feat(worker): share the MusicBrainz cache from scans/discovery

The worker's MB browser (discography + artist search, used by monitor sweeps,
library scans, and Last.fm discovery) re-fetched every time. Route it through
the same ApiCache table + logical keys the web app uses, so a discography one
process fetched warms the other, and repeat lookups stop hitting MB.

- apicache.py cached_api(): read-through, serve-stale-on-outage, never caches
  null, best-effort (a cache-DB error degrades to a live fetch, never breaks
  scan/sweep/discovery). Its own DEDICATED autocommit connection so it can't
  commit the worker's in-flight transaction, and autocommit avoids freezing
  Postgres now() (which would break TTL math).
- CachingMbBrowser decorates MusicBrainzBrowser, serializing to the SAME
  camelCase JSON shape the web writes (round-trip tested). registry.build_browser
  wraps it when a DSN is present; the Last.fm source's browser rides it too.
- Hourly prune drops ApiCache rows > 90d so the table stays bounded.
- worker 213 tests / 7-skip.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -42,3 +42,4 @@ Thumbs.db
 *.swp
 slskd-downloads/
 backups/
+.playwright-mcp/

web/prisma/migrations/20260714090112_add_api_cache/migration.sql

@@ -0,0 +1,11 @@
+-- CreateTable
+CREATE TABLE "ApiCache" (
+    "key" TEXT NOT NULL,
+    "json" JSONB NOT NULL,
+    "fetchedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "ApiCache_pkey" PRIMARY KEY ("key")
+);
+
+-- CreateIndex
+CREATE INDEX "ApiCache_fetchedAt_idx" ON "ApiCache"("fetchedAt");

web/prisma/schema.prisma

@@ -182,3 +182,15 @@ model ScanWorkItem {
   @@unique([scanId, path])
   @@index([scanId, done, artist, album])
 }
+
+// Generic read-through cache for external API responses shared by web + worker (keyed on
+// the LOGICAL operation, not the request URL, so both processes share entries). TTL is
+// applied at read time from fetchedAt in code (no expiresAt column → tunable, no migration).
+// Named generically so the Last.fm browse cache can ride the same table.
+model ApiCache {
+  key       String   @id
+  json      Json
+  fetchedAt DateTime @default(now())
+
+  @@index([fetchedAt])
+}

web/src/app/_ui/album-modal.tsx

@@ -6,6 +6,10 @@ import { CoverArt } from "./cover-art";
 
 type Track = { position: number; title: string; lengthMs: number | null };
 
+// Session-scoped, per-release-group tracklist cache: reopening the same album modal is
+// instant with zero network. Complements the server-side ApiCache (which spans sessions).
+const trackCache = new Map<string, Track[]>();
+
 function fmt(ms: number | null): string {
   if (ms == null) return "";
   const s = Math.round(ms / 1000);
@@ -39,15 +43,25 @@ export function AlbumModal({
 
   useEffect(() => {
     if (!open) return;
-    if (!album.rgMbid) {
+    const rgMbid = album.rgMbid;
+    if (!rgMbid) {
       setTracks("error");
       return;
     }
+    const hit = trackCache.get(rgMbid);
+    if (hit) {
+      setTracks(hit);
+      return;
+    }
     setTracks("loading");
     let cancelled = false;
-    fetch(`/api/mb/release-groups/${album.rgMbid}/tracks`)
+    fetch(`/api/mb/release-groups/${rgMbid}/tracks`)
       .then((r) => (r.ok ? r.json() : Promise.reject()))
-      .then((d) => !cancelled && setTracks(d.tracks))
+      .then((d) => {
+        if (cancelled) return;
+        trackCache.set(rgMbid, d.tracks);
+        setTracks(d.tracks);
+      })
       .catch(() => !cancelled && setTracks("error"));
     return () => {
       cancelled = true;

web/src/lib/apicache.test.ts

@@ -0,0 +1,54 @@
+import { describe, it, expect, vi } from "vitest";
+import { cached, normKey } from "./apicache";
+import { prisma } from "@/lib/db";
+
+describe("cached (read-through)", () => {
+  it("fetches once, then serves the cached value without re-fetching", async () => {
+    const fetchFresh = vi.fn(async () => ({ v: 1 }));
+    const first = await cached("k1", 3600, fetchFresh);
+    const second = await cached("k1", 3600, fetchFresh);
+    expect(first).toEqual({ v: 1 });
+    expect(second).toEqual({ v: 1 });
+    expect(fetchFresh).toHaveBeenCalledTimes(1); // second served from cache
+  });
+
+  it("re-fetches once the TTL has elapsed", async () => {
+    const fetchFresh = vi.fn(async () => ({ v: 2 }));
+    await cached("k2", 3600, fetchFresh);
+    // backdate the row beyond the ttl
+    await prisma.apiCache.update({ where: { key: "k2" }, data: { fetchedAt: new Date(Date.now() - 7200_000) } });
+    await cached("k2", 3600, fetchFresh);
+    expect(fetchFresh).toHaveBeenCalledTimes(2);
+  });
+
+  it("serves a stale value when the fresh fetch throws", async () => {
+    await cached("k3", 3600, async () => ({ v: "old" }));
+    await prisma.apiCache.update({ where: { key: "k3" }, data: { fetchedAt: new Date(Date.now() - 7200_000) } });
+    const result = await cached("k3", 3600, async () => {
+      throw new Error("upstream down");
+    });
+    expect(result).toEqual({ v: "old" }); // stale-on-outage
+  });
+
+  it("rethrows when the fetch fails and there is no cached row", async () => {
+    await expect(
+      cached("k4", 3600, async () => {
+        throw new Error("boom");
+      }),
+    ).rejects.toThrow("boom");
+    // nothing was cached
+    expect(await prisma.apiCache.findUnique({ where: { key: "k4" } })).toBeNull();
+  });
+
+  it("does not cache null results", async () => {
+    const fetchFresh = vi.fn(async () => null);
+    await cached("k5", 3600, fetchFresh);
+    await cached("k5", 3600, fetchFresh);
+    expect(fetchFresh).toHaveBeenCalledTimes(2); // null never cached → always re-fetched
+    expect(await prisma.apiCache.findUnique({ where: { key: "k5" } })).toBeNull();
+  });
+
+  it("normKey trims and lowercases", () => {
+    expect(normKey("  Radiohead ")).toBe("radiohead");
+  });
+});

web/src/lib/apicache.ts

@@ -0,0 +1,39 @@
+import { prisma } from "@/lib/db";
+
+// Read-through cache over the shared ApiCache table. Keyed on the LOGICAL operation (not the
+// request URL) so the worker and web app share entries. TTL is applied here from fetchedAt,
+// so TTLs are tunable in code with no migration. On a fetch failure a stale row is served.
+// Null/undefined results are NOT cached (keeps negative caching out + avoids Prisma JsonNull).
+
+export const CACHE_TTL = {
+  DAY: 86_400,
+  WEEK: 7 * 86_400,
+  MONTH: 30 * 86_400,
+};
+
+export async function cached<T>(key: string, ttlSec: number, fetchFresh: () => Promise<T>): Promise<T> {
+  const hit = await prisma.apiCache.findUnique({ where: { key } });
+  if (hit && (Date.now() - hit.fetchedAt.getTime()) / 1000 < ttlSec) {
+    return hit.json as T;
+  }
+  try {
+    const fresh = await fetchFresh();
+    if (fresh !== null && fresh !== undefined) {
+      await prisma.apiCache.upsert({
+        where: { key },
+        create: { key, json: fresh as object },
+        update: { json: fresh as object, fetchedAt: new Date() },
+      });
+    }
+    return fresh;
+  } catch (e) {
+    if (hit) return hit.json as T; // serve stale on an upstream outage
+    throw e;
+  }
+}
+
+/** Normalize a free-text search term so casing/whitespace don't fragment the cache. MUST
+ *  match the worker's normalization byte-for-byte (both: trim + lowercase). */
+export function normKey(s: string): string {
+  return s.trim().toLowerCase();
+}

web/src/lib/musicbrainz.ts

@@ -1,3 +1,5 @@
+import { cached, normKey, CACHE_TTL } from "@/lib/apicache";
+
 const MB_BASE = "https://musicbrainz.org/ws/2";
 const USER_AGENT = "Lyra/0.1 ( https://git.jger.nl/Jonathan/Lyra )";
 
@@ -30,28 +32,34 @@ function toReleaseGroup(g: any): ReleaseGroupInfo {
 }
 
 export async function searchArtists(query: string): Promise<ArtistHit[]> {
-  const data = await mbGet(`/artist?query=${encodeURIComponent(query)}&fmt=json&limit=8`);
-  return (data.artists ?? []).map((a: any) => ({
-    mbid: a.id,
-    name: a.name ?? "",
-    disambiguation: a.disambiguation ?? "",
-  }));
+  const q = normKey(query);
+  if (!q) return [];
+  return cached(`artist-search:${q}`, CACHE_TTL.DAY, async () => {
+    const data = await mbGet(`/artist?query=${encodeURIComponent(query)}&fmt=json&limit=8`);
+    return (data.artists ?? []).map((a: any) => ({
+      mbid: a.id,
+      name: a.name ?? "",
+      disambiguation: a.disambiguation ?? "",
+    }));
+  });
 }
 
 export async function browseReleaseGroups(artistMbid: string): Promise<ReleaseGroupInfo[]> {
-  const out: ReleaseGroupInfo[] = [];
-  let offset = 0;
-  for (;;) {
-    const data = await mbGet(
-      `/release-group?artist=${encodeURIComponent(artistMbid)}&fmt=json&limit=100&offset=${offset}`,
-    );
-    const groups: any[] = data["release-groups"] ?? [];
-    for (const g of groups) out.push(toReleaseGroup(g));
-    offset += groups.length;
-    const total = data["release-group-count"] ?? offset;
-    if (groups.length === 0 || offset >= total) break;
-  }
-  return out;
+  return cached(`artist-rgs:${artistMbid}`, CACHE_TTL.MONTH, async () => {
+    const out: ReleaseGroupInfo[] = [];
+    let offset = 0;
+    for (;;) {
+      const data = await mbGet(
+        `/release-group?artist=${encodeURIComponent(artistMbid)}&fmt=json&limit=100&offset=${offset}`,
+      );
+      const groups: any[] = data["release-groups"] ?? [];
+      for (const g of groups) out.push(toReleaseGroup(g));
+      offset += groups.length;
+      const total = data["release-group-count"] ?? offset;
+      if (groups.length === 0 || offset >= total) break;
+    }
+    return out;
+  });
 }
 
 function escapeLucenePhrase(s: string): string {
@@ -94,6 +102,12 @@ function albumPreferenceRank(g: any): number {
 }
 
 export async function searchReleaseGroup(artist: string, album: string): Promise<ReleaseGroupMatch | null> {
+  return cached(`rg-search:${normKey(artist)}|${normKey(album)}`, CACHE_TTL.WEEK, () =>
+    _searchReleaseGroup(artist, album),
+  );
+}
+
+async function _searchReleaseGroup(artist: string, album: string): Promise<ReleaseGroupMatch | null> {
   const art = escapeLucenePhrase(artist);
   const stripped = stripEditionQualifiers(album);
   // Decreasing precision: exact phrase on the given title, then on the edition-stripped
@@ -121,32 +135,36 @@ export async function searchReleaseGroup(artist: string, album: string): Promise
 export type Track = { position: number; title: string; lengthMs: number | null };
 
 export async function getArtistName(mbid: string): Promise<string> {
-  const data = await mbGet(`/artist/${encodeURIComponent(mbid)}?fmt=json`);
-  return data.name ?? "";
+  return cached(`artist-name:${mbid}`, CACHE_TTL.MONTH, async () => {
+    const data = await mbGet(`/artist/${encodeURIComponent(mbid)}?fmt=json`);
+    return data.name ?? "";
+  });
 }
 
 export async function browseReleaseGroupTracks(rgMbid: string): Promise<Track[]> {
-  const data = await mbGet(
-    `/release?release-group=${encodeURIComponent(rgMbid)}&inc=recordings&fmt=json&limit=25`,
-  );
-  const releases: any[] = data.releases ?? [];
-  if (releases.length === 0) return [];
-  // Prefer an Official release, then the earliest date.
-  const pick = [...releases].sort((a, b) => {
-    const ao = a.status === "Official" ? 0 : 1;
-    const bo = b.status === "Official" ? 0 : 1;
-    if (ao !== bo) return ao - bo;
-    return (a.date ?? "9999").localeCompare(b.date ?? "9999");
-  })[0];
-  const tracks: Track[] = [];
-  for (const medium of pick.media ?? []) {
-    for (const t of medium.tracks ?? []) {
-      tracks.push({
-        position: Number(t.position ?? tracks.length + 1),
-        title: t.title ?? t.recording?.title ?? "",
-        lengthMs: t.length ?? t.recording?.length ?? null,
-      });
+  return cached(`rg-tracks:${rgMbid}`, CACHE_TTL.MONTH, async () => {
+    const data = await mbGet(
+      `/release?release-group=${encodeURIComponent(rgMbid)}&inc=recordings&fmt=json&limit=25`,
+    );
+    const releases: any[] = data.releases ?? [];
+    if (releases.length === 0) return [];
+    // Prefer an Official release, then the earliest date.
+    const pick = [...releases].sort((a, b) => {
+      const ao = a.status === "Official" ? 0 : 1;
+      const bo = b.status === "Official" ? 0 : 1;
+      if (ao !== bo) return ao - bo;
+      return (a.date ?? "9999").localeCompare(b.date ?? "9999");
+    })[0];
+    const tracks: Track[] = [];
+    for (const medium of pick.media ?? []) {
+      for (const t of medium.tracks ?? []) {
+        tracks.push({
+          position: Number(t.position ?? tracks.length + 1),
+          title: t.title ?? t.recording?.title ?? "",
+          lengthMs: t.length ?? t.recording?.length ?? null,
+        });
+      }
     }
-  }
-  return tracks;
+    return tracks;
+  });
 }

web/src/test/setup.ts

@@ -17,4 +17,5 @@ beforeEach(async () => {
   await prisma.watchedArtist.deleteMany();
   await prisma.discoverySuggestion.deleteMany();
   await prisma.config.deleteMany();
+  await prisma.apiCache.deleteMany(); // read-through cache must not leak across tests
 });

worker/lyra_worker/_mbbrowser.py

@@ -1,6 +1,72 @@
+from lyra_worker.apicache import DAY, MONTH, cached_api, norm_key, open_cache_conn
 from lyra_worker.browser import ArtistHit, ReleaseGroupInfo
 
 
+# Serialize to the SAME camelCase JSON shape the web app stores (web/src/lib/musicbrainz.ts),
+# so a discography one process fetched warms the cache for the other.
+def _artist_hit_to_json(a: ArtistHit) -> dict:
+    return {"mbid": a.mbid, "name": a.name, "disambiguation": a.disambiguation}
+
+
+def _artist_hit_from_json(d: dict) -> ArtistHit:
+    return ArtistHit(mbid=d["mbid"], name=d.get("name", ""), disambiguation=d.get("disambiguation", ""))
+
+
+def _rg_to_json(r: ReleaseGroupInfo) -> dict:
+    return {
+        "rgMbid": r.rg_mbid,
+        "title": r.title,
+        "primaryType": r.primary_type or None,
+        "secondaryTypes": list(r.secondary_types),
+        "firstReleaseDate": r.first_release_date or None,
+    }
+
+
+def _rg_from_json(d: dict) -> ReleaseGroupInfo:
+    return ReleaseGroupInfo(
+        rg_mbid=d["rgMbid"],
+        title=d.get("title", "") or "",
+        primary_type=d.get("primaryType") or "",
+        secondary_types=tuple(d.get("secondaryTypes") or ()),
+        first_release_date=d.get("firstReleaseDate") or "",
+    )
+
+
+class CachingMbBrowser:
+    """Wraps an MbBrowser with the shared ApiCache read-through, keyed on the same logical
+    strings the web side uses (`artist-search:{q}`, `artist-rgs:{mbid}`). Uses a dedicated,
+    lazily (re)opened cache connection; if it can't connect, it just calls through uncached."""
+
+    def __init__(self, inner, dsn: str):
+        self._inner = inner
+        self._dsn = dsn
+        self._conn = None
+
+    def _cache_conn(self):
+        if self._conn is not None and not self._conn.closed:
+            return self._conn
+        self._conn = open_cache_conn(self._dsn)
+        return self._conn
+
+    def search_artist(self, name: str) -> list[ArtistHit]:
+        conn = self._cache_conn()
+        if conn is None:
+            return self._inner.search_artist(name)
+        key = f"artist-search:{norm_key(name)}"
+        rows = cached_api(conn, key, DAY,
+                          lambda: [_artist_hit_to_json(a) for a in self._inner.search_artist(name)])
+        return [_artist_hit_from_json(d) for d in rows]
+
+    def browse_release_groups(self, artist_mbid: str) -> list[ReleaseGroupInfo]:
+        conn = self._cache_conn()
+        if conn is None:
+            return self._inner.browse_release_groups(artist_mbid)
+        key = f"artist-rgs:{artist_mbid}"
+        rows = cached_api(conn, key, MONTH,
+                          lambda: [_rg_to_json(r) for r in self._inner.browse_release_groups(artist_mbid)])
+        return [_rg_from_json(d) for d in rows]
+
+
 class MusicBrainzBrowser:
     """Real MbBrowser via musicbrainzngs (imported lazily; not unit-tested offline)."""
 

worker/lyra_worker/apicache.py

@@ -0,0 +1,82 @@
+"""Read-through cache over the shared ApiCache table (same table + logical keys the web app
+uses, so web and worker share entries). TTL applied here from fetchedAt; serves a stale row
+if the fresh fetch fails; never caches None.
+
+Best-effort: all DB access is guarded so a cache-layer problem NEVER breaks the caller
+(scan/sweep/discovery) — on a DB error it just degrades to a live fetch. It is given a
+DEDICATED connection (not the worker's main conn) so its commits can't commit the worker's
+in-flight transaction."""
+from typing import Callable, TypeVar
+
+import psycopg
+from psycopg.types.json import Jsonb
+
+T = TypeVar("T")
+
+DAY = 86_400
+WEEK = 7 * 86_400
+MONTH = 30 * 86_400
+
+
+def _read(conn, key: str):
+    try:
+        with conn.cursor() as cur:
+            cur.execute(
+                'SELECT json, extract(epoch from (now() - "fetchedAt")) FROM "ApiCache" WHERE key = %s',
+                (key,),
+            )
+            return cur.fetchone()
+    except Exception:
+        try:
+            conn.rollback()
+        except Exception:
+            pass
+        return None
+
+
+def _write(conn, key: str, value) -> None:
+    try:
+        with conn.cursor() as cur:
+            cur.execute(
+                'INSERT INTO "ApiCache" (key, json, "fetchedAt") VALUES (%s, %s, now()) '
+                'ON CONFLICT (key) DO UPDATE SET json = EXCLUDED.json, "fetchedAt" = now()',
+                (key, Jsonb(value)),
+            )
+        conn.commit()
+    except Exception:
+        try:
+            conn.rollback()
+        except Exception:
+            pass
+
+
+def cached_api(conn, key: str, ttl_sec: int, fetch_fresh: Callable[[], T]) -> T:
+    row = _read(conn, key)
+    if row is not None and row[1] < ttl_sec:
+        return row[0]
+    try:
+        fresh = fetch_fresh()
+    except Exception:
+        if row is not None:
+            return row[0]  # serve stale on an upstream outage
+        raise
+    if fresh is not None:
+        _write(conn, key, fresh)
+    return fresh
+
+
+def norm_key(s: str) -> str:
+    """Match the web side's normKey (trim + lowercase) so search keys don't fragment."""
+    return s.strip().lower()
+
+
+def open_cache_conn(dsn: str):
+    """A dedicated AUTOCOMMIT connection for cache I/O, or None if it can't be opened (the
+    cache then simply no-ops). Kept separate from the worker's main connection. Autocommit is
+    essential: a read that hits the TTL returns without committing, and a non-autocommit conn
+    would leave that transaction open — freezing Postgres `now()` at the transaction start
+    (now() = transaction timestamp), which would break every subsequent TTL comparison."""
+    try:
+        return psycopg.connect(dsn, autocommit=True)
+    except Exception:
+        return None

worker/lyra_worker/main.py

@@ -23,6 +23,7 @@ IDLE_SLEEP = 2.0
 HEARTBEAT_SECONDS = 15.0  # how often to stamp worker.heartbeat (its updatedAt = liveness)
 MONITOR_TICK_SECONDS = 60.0
 DISCOVER_TICK_SECONDS = 300.0  # check the discovery interval every 5 min (interval itself is hours)
+CACHE_PRUNE_SECONDS = 3600.0  # prune the shared ApiCache at most hourly (drops rows > 90d old)
 DEST_ROOT = "/music"  # must match run_pipeline's default library root
 # Per-job download staging. Defaults inside the library (/music/.staging) to preserve prior
 # behavior; set STAGING_ROOT (mounted as a separate volume) to keep temp download I/O off a
@@ -167,6 +168,23 @@ def maybe_run_discovery(conn, sources, browser, config, dcfg, now, last_discover
     return now if due else last_discover_tick
 
 
+def prune_api_cache(conn) -> None:
+    """Drop ApiCache rows older than 90 days so the shared cache stays bounded. Best-effort:
+    a failure here (e.g. a transient error) must never take down the worker loop."""
+    try:
+        with conn.cursor() as cur:
+            cur.execute('DELETE FROM "ApiCache" WHERE "fetchedAt" < now() - interval \'90 days\'')
+        conn.commit()
+    except psycopg.OperationalError:
+        raise  # a real disconnect — let the loop's reconnect handler deal with it
+    except Exception as e:
+        print(f"worker: ApiCache prune failed: {e}", flush=True)
+        try:
+            conn.rollback()
+        except Exception:
+            pass
+
+
 def _require_secret_key() -> None:
     """Fail fast (or warn loudly) on a missing/placeholder/public LYRA_SECRET_KEY before
     the worker touches any encrypted credential."""
@@ -192,9 +210,10 @@ def run_forever() -> None:
     adapters = build_adapters(get_config(conn))
     resolver = build_resolver()
     tagger = build_tagger()
-    browser = build_browser()
+    _dsn = os.environ.get("DATABASE_URL")
+    browser = build_browser(_dsn)  # shared ApiCache read-through when a DSN is present
     probe = build_probe()
-    similarity_sources = build_similarity_sources(get_config(conn))
+    similarity_sources = build_similarity_sources(get_config(conn), _dsn)
     # Backfill: releases pressed while the monitor was off never got reconciled, so they linger
     # on the wanted list. Fix any already-owned ones once at startup.
     fixed = fulfill_owned_releases(conn, MonitorConfig.from_config(get_config(conn)))
@@ -205,6 +224,7 @@ def run_forever() -> None:
     last_tick = 0.0
     last_discover_tick = 0.0
     last_heartbeat = 0.0
+    last_prune = 0.0
     try:
         while True:
             # A transient DB connection loss (e.g. AdminShutdown when Postgres restarts) raises
@@ -218,6 +238,9 @@ def run_forever() -> None:
                 if now - last_heartbeat >= HEARTBEAT_SECONDS:
                     _set_config(conn, "worker.heartbeat", "1")  # value irrelevant; updatedAt is the signal
                     last_heartbeat = now
+                if now - last_prune >= CACHE_PRUNE_SECONDS:
+                    prune_api_cache(conn)  # keep the shared ApiCache bounded
+                    last_prune = now
                 if mcfg.enabled and now - last_tick >= MONITOR_TICK_SECONDS:
                     try:
                         sweep(conn, browser, mcfg)

worker/lyra_worker/registry.py

@@ -1,4 +1,4 @@
-from lyra_worker._mbbrowser import MusicBrainzBrowser
+from lyra_worker._mbbrowser import CachingMbBrowser, MusicBrainzBrowser
 from lyra_worker._musicbrainz import MusicBrainzResolver
 from lyra_worker._mutagen import MutagenTagger
 from lyra_worker._probe import MutagenProbe
@@ -42,9 +42,12 @@ def build_tagger() -> Tagger:
     return MutagenTagger()
 
 
-def build_browser() -> MbBrowser:
-    """The MusicBrainz browser used by the background monitor."""
-    return MusicBrainzBrowser()
+def build_browser(dsn: str | None = None) -> MbBrowser:
+    """The MusicBrainz browser used by the background monitor. When a DSN is given, wrap it
+    in the shared ApiCache read-through so scans/discovery stop re-fetching discographies
+    (and warm the same cache the web modals read)."""
+    inner = MusicBrainzBrowser()
+    return CachingMbBrowser(inner, dsn) if dsn else inner
 
 
 def build_probe() -> AudioProbe:
@@ -52,14 +55,15 @@ def build_probe() -> AudioProbe:
     return MutagenProbe()
 
 
-def build_similarity_sources(config: dict) -> list[SimilaritySource]:
+def build_similarity_sources(config: dict, dsn: str | None = None) -> list[SimilaritySource]:
     """Similarity sources for discovery. ListenBrainz needs no credentials, so it is
     always constructed; per-run reachability is checked via each source's health().
-    Last.fm is added only when an API key is configured."""
+    Last.fm is added only when an API key is configured. Its MB name→MBID lookups ride the
+    shared ApiCache when a DSN is given."""
     sources: list[SimilaritySource] = [
         ListenBrainzSource(base_url=config.get("discover.listenBrainzUrl") or "")
     ]
     key = config.get("lastfm.api_key")
     if key:
-        sources.append(LastfmSource(api_key=key, browser=MusicBrainzBrowser()))
+        sources.append(LastfmSource(api_key=key, browser=build_browser(dsn)))
     return sources

worker/tests/conftest.py

@@ -30,6 +30,7 @@ def conn():
         cur.execute('DELETE FROM "DiscoverySuggestion"')
         cur.execute('DELETE FROM "DiscoverySeedContribution"')
         cur.execute('DELETE FROM "ScanWorkItem"')
+        cur.execute('DELETE FROM "ApiCache"')
         cur.execute('DELETE FROM "Config"')
     connection.commit()
     yield connection
@@ -47,6 +48,7 @@ def conn():
         cur.execute('DELETE FROM "DiscoverySuggestion"')
         cur.execute('DELETE FROM "DiscoverySeedContribution"')
         cur.execute('DELETE FROM "ScanWorkItem"')
+        cur.execute('DELETE FROM "ApiCache"')
         cur.execute('DELETE FROM "Config"')
     connection.commit()
     connection.close()

worker/tests/test_apicache.py

@@ -0,0 +1,104 @@
+"""Read-through + shared-shape tests for the worker cache. cached_api uses the real
+lyra_test DB (via the conn fixture); MB is never hit (fetch_fresh is a fake)."""
+from lyra_worker.apicache import cached_api, norm_key
+from lyra_worker._mbbrowser import (
+    _artist_hit_from_json, _artist_hit_to_json, _rg_from_json, _rg_to_json,
+)
+from lyra_worker.browser import ArtistHit, ReleaseGroupInfo
+
+
+def _backdate(conn, key, seconds):
+    with conn.cursor() as cur:
+        cur.execute(
+            'UPDATE "ApiCache" SET "fetchedAt" = now() - make_interval(secs => %s) WHERE key = %s',
+            (seconds, key),
+        )
+    conn.commit()
+
+
+def test_read_through_fetches_once(conn):
+    calls = []
+
+    def fresh():
+        calls.append(1)
+        return {"v": 1}
+
+    assert cached_api(conn, "k1", 3600, fresh) == {"v": 1}
+    assert cached_api(conn, "k1", 3600, fresh) == {"v": 1}
+    assert len(calls) == 1  # second served from cache
+
+
+def test_refetches_after_ttl(conn):
+    calls = []
+
+    def fresh():
+        calls.append(1)
+        return {"v": len(calls)}
+
+    cached_api(conn, "k2", 3600, fresh)
+    _backdate(conn, "k2", 7200)  # older than the ttl
+    cached_api(conn, "k2", 3600, fresh)
+    assert len(calls) == 2
+
+
+def test_serves_stale_on_fetch_error(conn):
+    cached_api(conn, "k3", 3600, lambda: {"v": "old"})
+    _backdate(conn, "k3", 7200)
+
+    def boom():
+        raise RuntimeError("mb down")
+
+    assert cached_api(conn, "k3", 3600, boom) == {"v": "old"}
+
+
+def test_reraises_when_no_cached_row(conn):
+    def boom():
+        raise RuntimeError("mb down")
+
+    try:
+        cached_api(conn, "k4-missing", 3600, boom)
+    except RuntimeError as e:
+        assert "mb down" in str(e)
+    else:
+        raise AssertionError("expected the fetch error to propagate")
+
+
+def test_does_not_cache_none(conn):
+    calls = []
+
+    def fresh():
+        calls.append(1)
+        return None
+
+    cached_api(conn, "k5", 3600, fresh)
+    cached_api(conn, "k5", 3600, fresh)
+    assert len(calls) == 2  # None never cached
+    with conn.cursor() as cur:
+        cur.execute('SELECT 1 FROM "ApiCache" WHERE key = %s', ("k5",))
+        assert cur.fetchone() is None
+
+
+def test_norm_key():
+    assert norm_key("  Radiohead ") == "radiohead"
+
+
+def test_release_group_json_roundtrip():
+    r = ReleaseGroupInfo(rg_mbid="rg1", title="OK Computer", primary_type="Album",
+                         secondary_types=("Live",), first_release_date="1997-05-21")
+    assert _rg_from_json(_rg_to_json(r)) == r
+    # a bare group (no type/date) round-trips to the worker's empty-string defaults
+    bare = ReleaseGroupInfo(rg_mbid="rg2", title="X")
+    assert _rg_from_json(_rg_to_json(bare)) == bare
+
+
+def test_artist_hit_json_roundtrip():
+    a = ArtistHit(mbid="a1", name="Radiohead", disambiguation="UK band")
+    assert _artist_hit_from_json(_artist_hit_to_json(a)) == a
+
+
+def test_worker_shape_matches_web_camelcase():
+    # the stored dict MUST use the same camelCase keys the web app writes/reads
+    d = _rg_to_json(ReleaseGroupInfo(rg_mbid="rg", title="T", primary_type="Album",
+                                     secondary_types=(), first_release_date="2000"))
+    assert set(d) == {"rgMbid", "title", "primaryType", "secondaryTypes", "firstReleaseDate"}
+    assert set(_artist_hit_to_json(ArtistHit(mbid="a", name="N"))) == {"mbid", "name", "disambiguation"}