Companion to music-mirror, which keeps an MP3 copy of a lossless library for an iPod. This one answers the question that mirror cannot: which of it is worth carrying, and which of it has not been played in years.

This is stage two. It ingests the scrobble history, indexes the library from Lidarr, and matches one to the other. There are no playlists yet, and it writes nothing back — every Lidarr call is a GET. See "Where this is going" below.

What it does today

Pulls the full Last.fm scrobble history into SQLite, then keeps it current.
Tracks loved tracks separately, as the protected set for later stages.
Indexes every artist, album and track Lidarr knows about, with file paths and the date each file landed.
Ties the two together and reports how well it managed.

Matching

Two tiers, and no third.

Tier	Key	Notes
`mbid`	MusicBrainz recording id	Exact. Last.fm's per-scrobble `mbid` against Lidarr's `ForeignRecordingId`
`name`	Normalised artist and title	Everything the first tier could not carry
`none`	—	Recorded as a miss, never guessed at

The name tier does almost all of the work. A recording MBID is exact when it lands, but MusicBrainz holds a separate recording per release, and Last.fm and Lidarr rarely pick the same one: on a real library, two thirds of scrobbles carry a recording id and barely a twentieth of them join on it. The report counts how many carried an id and matched on name anyway, which is the measure of that disagreement.

So the normalisation is the load-bearing part, because the two sides disagree in predictable ways. It folds case and accents, drops guest credits (Yellowcard feat. Tay Jardine against a tag of Yellowcard), strips a trailing version suffix ((Remastered 2011), - Live), expands &, and removes a leading The. Punctuation gets two different rules that pull against each other and are both required: apostrophes are deleted, so Don't meets Dont, while every other mark becomes a space, so AC/DC, AC-DC and AC DC all meet as well.

It leans towards collapsing too much. A false match makes something look played; a missed match makes something look abandoned. Only one of those deletes music.

Indexing quirks

Albums are fetched from the unfiltered GET /api/v1/album first: one request, and the only path that skips albums whose artist metadata is missing rather than dereferencing it.

That is not enough on its own. Every album endpoint maps through a resource that picks the release with SingleOrDefault(x => x.Monitored), which throws for an album with two monitored releases and takes the whole response with it:

HTTP 500: Sequence contains more than one element

When the bulk call dies that way, the indexer falls back to one request per artist. It cannot avoid the exception, but it confines it to whichever artist owns the offending album and names them in the log — which is the only practical way to find it in a large library. Open that artist in Lidarr and check the Releases tab of each album: exactly one release may be monitored.

Losing an artist's albums does not cost their tracks, which come from a different endpoint with a different mapper, so matching is unaffected. A cull would not be, and the report says so.

Talking to Lidarr

Indexing is two requests per artist, and more when the album fallback fires. On a large library that is thousands of requests in a few minutes. urllib opens a new TCP connection and performs a new DNS lookup for every one of them, which is enough to exhaust a container's resolver and produce [Errno -3] Try again on everything at once. The client therefore holds one connection open per host and resolves once.

Transient failures — a dropped connection, a resolver hiccup, 429, 502, 503, 504 — are retried with a backoff. An HTTP 500 is not: it is an unhandled exception inside Lidarr's own serialisation and will be raised again identically. That distinction also decides whether a failure is worth investigating; a library-wide outage is not probed artist by artist, because doing so multiplies the load that caused it.

A local address is preferable to a public hostname here. It removes DNS, the reverse proxy and its timeouts from a path that needs none of them.

Tracks and files have no unfiltered endpoint — Lidarr rejects a call with no filter — so they stay per artist. If one artist cannot be served, that artist is skipped and the run continues, but the count is recorded and the coverage report says so loudly. A missing artist makes their played music look cold, so an incomplete index must never be culled against.

Reading the coverage report

Matched against unmatched is the wrong comparison — most unmatched listening is music that was never in the library, which says nothing at all about the matcher. The line to watch is:

unmatched by an artist the library holds: N pairs, M plays

That is a track that was played by an artist the library holds. It is then split three ways, because owning an artist is a weak proxy for owning a track and a shared title is a weak proxy for a shared song:

the library's own title credits the scrobbled artist — Voodoo People (Pendulum Remix) against a play credited to Pendulum. Same song, filed under the original artist. These are the genuine misses.
the same title under an unrelated artist — a collision, not a miss. Across fifty thousand tracks these are constant: Everyday is Rusko and also Def Leppard, Kaleidoscope is Delta Heavy and also Chappell Roan. Matching on title alone would be far worse than missing them, which is why there is no such tier.
the title is nowhere in the library — never bought.

Only the first is worth chasing. Counting all three as matcher failures overstates the problem and would over-block the cull.

How the ingest works

Two halves, both taking their bounds from the database rather than from a saved cursor, so an interrupted run resumes from what it actually has.

Half	Window	Purpose
Catch-up	`newest held` → `now`	New scrobbles since the last pass
Backfill	start → `oldest held`	Walks towards the beginning of the history

The backfill asks repeatedly for the newest page of everything at or before a cursor, and moves the cursor to the oldest scrobble that came back. When a whole page shares a single second the cursor cannot move without stepping over the rest of that second, so it takes the next page of the same window instead.

Both windows are bounded at both ends, so paging cannot shift under the fetch while new scrobbles arrive mid-run.

Three details of the API that are easy to get wrong, all handled:

The currently-playing track is prepended to the first page with no timestamp at all. Stored once, it would come back on every pass forever.
A lone result is returned as a bare object, not a one-item list.
MBIDs are empty strings rather than absent when unknown, and an empty string looks like a usable join key right up until it silently matches everything.

Scrobbles have no identifier, so the primary key is timestamp plus artist plus track. Two plays of the same track in the same second collapse into one; they are genuinely indistinguishable, and a surrogate key would make re-ingest non-idempotent, which is a far worse trade.

Retries cover error 29 (rate limit) and the backend failures, 8, 11 and 16, with an exponential backoff. An invalid or suspended key fails immediately rather than retrying four more times to reach the same conclusion.

Usage

music-curator                 # one pass, then report
music-curator --interval 6h   # keep running
music-curator --report-only   # report on the store, fetch nothing

Option	Environment variable	Default	Meaning
`--user`	`MUSIC_CURATOR_LASTFM_USER`	—	Last.fm username to read
`--api-key`	`MUSIC_CURATOR_LASTFM_API_KEY`	—	Last.fm API key
`--db`	`MUSIC_CURATOR_DB`	`/data/curator.db`	Path to the SQLite store
`--interval`	`MUSIC_CURATOR_INTERVAL`	unset	Repeat forever, e.g. `45m`, `6h`, `1d`
`--request-delay`	`MUSIC_CURATOR_REQUEST_DELAY`	`0.25`	Seconds between API requests
`--backfill-limit`	`MUSIC_CURATOR_BACKFILL_LIMIT`	`0`	Cap backfill requests per pass; 0 for no cap
`--lidarr-url`	`MUSIC_CURATOR_LIDARR_URL`	unset	Lidarr base URL, e.g. `http://lidarr:8686`
`--lidarr-api-key`	`MUSIC_CURATOR_LIDARR_API_KEY`	unset	Lidarr API key
`--skip-index`	—	off	Match against the index already held
`--report-only`	—	off	Report without fetching

A Last.fm API key is all that is needed. None of the endpoints used here authenticate a user, so there is no shared secret, no session key and no signing.

The first pass over a long history is thousands of requests at 200 scrobbles each. --backfill-limit spreads that over several passes if you would rather not do it in one.

Running it on TrueNAS Scale

compose.yaml is a Custom App definition. Adjust the host path and the user: to match your pool, set the API key through the TrueNAS UI rather than in the file, then add it as a custom app. The image is published to this Gitea's registry on every release:

code.emmathe.dev/lyrathorpe/music-curator:latest

Give the store its own dataset. It is derived data and can be rebuilt from Last.fm, but rebuilding means downloading the whole history again.

Tests

docker build --target test .   # what CI runs
pytest                         # needs pytest on PATH

The suite runs against fake transports for both services. The Last.fm one reproduces its paging, its from/to semantics and its awkward response shapes; the Lidarr one serves a canned library split across the same four endpoints the indexer calls, so the stitching is exercised rather than stubbed. No network, no credentials, no rate limit. On a Nix machine:

nix shell nixpkgs#python3Packages.pytest -c pytest

Where this is going

Stage	Status
Last.fm ingest and store	done
Lidarr index and the scrobble-to-track matcher	done
M3U playlists written into the mirror	next
Cold-music report, unmonitoring what is not played	last

The cull will unmonitor cold albums in Lidarr and tag their artists. It will never delete files: Lidarr's AlbumResource has no tags at all, so tagging only works per artist, and an artist is only tagged when every one of their albums qualifies. It will also refuse to run at all if the matcher's coverage is poor, because an unmatched track is not an unplayed track.