ML-151: document Assets.Cache TTL and invalidation strategy

Expand @moduledoc with cache key structure, 7-day TTL via
@one_week_seconds, periodic pruning by PruneAssetCache (every 12h),
and rationale for TTL-only invalidation (content-addressable immutable
assets). Update architecture.md entry accordingly.

backlog/tasks/ml-151 - Document-Assets.Cache-TTL-and-cache-invalidation-strategy.md

@@ -1,9 +1,10 @@
 ---
 id: ML-151
 title: Document Assets.Cache TTL and cache invalidation strategy
-status: To Do
+status: Done
 assignee: []
 created_date: '2026-04-30 10:48'
+updated_date: '2026-04-30 12:04'
 labels:
   - documentation
   - assets
@@ -28,7 +29,35 @@ Add a `@moduledoc` to `Assets.Cache` that explains:
 
 ## Acceptance Criteria
 <!-- AC:BEGIN -->
-- [ ] #1 `Assets.Cache` `@moduledoc` documents the TTL value and where it is configured
-- [ ] #2 The invalidation strategy (TTL-based expiry) is explained with rationale
-- [ ] #3 Any architectural implications are captured in `docs/architecture.md`
+- [x] #1 `Assets.Cache` `@moduledoc` documents the TTL value and where it is configured
+- [x] #2 The invalidation strategy (TTL-based expiry) is explained with rationale
+- [x] #3 Any architectural implications are captured in `docs/architecture.md`
 <!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+## Plan
+
+### What we're documenting
+
+`Assets.Cache` is an ETS-based cache for transformed binary asset data. Assets are stored by SHA256 hash and are immutable (write-once). The cache key is `{payload, format}`.
+
+### Key facts to document
+
+- **TTL**: `@one_week_seconds` = 7 days, defined as a module attribute
+- **Pruning**: `prune/0` defaults to 7-day TTL, called every 12h by `PruneAssetCache` Oban cron worker
+- **Invalidation strategy**: TTL-based expiry only — no explicit invalidation
+
+### Rationale
+
+Explicit invalidation isn't needed because:
+1. Assets are content-addressable (SHA256) and immutable — updates create new hashes, so old entries are never requested again
+2. Periodic pruning (every 12h) cleans up stale entries
+3. ETS table is in-memory, cleared on restart
+
+### Changes
+
+1. Expand `@moduledoc` in `lib/music_library/assets/cache.ex`
+2. Expand the `Assets.Cache` entry in `docs/architecture.md` Business Logic Modules table
+<!-- SECTION:PLAN:END -->

docs/architecture.md

@@ -123,7 +123,7 @@ Last.fm schemas (separate, not Ecto-persisted to main DB):
 | `Req.RateLimiter` | ETS-backed Req request step enforcing per-API minimum intervals between requests |
 | `Req.RateLimiter.Clock` | Behaviour for time operations (allows test clock injection) |
 | `Req.RateLimiter.SystemClock` | Real clock implementation using System.monotonic_time |
-| `Assets.Cache` | ETS-based asset cache with TTL |
+| `Assets.Cache` | ETS-based asset cache with TTL (7-day TTL, TTL-only invalidation since assets are content-addressable and immutable) |
 | `Assets.Image` / `Assets.Transform` | Image processing via Vix (libvips) |
 | `Colors.Extractor` | Behaviour for dominant color extraction (configurable, allows test stubbing) |
 | `Colors.KMeansExtractor` | Color extraction via K-Means clustering (dominant_colors library), implements `Colors.Extractor` |

lib/music_library/assets/cache.ex

@@ -1,6 +1,37 @@
 defmodule MusicLibrary.Assets.Cache do
   @moduledoc """
   ETS-based asset cache with TTL for serving frequently accessed images.
+
+  ## Cache key
+
+  Each entry is keyed by `{payload, format}` where `payload` is a transform
+  parameter string (encoding width and asset hash) and `format` is the output
+  MIME type (e.g. `"image/webp"`).
+
+  ## TTL and pruning
+
+  The TTL is configured via the `@one_week_seconds` module attribute
+  (`60 * 60 * 24 * 7` = 7 days). The `prune/0` function deletes all entries
+  whose `inserted_at` timestamp is older than this threshold.
+
+  Pruning runs automatically every 12 hours via the `PruneAssetCache` Oban
+  cron worker.
+
+  ## Invalidation strategy
+
+  This cache uses **TTL-based expiry only** — there is no explicit
+  invalidation. This is sufficient because:
+
+  1. Assets are content-addressable (SHA256 hashes) and immutable once
+     written. When an image is replaced (e.g. a new cover is uploaded),
+     the new asset gets a new hash, producing a different cache key. The
+     old entry is never requested again and naturally expires.
+
+  2. The ETS table is in-memory and cleared on application restart.
+
+  Explicit invalidation would add complexity (tracking which cache entries
+  derive from which original asset hash) with no practical benefit given
+  the above properties.
   """
 
   @spec new() :: :ets.table()