Claudio Ortolina
58 lines
2.5 KiB
Markdown
58 lines
2.5 KiB
Markdown
---
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id: ML-11
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title: Telemetry.Storage GenServer funnels synchronous SQLite writes
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status: Done
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assignee: []
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created_date: "2026-04-20 08:49"
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updated_date: "2026-04-24 09:29"
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labels: []
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dependencies: []
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references:
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- "https://github.com/cloud8421/music_library/issues/172"
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priority: medium
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---
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## Description
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<!-- SECTION:DESCRIPTION:BEGIN -->
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_GitHub: created 2026-04-16 · updated 2026-04-16_
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## Summary
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`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.
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## Evidence
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- `lib/music_library_web/telemetry/storage.ex:51` — `GenServer.cast(__MODULE__, {:store, ...})`
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- `lib/music_library_web/telemetry/storage.ex:56-69` — `handle_cast/2` runs `insert_and_prune/4` synchronously
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- `lib/music_library_web/telemetry/storage.ex:89-107` — two SQLite queries per event: `INSERT` + `DELETE`-preserving-last-N subquery
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- `TelemetryRepo` pool is 2 per `docs/production-infrastructure.md`, but the GenServer writes serially so pool size is irrelevant.
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- `lib/music_library_web/telemetry/storage.ex:108-109, 125-126` — bare `catch _, _ -> :ok` swallows every error class including EXIT signals.
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## Relation to #105
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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.
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## Fix (options)
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1. **Buffer-and-flush**: accumulate datapoints in-memory, flush batch every N seconds in a single transaction.
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2. **Task.Supervisor per cast**: drain mailbox immediately, do I/O in a supervised task.
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3. **ETS ring buffer + periodic flusher**: read path hits ETS; single-writer flusher persists to SQLite at a low cadence.
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Whichever option is chosen, replace the bare `catch _, _` with either a targeted `rescue` or `Logger.debug` so failures become observable.
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## Acceptance Criteria
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<!-- AC:BEGIN -->
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- Telemetry write path does not block on the GenServer mailbox
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- Error path surfaces at least once per distinct failure (not silently swallowed)
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- Existing dashboard queries continue to work
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<!-- SECTION:DESCRIPTION:END -->
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- [x] #1 Telemetry write path does not block on the GenServer mailbox
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- [x] #2 Error path surfaces at least once per distinct failure (not silently swallowed)
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- [x] #3 Existing dashboard queries continue to work
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<!-- AC:END -->
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