Near-identical viewports from different collaborators (slightly
different pan/zoom because Yjs awareness doesn't enforce pixel-exact
alignment) previously produced byte-different Overpass queries and
therefore different server-side cache keys. Each client missed cache
and triggered its own upstream fetch.
buildQuery now quantizes the bbox to a 0.01° grid (~1 km), expanding
outward (south/west floor, north/east ceil) so the viewport is always
covered, and formats coords with 3 decimals for a stable string.
Trade-offs:
- Slightly larger query bbox: ≤ one grid cell (≈ 1 km) of padding on
each side. For a zoom-12 viewport (~11 km wide) that's <10% extra
data; bounded by the existing `[maxsize:1048576]` + `out 100` limits.
- Coordinates in the query are now 3-decimal strings regardless of the
caller's precision — existing buildQuery test updated to match.
Two new tests cover the cache-alignment property and the outward-
expansion invariant. Hit ratio impact will be visible on the Grafana
"Overpass Cache Hit Ratio" stat added in the prior PR.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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