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ML-169.3: performed pubsub audit

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Claudio Ortolina
2026-05-19 08:20:11 +01:00
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backlog/docs/audits/phase2-pubsub-lifecycle/doc-25 - Audit-Report-PubSub-Subscription-Lifecycle-Phase-2.md
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---
id: doc-25
title: "Audit Report: PubSub Subscription Lifecycle (Phase 2)"
type: other
created_date: "2026-05-19 07:16"
tags:
- audit
- pubsub
- subscribe
- liveview
- reconnect
---
# Phase 2 PubSub Audit: Subscription Lifecycle
**Date:** 2026-05-19
**Scope:** All `Phoenix.PubSub.subscribe/2`, `unsubscribe/2`, `broadcast/3` call sites matched against consumers and navigation paths. Reconnect/death lifecycle verified against Phoenix.PubSub and LiveView behavior.
## Executive Summary
**Result: PASS — No bugs or stale subscriptions found.** All 8 subscribe sites have correct unsubscribe or auto-cleanup coverage. The pre-flagged `ListeningStats.subscribe()` double-subscription concern is a false positive: Phoenix.PubSub internally deduplicates per PID, and LiveView reconnect reuses the same PID within the grace period.
---
## PubSub Topic Inventory
| Topic | Subscribe (lines) | Unsubscribe (lines) | Broadcast (lines) | Consumers (lines) |
| -------------------------- | --------------------------------------------------------------------------------------- | ----------------------------------------- | ------------------------------------------------ | --------------------------------------------------------------- |
| `"records:#{id}"` | `records.ex:109`, `record_actions.ex:108` | `records.ex:117`, `record_actions.ex:107` | `records.ex:123` via `notify_update/1` | `collection_live/show.ex:474`, `wishlist_live/show.ex:367` |
| `"records:index_changed"` | `records.ex:143`, `collection_live/index.ex:247`, `wishlist_live/index.ex:180` | (auto-clean on PID death) | `records.ex:136` via `broadcast_index_changed/0` | `collection_live/index.ex:308`, `wishlist_live/index.ex:224` |
| `"listening_stats:update"` | `listening_stats.ex:74`, `stats_live/index.ex:80`, `scrobbled_tracks_live/index.ex:235` | (auto-clean on PID death) | `listening_stats.ex:67` | `stats_live/index.ex:176`, `scrobbled_tracks_live/index.ex:281` |
---
## Acceptance Criterion #1: Records.subscribe/1 and unsubscribe/1 Call Sites
**Status: ✅ CONFIRMED — only called via manage_subscription/2 in Show LiveViews.**
### Subscribe call sites
| Source | File | Line | Context |
| --------------------------- | ------------------- | ---- | ------------------------------ |
| `Records.subscribe/1` (def) | `records.ex` | 109 | Function definition |
| `Records.subscribe(new_id)` | `record_actions.ex` | 108 | Inside `manage_subscription/2` |
### Unsubscribe call sites
| Source | File | Line | Context |
| -------------------------------- | ------------------- | ---- | ------------------------------ |
| `Records.unsubscribe/1` (def) | `records.ex` | 117 | Function definition |
| `Records.unsubscribe(record.id)` | `record_actions.ex` | 107 | Inside `manage_subscription/2` |
### Callers of manage_subscription/2
| LiveView | File | Line | Hook |
| ------------------- | ---------------------------------------------------- | ---- | ----------------- |
| CollectionLive.Show | `lib/music_library_web/live/collection_live/show.ex` | 361 | `handle_params/3` |
| WishlistLive.Show | `lib/music_library_web/live/wishlist_live/show.ex` | 298 | `handle_params/3` |
**Verification:** Grep for `Records.subscribe` and `Records.unsubscribe` across `lib/` (excluding tests) returns only the definition in `records.ex` and the calls in `record_actions.ex`. No raw subscribe/unsubscribe bypass exists anywhere.
---
## Acceptance Criterion #2: manage_subscription/2 Navigation Correctness
**Status: ✅ VERIFIED — correct across all navigation paths.**
### Path coverage analysis
| Navigation scenario | Lifecycle | Behavior | Correct? |
| -------------------------------------- | ------------------------------------------------- | ----------------------------------------------------------------------------------------------------- | -------- |
| Direct URL load | `mount` → `handle_params` | `socket.assigns[:record]` is nil → subscribes only (no old record) | ✅ |
| `push_navigate` to record | `handle_params` fires with new ID | Unsubscribes old → subscribes new | ✅ |
| `push_patch` to record | `handle_params` fires with new ID | Same as push_navigate | ✅ |
| Browser back/forward | `handle_params` fires (LiveView handles popstate) | Same as push_navigate | ✅ |
| LiveView reconnect (same page) | `mount` → `handle_params` (same PID) | Unsubscribes old → resubscribes same record. PubSub MapSet deduplicates PID, so it's a harmless no-op | ✅ |
| LiveView reconnect (timeout → new PID) | Old PID dies → auto-cleanup → new mount | New PID subscribes fresh. No overlap. | ✅ |
### Code path analysis
```elixir
# lib/music_library_web/live_helpers/record_actions.ex:105-109
def manage_subscription(socket, new_id) do
if Phoenix.LiveView.connected?(socket) do
if socket.assigns[:record], do: Records.unsubscribe(socket.assigns.record.id)
Records.subscribe(new_id)
end
:ok
end
```
- **connected?(socket) guard:** Prevents subscribing during static/dead render. Subscription only happens when the WebSocket is connected. ✅
- **nil-safe unsubscribe:** Uses `socket.assigns[:record]` (access syntax) not `socket.assigns.record` (raises on nil). First mount with no record is safe. ✅
- **Called before record fetch:** `manage_subscription` is called at the top of `handle_params`, before `Records.get_record!(id)`. This means `socket.assigns[:record]` still holds the _old_ record (from the previous `handle_params` call), which is the correct one to unsubscribe from. ✅
---
## Acceptance Criterion #3: ListeningStats.subscribe() Reconnect Risk
**Status: ✅ CONFIRMED SAFE — no double-subscription risk.**
### Pre-flagged concern
`ListeningStats.subscribe()` is called in `mount/3` by `StatsLive.Index` and `ScrobbledTracksLive.Index` but is never explicitly unsubscribed.
### Why this is safe
**1. Phoenix.PubSub internal deduplication**
Phoenix.PubSub maintains a MapSet of PIDs per topic. Subscribing the same PID to the same topic twice is a no-op. Source: [Phoenix.PubSub docs](https://hexdocs.pm/phoenix_pubsub/Phoenix.PubSub.html#subscribe/2).
```elixir
# This is harmless on reconnect — the PID is already in the topic's MapSet
ListeningStats.subscribe() # no-op on same PID for same topic
```
**2. LiveView reconnect reuses the same PID**
When a WebSocket disconnects, Phoenix LiveView keeps the process alive for a configurable reconnect timeout (default 60s in this project). If the client reconnects within this window, `mount/3` is called on the **same PID**. Since the PID is already subscribed, the second `subscribe` call is a no-op.
**3. PID death cleans up subscriptions**
If the LiveView crashes or the reconnect timeout expires, the process dies. Phoenix.PubSub's process monitor (`:erlang.monitor/2`) automatically removes the PID from all topic MapSets. A new connection spawns a new PID with a fresh subscription.
**4. connected?(socket) guard prevents dead-render subscription**
```elixir
# stats_live/index.ex:79-81
if connected?(socket) do
ListeningStats.subscribe()
end
```
No subscription is created during the initial static render (dead mount).
### Reconnect sequence diagram
```
Scenario: WebSocket drops, client reconnects within 60s
PID-1 subscribes to "listening_stats:update" (mount)
↓
WebSocket disconnects
PID-1 stays alive (waiting for reconnect)
PubSub retains PID-1 in topic MapSet
↓
Client reconnects within 60s
mount/3 called on PID-1
ListeningStats.subscribe() called again
PubSub: PID-1 already in MapSet → no-op
↓
Result: single subscription for PID-1 ✅
Scenario: LiveView crashes hard
PID-1 subscribes to "listening_stats:update" (mount)
↓
PID-1 crashes
PubSub auto-removes PID-1 from topic MapSet
↓
Supervisor restarts LiveView → PID-2 spawns
mount/3 called on PID-2
ListeningStats.subscribe() called
↓
Result: single subscription for PID-2 ✅
```
---
## Acceptance Criterion #4: Termination Cleanup
**Status: ✅ VERIFIED — PID death covers all cleanup. No manual gaps.**
### Cleanup mechanism
Phoenix.PubSub uses `Process.monitor/1` to track subscriber PIDs. When a process exits (gracefully or crashes), the monitor fires and Phoenix.PubSub removes all of that PID's subscriptions. No explicit `unsubscribe` in `terminate/2` is needed.
### Per-topic cleanup summary
| Topic | Subscribers | Manual unsubscribe? | PID-death cleanup? | Gap? |
| -------------------------- | ------------------------------------------ | -------------------------------------------------------- | ------------------ | ---- |
| `"records:#{id}"` | CollectionLive.Show, WishlistLive.Show | Yes — `manage_subscription/2` unsubscribes on navigation | Yes — covers crash | None |
| `"records:index_changed"` | CollectionLive.Index, WishlistLive.Index | No | Yes | None |
| `"listening_stats:update"` | StatsLive.Index, ScrobbledTracksLive.Index | No | Yes | None |
### Design note
Adding `Phoenix.PubSub.unsubscribe(MusicLibrary.PubSub, "...")` to `on_mount/2` or a LiveView `terminate/2` would make the cleanup explicit, but this would be defensive coding against a mechanism that already works correctly. **No change recommended.**
---
## Acceptance Criterion #5: notify_update/1 Broadcast Sites
**Status: ✅ VERIFIED — correctly scoped and consumed.**
### Broadcast producers
All broadcasts go to `"records:#{record.id}"` with message `{:update, record}`.
| Worker | File | Line | Trigger |
| ------------------------------------- | -------------------------------------------------------------- | ---- | -------------------------------------------------- |
| `Worker.GenerateRecordEmbedding` | `lib/music_library/worker/generate_record_embedding.ex` | 16 | After embedding generation completes |
| `Worker.PopulateGenres` | `lib/music_library/worker/populate_genres.ex` | 15 | After genres populated + embedding generated async |
| `Worker.RecordRefreshMusicBrainzData` | `lib/music_library/worker/record_refresh_music_brainz_data.ex` | 14 | After MusicBrainz data refresh |
| `Worker.RefreshCover` | `lib/music_library/worker/refresh_cover.ex` | 14 | After cover image refreshed/resized |
### Broadcast consumers
```elixir
# collection_live/show.ex:474-483
def handle_info({:update, record}, socket) do
if record.id == socket.assigns.record.id do
{:noreply, socket |> RecordActions.handle_record_updated(record) |> assign_similar_records()}
else
{:noreply, socket}
end
end
# wishlist_live/show.ex:367-373
def handle_info({:update, record}, socket) do
if record.id == socket.assigns.record.id do
{:noreply, RecordActions.handle_record_updated(socket, record)}
else
{:noreply, socket}
end
end
```
**Guard clause analysis:** Both consumers verify `record.id == socket.assigns.record.id` before applying the update. This prevents a Show LiveView displaying record A from being updated by a worker that modified record B. ✅
**Topic scoping:** Since the topic is `"records:#{record.id}"`, only LiveViews subscribed to that specific record's topic will receive the broadcast. A Show LiveView for record A never receives updates for record B. ✅
### broadcast_index_changed/0
| Producer | File | Line | Trigger |
| ------------------------------------------ | ------------------------------------------------------------------- | ---- | ---------------------- |
| `Worker.ImportFromMusicbrainzRelease` | `lib/music_library/worker/import_from_musicbrainz_release.ex` | 24 | After import completes |
| `Worker.ImportFromMusicbrainzReleaseGroup` | `lib/music_library/worker/import_from_musicbrainz_release_group.ex` | 27 | After import completes |
**Consumers** (both with guard clause):
```elixir
# collection_live/index.ex:308-313
def handle_info(:records_index_changed, socket)
when socket.assigns.live_action in [:index, :edit] do
{:noreply, IndexActions.handle_index_changed(socket)}
end
def handle_info(:records_index_changed, socket), do: {:noreply, socket}
```
The guard clause `when socket.assigns.live_action in [:index, :edit]` prevents index refresh when the user is on the import modal or scanning barcodes — these actions have their own lifecycle. ✅
---
## Complete Call Site Matrix
| # | File:Line | Call | Category | Verified? |
| --- | --------------------------------------------- | ------------------------------------------- | --------------------- | --------- |
| 1 | `records.ex:109-110` | `subscribe/1` def | Definition | ✅ |
| 2 | `records.ex:117-118` | `unsubscribe/1` def | Definition | ✅ |
| 3 | `records.ex:122-127` | `notify_update/1` via `broadcast` | Broadcast | ✅ |
| 4 | `records.ex:135-136` | `broadcast_index_changed/0` via `broadcast` | Broadcast | ✅ |
| 5 | `records.ex:143-144` | `subscribe_to_index/0` def | Definition | ✅ |
| 6 | `listening_stats.ex:67` | `broadcast` in `update/1` | Broadcast | ✅ |
| 7 | `listening_stats.ex:73-74` | `subscribe/0` def | Definition | ✅ |
| 8 | `record_actions.ex:107` | `unsubscribe` in `manage_subscription` | Unsubscribe | ✅ |
| 9 | `record_actions.ex:108` | `subscribe` in `manage_subscription` | Subscribe | ✅ |
| 10 | `collection_live/index.ex:247` | `subscribe_to_index` in mount | Subscribe | ✅ |
| 11 | `collection_live/show.ex:361` | `manage_subscription` | Subscribe/Unsubscribe | ✅ |
| 12 | `wishlist_live/index.ex:180` | `subscribe_to_index` in mount | Subscribe | ✅ |
| 13 | `wishlist_live/show.ex:298` | `manage_subscription` | Subscribe/Unsubscribe | ✅ |
| 14 | `stats_live/index.ex:80` | `subscribe` in mount | Subscribe | ✅ |
| 15 | `scrobbled_tracks_live/index.ex:235` | `subscribe` in mount | Subscribe | ✅ |
| 16 | `collection_live/show.ex:474` | `handle_info({:update, record}, ...)` | Consumer | ✅ |
| 17 | `wishlist_live/show.ex:367` | `handle_info({:update, record}, ...)` | Consumer | ✅ |
| 18 | `collection_live/index.ex:308` | `handle_info(:records_index_changed, ...)` | Consumer | ✅ |
| 19 | `wishlist_live/index.ex:224` | `handle_info(:records_index_changed, ...)` | Consumer | ✅ |
| 20 | `stats_live/index.ex:167,176` | `handle_info(%{track_count: _}, ...)` | Consumer | ✅ |
| 21 | `scrobbled_tracks_live/index.ex:281,286` | `handle_info(%{track_count: _}, ...)` | Consumer | ✅ |
| 22 | `generate_record_embedding.ex:16` | `notify_update` | Broadcast | ✅ |
| 23 | `populate_genres.ex:15` | `notify_update` | Broadcast | ✅ |
| 24 | `record_refresh_music_brainz_data.ex:14` | `notify_update` | Broadcast | ✅ |
| 25 | `refresh_cover.ex:14` | `notify_update` | Broadcast | ✅ |
| 26 | `import_from_musicbrainz_release.ex:24` | `broadcast_index_changed` | Broadcast | ✅ |
| 27 | `import_from_musicbrainz_release_group.ex:27` | `broadcast_index_changed` | Broadcast | ✅ |
---
## Recommendations
| # | Finding | Severity | Recommendation |
| --- | --------------------------------------------------------------------------------------------------------------------------------------- | -------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| 1 | No explicit `unsubscribe` in LiveView `terminate/2` for index or listening_stats subscriptions | ℹ️ Info | Not needed — PID death auto-cleans. Adding would be defensive but offers no functional benefit. |
| 2 | `manage_subscription/2` redundantly unsubscribes+resubscribes on LiveView reconnect to the same record | ℹ️ Info | Harmless no-op (PubSub deduplicates by PID). A same-record check (`if socket.assigns[:record] && socket.assigns.record.id != new_id`) would be a micro-optimization. |
| 3 | `Records.notify_update/1` broadcasts `record` struct over PubSub — contains all fields including potentially large ones (cover binary?) | ℹ️ Info | Struct is already loaded from DB by the worker, and the struct is small (references, not blobs). Covers are stored on disk, not in the struct. No concern. |
**No issues require code changes.**
backlog/tasks/ml-169.3 - Phase-2-PubSub-subscription-lifecycle-audit.md
+26 -8
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@@ -1,10 +1,10 @@
--- ---
id: ML-169.3 id: ML-169.3
title: "Phase 2: PubSub subscription lifecycle audit" title: "Phase 2: PubSub subscription lifecycle audit"
status: To Do status: In Progress
assignee: [] assignee: []
created_date: "2026-05-08 08:59" created_date: "2026-05-08 08:59"
updated_date: "2026-05-11 06:46" updated_date: "2026-05-19 07:16"
labels: labels:
- audit - audit
- ready - ready
@@ -13,6 +13,10 @@ references:
- lib/music_library/records.ex - lib/music_library/records.ex
- lib/music_library/listening_stats.ex - lib/music_library/listening_stats.ex
- lib/music_library_web/live_helpers/record_actions.ex - lib/music_library_web/live_helpers/record_actions.ex
modified_files:
- >-
docs/audits/phase2-pubsub-lifecycle/doc-25 -
Audit-Report-PubSub-Subscription-Lifecycle-Phase-2.md
parent_task_id: ML-169 parent_task_id: ML-169
priority: medium priority: medium
--- ---
@@ -31,10 +35,24 @@ Pre-flagged concern: ListeningStats.subscribe() is called in mount by StatsLive.
<!-- AC:BEGIN --> <!-- AC:BEGIN -->
- [ ] #1 Records.subscribe/1 and Records.unsubscribe/1 call sites audited — confirmed only called via manage_subscription/2 in Show LiveViews - [x] #1 Records.subscribe/1 and Records.unsubscribe/1 call sites audited — confirmed only called via manage_subscription/2 in Show LiveViews
- [ ] #2 manage_subscription/2 verified correct across all navigation paths (direct URL, browser back/forward, push_navigate, push_patch) - [x] #2 manage_subscription/2 verified correct across all navigation paths (direct URL, browser back/forward, push_navigate, push_patch)
- [ ] #3 ListeningStats.subscribe() double-subscription risk on reconnect evaluated — either confirmed safe by PID monitoring or flagged with fix - [x] #3 ListeningStats.subscribe() double-subscription risk on reconnect evaluated — either confirmed safe by PID monitoring or flagged with fix
- [ ] #4 Termination cleanup verified: Phoenix.PubSub auto-cleans on PID death confirmed; any manual unsubscribe gaps documented - [x] #4 Termination cleanup verified: Phoenix.PubSub auto-cleans on PID death confirmed; any manual unsubscribe gaps documented
- [ ] #5 Records.notify_update/1 broadcast sites audited against intended behavior (coordinates with Phase 3 worker audit) - [x] #5 Records.notify_update/1 broadcast sites audited against intended behavior (coordinates with Phase 3 worker audit)
- [ ] #6 Findings report written as a Backlog.md document with file:line references, severity ratings, and fix recommendations - [x] #6 Findings report written as a Backlog.md document with file:line references, severity ratings, and fix recommendations
<!-- AC:END --> <!-- AC:END -->
## Implementation Notes
<!-- SECTION:NOTES:BEGIN -->
Audit complete. Report written at docs/audits/phase2-pubsub-lifecycle/doc-25.
Key findings:
- 27 call sites across subscribe, unsubscribe, broadcast, and consumer handle_info verified
- All 8 subscribe sites have cleanup coverage (explicit unsubscribe or PID-death auto-clean)
- Pre-flagged ListeningStats.subscribe() double-subscription risk is a false positive: Phoenix.PubSub MapSet deduplicates per PID, and LiveView reconnect reuses the same PID within grace period
- No code changes required
<!-- SECTION:NOTES:END -->