# ADR-031 — ASL + TGK Index Separation

Status: Accepted
Date: 2026-02-21
Decision Owner: Architecture

## Context

ASL index/log provide artifact visibility, replay, and tombstone semantics.
TGK store provides graph query semantics over TGK edge artifacts. We must decide
whether to **merge** ASL and TGK indexes into one unified index structure, or
keep them separated with explicit cross-layer invariants.

## Decision

We **reject** a merged ASL+TGK index.

ASL index/log and TGK index remain **separate layers** with strict invariants
that keep them consistent.

## Rationale

* ASL index is about **artifact visibility and replay**; TGK index is about
  **graph query acceleration**. Their update streams and invariants differ.
* Keeping them separate preserves clean layering and deterministic replay.
* TGK indexes can be rebuilt or swapped without changing ASL semantics.
* A merged index risks coupling graph query policies with storage visibility,
  complicating federation and policy enforcement.

## Invariants (Normative)

1. **Visibility alignment:** TGK indexes MUST only index artifacts visible under
   ASL log replay at the same `(snapshot_id, log_position)`.
2. **Tombstone enforcement:** TGK indexes MUST respect ASL tombstones and
   tombstone lifts at their `logseq`.
3. **No new identity:** TGK nodes/edges MUST use ASL `Reference` identities;
   TGK MUST NOT introduce a distinct object identity scheme.
4. **Deterministic derivation:** TGK index updates MUST be deterministically
   derived from ASL-admitted records (segments/log). No side-channel writes.
5. **Replay equivalence:** Replaying ASL log + sealed segments MUST yield the
   same TGK index state (up to encoding) as an online build.
6. **Snapshot consistency:** TGK queries at a snapshot MUST be consistent with
   ASL `IndexState` for that snapshot.

## Consequences

* ASL and TGK index encodings remain separate specs.
* Conformance tests for ASL and TGK can evolve independently, with explicit
  cross-layer checks for the invariants above.
* TGK index rebuilds remain possible without touching ASL log/segment formats.