Clicks on the map were hanging for 1-3s, triggering the Phoenix topbar
progress bar visible to the user. Three hot-path improvements:
1. ScoresFile.read_point now uses a two-range :file.pread/2 — reads the
33-byte header + one cell byte (~60 bytes) instead of the full ~93 KB
file. point_forecast walks 19 forecast hours per click, so the old
full-read path was the dominant cost on cold cache.
2. ProfilesFile.read/1 and list_valid_times/0 cached via
Microwaveprop.Cache (5s TTL). Decoded profile maps are ~10 MB each
(92k cells); timeline scrub that re-clicks the same valid_time
within seconds now hits ETS instead of re-gunzipping + decoding.
Cache keys include base_dir so test setup that swaps dirs doesn't
see stale entries. Writes + prune + retain_window invalidate.
3. Endpoint log_level now filters /metrics the same way it already
filters /health — Prometheus scrapes every 5s and was producing
visible log spam.
Pod-local ETS is right for this workload (per-click read, tiny working
set); Valkey / shared memstore would not help here since each pod needs
its own fast-path lookup. File a follow-up if cross-pod score lookups
ever show up in flame graphs.
Graham McIntire
bd3b1148a4