Done cleanup

2026-04-25 22:27:58 +01:00
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+---
+id: ML-11
+title: Telemetry.Storage GenServer funnels synchronous SQLite writes
+status: Done
+assignee: []
+created_date: '2026-04-20 08:49'
+updated_date: '2026-04-24 09:29'
+labels: []
+dependencies: []
+references:
+  - 'https://github.com/cloud8421/music_library/issues/172'
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+_GitHub: created 2026-04-16 · updated 2026-04-16_
+
+## Summary
+
+`MusicLibraryWeb.Telemetry.Storage` is a single named GenServer that receives a cast for every telemetry event and performs two synchronous SQLite writes per event. A page with 10 DB queries plus a render can push 30+ messages into this one mailbox, all queued behind serial disk I/O.
+
+## Evidence
+
+- `lib/music_library_web/telemetry/storage.ex:51` — `GenServer.cast(__MODULE__, {:store, ...})`
+- `lib/music_library_web/telemetry/storage.ex:56-69` — `handle_cast/2` runs `insert_and_prune/4` synchronously
+- `lib/music_library_web/telemetry/storage.ex:89-107` — two SQLite queries per event: `INSERT` + `DELETE`-preserving-last-N subquery
+- `TelemetryRepo` pool is 2 per `docs/production-infrastructure.md`, but the GenServer writes serially so pool size is irrelevant.
+- `lib/music_library_web/telemetry/storage.ex:108-109, 125-126` — bare `catch _, _ -> :ok` swallows every error class including EXIT signals.
+
+## Relation to #105
+
+Issue #105 was closed as NOT_PLANNED with the rationale that bare catches are "reasonable for telemetry resilience". New framing: the catches mask a more fundamental design: the mailbox + synchronous-write pattern itself.
+
+## Fix (options)
+
+1. **Buffer-and-flush**: accumulate datapoints in-memory, flush batch every N seconds in a single transaction.
+2. **Task.Supervisor per cast**: drain mailbox immediately, do I/O in a supervised task.
+3. **ETS ring buffer + periodic flusher**: read path hits ETS; single-writer flusher persists to SQLite at a low cadence.
+
+Whichever option is chosen, replace the bare `catch _, _` with either a targeted `rescue` or `Logger.debug` so failures become observable.
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- Telemetry write path does not block on the GenServer mailbox
+- Error path surfaces at least once per distinct failure (not silently swallowed)
+- Existing dashboard queries continue to work
+<!-- SECTION:DESCRIPTION:END -->
+
+- [x] #1 Telemetry write path does not block on the GenServer mailbox
+- [x] #2 Error path surfaces at least once per distinct failure (not silently swallowed)
+- [x] #3 Existing dashboard queries continue to work
+<!-- AC:END -->