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docs(decisions): Q-098 — persistence of generated river/city mapping outputs

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Looking-ahead question from the Phase 4 markers strip. Generated mapping (rivers via
drainage, cities via econ sim + placement) is deterministic so always recomputable,
but expensive — should be computed once per body and kept, not volatile. Refines D-203
(LRU evicts) and D-200 (build-time bake vs runtime persist). Gates the atlas viewer
(#960). To resolve during the execution-model work (#952).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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jpmschweitzer
2026-05-22 19:55:01 +02:00
co-authored by Claude Opus 4.7
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@@ -330,4 +330,12 @@ Technical foundation questions: engine, protocols, data structures, performance,
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*46 questions (8 resolved, 1 partially resolved, 37 open). Last updated: 2026-04-05.*
### Q-098: Persistence of generated river/city mapping outputs
- **Status:** Open — to resolve during Phase 4 execution-model work (#952)
- **Question:** The deterministic cascade can recompute river courses (D8 drainage, D-208) and city placements (economic sim + attractor matching, D-211) from seed at any time, so persisting them is a *cost* optimization, not a correctness need. But the compute is expensive — recomputing per session or per atlas view is waste. How are these mapping outputs persisted so they are computed **once per body and kept**? D-203's BodyWorldState cache is an LRU — it *evicts* (volatile). The fork: **(a)** build-time bake into `systems.db` (D-200 build-time tier — precompute all, ship); **(b)** lazy compute + persist at runtime (cache DB / savegame — compute on first visit, keep); **(c)** hybrid. Whatever the answer, the in-memory LRU should sit over a *durable* store so eviction triggers a cheap reload, not a recompute.
- **Context:** Raised 2026-05-22 looking ahead from the Phase 4 markers strip (#951). Refines D-200 (three-tier execution) and D-203 (LRU cache). Gates the Atlas layer viewer (#960), which needs persisted mapping to render without recomputing. Determinism (#952) guarantees recompute is always a valid fallback.
- **Cross-reference:** [D-200](architecture.md#d-200), [D-203](architecture.md#d-203), [D-208](architecture.md#d-208), [D-211](architecture.md#d-211), #952 (determinism harness), #960 (atlas viewer)
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*47 questions (8 resolved, 1 partially resolved, 38 open). Last updated: 2026-05-22.*