations
  *content-agnostic* — a node is an opaque id string; domains own what ids mean.
- **Shared with acl-sx:** both run on Datalog and both lean on the same recursive-
  reachability shape (`reach(X,Y) :- edge(X,Y).` / `reach(X,Y) :- edge(X,Z), reach(Z,Y).`).
  Watch for it; flag convergence in the Progress log, but **do not extract** —
  `plans/mod-on-sx.md` records why cross-subsystem extraction waits for the
  architecture integrator with all consumers in view.
- **Commits:** one feature per commit. Keep Progress log updated and tick boxes.

## Architecture sketch

```
relate(src, dst, kind)                  query
        │                                  │
        ▼                                  ▼
lib/relations/schema.sx          lib/relations/engine.sx
  — rel(Src,Dst,Kind) facts         — children/parents/ancestors/descendants
  — kind vocabulary                 — reachable?(A,B), cycle?(X)
        │                                  ▲
        ▼                                  │
lib/relations/api.sx             lib/relations/explain.sx
  — relate / unrelate               — path(A,B): the connecting chain
  — registry over a live db           (from the Datalog derivation)
        │
        ▼
lib/relations/federation.sx
  — cross-instance links via fed-sx (replicated rel facts, peer-trust gated)
```

## Phase 1 — Schema + direct relations

- [ ] `lib/relations/schema.sx` — `rel(Src, Dst, Kind)` fact projection; a small
  kind vocabulary (`parent`, `member`, `reply`, `variant`, `origin`, …) kept open
- [ ] `lib/relations/api.sx` — `(relations/relate src dst kind)` / `(unrelate …)`
  over a live Datalog db (assert/retract); `(children-of db node kind)`,
  `(parents-of db node kind)`, `(related db node kind)`
- [ ] `lib/relations/tests/direct.sx` — assert/retract, direct children/parents,
  kind filtering, unknown node → empty
- [ ] `lib/relations/conformance.sh` + scoreboard

## Phase 2 — Reachability + cycles

- [ ] recursive reachability rules: `ancestors`, `descendants`, `reachable?(A,B)`
  (transitive closure over a kind, the acl inheritance shape)
- [ ] `roots` / `leaves` (no parents / no children) for a kind
- [ ] cycle detection: `cycle?(X)` ⇔ `reachable(X, X)`; `acyclic?(db, kind)`
- [ ] `lib/relations/tests/reach.sx` — deep chains, diamonds, disconnected nodes,
  self-loops, multi-kind isolation

## Phase 3 — Typed relations + path explanation

- [ ] multiple kinds coexisting; mixed-kind vs single-kind reachability
- [ ] `lib/relations/explain.sx` — `(path db a b kind)` returns the connecting
  chain (the relationship equivalent of acl's proof tree), nil if unreachable
- [ ] `(distance db a b kind)` (hops) + shortest-path selection
- [ ] `lib/relations/tests/path.sx` — path correctness, shortest among many, no-path

## Phase 4 — Federation

- [ ] cross-instance relationships — a peer asserts `rel(local, remote, kind)`;
  replicate rel facts via fed-sx (mock the transport in tests)
- [ ] trust gating — a peer's link binds locally only under a local trust fact
  (mirror acl's non-transitive `trust`/gate-in-engine model; do NOT copy acl code,
  re-derive the shape)
- [ ] revocation — retract a replicated link; reachability re-saturates
- [ ] `lib/relations/tests/fed.sx` — federated reachability chains, trust gating,
  revocation

## Progress log

(loop fills this in)

## Blockers

(loop fills this in)