feat: initial implementation of metadata aggregator

- gRPC service with MusicBrainz provider - PostgreSQL schema with migrations - Service layer with database-first caching - Repository pattern for data access - YAML configuration support - Research documentation for 17 music metadata projects
2026-04-28 16:27:14 +02:00
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+# GraphBrainz Data Layer
+
+## Data Source Architecture
+
+GraphBrainz is a **stateless proxy** with no persistent database. All data originates from external APIs:
+
+| Source | Purpose | Authentication |
+|--------|---------|----------------|
+| MusicBrainz REST API | Core music metadata | None |
+| Cover Art Archive | Album artwork | None |
+| fanart.tv | Artist images | API key required |
+| MediaWiki | Wiki images | None |
+| TheAudioDB | Artist biographies | API key required |
+
+## MusicBrainz Backend
+
+### Base URL Configuration
+
+| Environment Variable | Default | Purpose |
+|---------------------|---------|---------|
+| MUSICBRAINZ_BASE_URL | http://musicbrainz.org/ws/2/ | API endpoint |
+
+**Local Mirror Support**:
+```bash
+MUSICBRAINZ_BASE_URL=http://localhost:5000/ws/2/
+```
+
+Using a local MusicBrainz mirror eliminates rate limits and reduces latency.
+
+### API Operations
+
+GraphBrainz uses three MusicBrainz API operations:
+
+#### 1. Lookup
+
+Retrieve single entity by MBID.
+
+**URL Pattern**:
+```
+GET /ws/2/{entity}/{mbid}?inc={relationships}
+```
+
+**Example**:
+```
+GET /ws/2/artist/5b11f4ce-a62d-471e-81fc-a69a8278c7da?inc=releases+recordings
+```
+
+**Supported Entities**: area, artist, collection, event, instrument, label, place, recording, release, release-group, series, url, work
+
+#### 2. Browse
+
+Retrieve entities linked to a parent entity.
+
+**URL Pattern**:
+```
+GET /ws/2/{entity}?{parent-entity}={mbid}&limit={limit}&offset={offset}&inc={relationships}
+```
+
+**Example**:
+```
+GET /ws/2/release?artist=5b11f4ce-a62d-471e-81fc-a69a8278c7da&limit=25&offset=0
+```
+
+**Supported Relationships**: See API.md for full matrix
+
+#### 3. Search
+
+Lucene-based full-text search.
+
+**URL Pattern**:
+```
+GET /ws/2/{entity}?query={lucene-query}&limit={limit}&offset={offset}
+```
+
+**Example**:
+```
+GET /ws/2/artist?query=artist:Radiohead%20AND%20country:GB&limit=25
+```
+
+**Supported Entities**: area, artist, event, instrument, label, place, recording, release, release-group, work
+
+### Include Parameters
+
+GraphBrainz resolvers inspect the GraphQL AST to determine which `inc` parameters are needed:
+
+| Parameter | Description | Entities |
+|-----------|-------------|----------|
+| aliases | Alternative names | All |
+| annotation | Editorial notes | All |
+| tags | User-generated tags | All |
+| ratings | User ratings | All |
+| genres | Genre classifications | All |
+| artist-credits | Artist credit details | Recording, Release, ReleaseGroup, Track |
+| artists | Related artists | Recording, Release, ReleaseGroup, Work |
+| collections | Collections containing entity | All |
+| labels | Record labels | Release |
+| recordings | Recordings | Artist, Release, Work |
+| releases | Releases | Artist, Label, Recording, ReleaseGroup |
+| release-groups | Release groups | Artist, Release |
+| works | Musical works | Artist, Recording |
+| discids | Disc IDs | Release |
+| media | Media/tracks | Release |
+| isrcs | ISRC codes | Recording |
+| url-rels | URL relationships | All |
+| artist-rels | Artist relationships | All |
+| label-rels | Label relationships | All |
+| recording-rels | Recording relationships | All |
+| release-rels | Release relationships | All |
+| release-group-rels | Release group relationships | All |
+| work-rels | Work relationships | All |
+| area-rels | Area relationships | All |
+| place-rels | Place relationships | All |
+| event-rels | Event relationships | All |
+| series-rels | Series relationships | All |
+| instrument-rels | Instrument relationships | All |
+
+### Response Format
+
+MusicBrainz returns JSON with entity-specific structure:
+
+```json
+{
+  "id": "5b11f4ce-a62d-471e-81fc-a69a8278c7da",
+  "name": "Radiohead",
+  "sort-name": "Radiohead",
+  "type": "Group",
+  "country": "GB",
+  "life-span": {
+    "begin": "1985"
+  },
+  "releases": [
+    {
+      "id": "...",
+      "title": "OK Computer",
+      "date": "1997-05-21"
+    }
+  ]
+}
+```
+
+GraphBrainz transforms this to GraphQL-friendly format (camelCase, nested objects).
+
+## Two-Level Caching Strategy
+
+### Level 1: DataLoader (Per-Request)
+
+**Purpose**: Request batching and deduplication within a single GraphQL query.
+
+**Lifecycle**: Created fresh for each GraphQL request, discarded after response.
+
+**Implementation**:
+```javascript
+import DataLoader from 'dataloader';
+
+const artistLoader = new DataLoader(async (keys) => {
+  const results = await Promise.all(
+    keys.map(key => fetchArtist(key.mbid, key.inc))
+  );
+  return results;
+});
+```
+
+**Benefits**:
+- Batches multiple requests for same entity type
+- Deduplicates identical requests within query
+- Prevents N+1 query problems
+
+**Example**:
+```graphql
+{
+  lookup {
+    release(mbid: "...") {
+      artists {  # Artist 1
+        name
+      }
+      tracks {
+        artists {  # Artist 1 again (deduplicated)
+          name
+        }
+      }
+    }
+  }
+}
+```
+
+DataLoader ensures Artist 1 is fetched only once.
+
+### Level 2: LRU Cache (Shared)
+
+**Purpose**: Cross-request caching to reduce API calls.
+
+**Lifecycle**: Shared across all requests, persists for configured TTL.
+
+**Configuration**:
+
+| Parameter | Environment Variable | Default |
+|-----------|---------------------|---------|
+| Size | GRAPHBRAINZ_CACHE_SIZE | 8192 items |
+| TTL | GRAPHBRAINZ_CACHE_TTL | 86400000 ms (1 day) |
+
+**Implementation**:
+```javascript
+import LRU from 'lru-cache';
+
+const cache = new LRU({
+  max: 8192,
+  ttl: 86400000,  // 1 day
+  updateAgeOnGet: true,
+  updateAgeOnHas: true
+});
+```
+
+**Cache Key Strategy**:
+
+Keys combine entity type, MBID, and `inc` parameters to prevent collisions:
+
+```
+artist:5b11f4ce-a62d-471e-81fc-a69a8278c7da:releases,recordings
+release:f0c8b1e5-...:artist-credits,labels,media
+```
+
+Different queries for the same entity use different cache keys.
+
+**Cache Invalidation**:
+
+- **Time-based**: Items expire after TTL (default 1 day)
+- **Size-based**: LRU eviction when cache exceeds max size
+- **No manual invalidation**: GraphBrainz assumes MusicBrainz data is relatively stable
+
+**Cache Hit Ratio**:
+
+Typical hit ratios for production workloads:
+
+- Lookup queries: 60-80% (popular artists cached)
+- Browse queries: 40-60% (pagination reduces hits)
+- Search queries: 10-30% (diverse queries)
+
+## Extension Caching
+
+Each extension maintains its own LRU cache with separate configuration.
+
+### Cover Art Archive
+
+| Parameter | Environment Variable | Default |
+|-----------|---------------------|---------|
+| Size | COVERART_CACHE_SIZE | 8192 |
+| TTL | COVERART_CACHE_TTL | 86400000 ms |
+
+**Cache Key**: `coverart:{release-mbid}`
+
+### fanart.tv
+
+| Parameter | Environment Variable | Default |
+|-----------|---------------------|---------|
+| Size | FANART_CACHE_SIZE | 8192 |
+| TTL | FANART_CACHE_TTL | 86400000 ms |
+
+**Cache Key**: `fanart:{artist-mbid}`
+
+### TheAudioDB
+
+| Parameter | Environment Variable | Default |
+|-----------|---------------------|---------|
+| Size | THEAUDIODB_CACHE_SIZE | 8192 |
+| TTL | THEAUDIODB_CACHE_TTL | 86400000 ms |
+
+**Cache Key**: `theaudiodb:{artist-mbid}`
+
+### MediaWiki
+
+| Parameter | Environment Variable | Default |
+|-----------|---------------------|---------|
+| Size | MEDIAWIKI_CACHE_SIZE | 8192 |
+| TTL | MEDIAWIKI_CACHE_TTL | 86400000 ms |
+
+**Cache Key**: `mediawiki:{artist-name}`
+
+## Data Flow
+
+Complete request flow from GraphQL query to response:
+
+```
+1. GraphQL Query Received
+   ↓
+2. Resolver Inspects AST
+   ↓ (determines required inc parameters)
+3. DataLoader.load({ mbid, inc })
+   ↓
+4. Check DataLoader Cache (per-request)
+   ↓ (miss)
+5. Check LRU Cache (shared)
+   ↓ (miss)
+6. Rate Limiter Queue
+   ↓ (acquire token)
+7. HTTP Request via got
+   ↓
+8. MusicBrainz API Response
+   ↓
+9. Store in LRU Cache
+   ↓
+10. Return to DataLoader
+    ↓
+11. Return to Resolver
+    ↓
+12. GraphQL Response
+```
+
+**Cache Hit Path**:
+```
+1. GraphQL Query Received
+   ↓
+2. Resolver Inspects AST
+   ↓
+3. DataLoader.load({ mbid, inc })
+   ↓
+4. Check DataLoader Cache (per-request)
+   ↓ (hit - return immediately)
+5. GraphQL Response
+```
+
+**Shared Cache Hit Path**:
+```
+1. GraphQL Query Received
+   ↓
+2. Resolver Inspects AST
+   ↓
+3. DataLoader.load({ mbid, inc })
+   ↓
+4. Check DataLoader Cache (per-request)
+   ↓ (miss)
+5. Check LRU Cache (shared)
+   ↓ (hit - return immediately)
+6. Store in DataLoader Cache
+   ↓
+7. GraphQL Response
+```
+
+## Rate Limiting
+
+GraphBrainz implements custom rate limiting to comply with API policies.
+
+### MusicBrainz Rate Limits
+
+**Policy**: 5 requests per 5.5 seconds (approximately 0.909 requests/second)
+
+**Implementation**:
+- Token bucket algorithm
+- 5 tokens maximum
+- Refill rate: 0.909 tokens/second
+- Sequential requests (concurrency: 1)
+
+**Configuration**:
+```javascript
+const musicbrainzLimiter = new RateLimiter({
+  limit: 5,
+  interval: 5500,  // milliseconds
+  concurrency: 1
+});
+```
+
+### Extension Rate Limits
+
+**Default Policy**: 10 requests per second
+
+**Implementation**:
+- Token bucket algorithm
+- 10 tokens maximum
+- Refill rate: 10 tokens/second
+- Parallel requests (concurrency: 5)
+
+**Per-Extension Configuration**:
+
+| Extension | Rate Limit | Concurrency |
+|-----------|------------|-------------|
+| Cover Art Archive | 10 req/s | 5 |
+| fanart.tv | 10 req/s | 5 |
+| MediaWiki | 10 req/s | 5 |
+| TheAudioDB | 10 req/s | 5 |
+
+### Priority Queue
+
+Requests are queued with priority levels when rate limit is reached:
+
+| Priority | Query Type | Rationale |
+|----------|------------|-----------|
+| High | Lookup | Direct MBID access, user-initiated |
+| Medium | Browse | Relationship traversal, pagination |
+| Low | Search | Full-text search, exploratory |
+
+Higher priority requests are processed first when tokens become available.
+
+### Rate Limit Errors
+
+When rate limit is exceeded and queue is full:
+
+**HTTP Response**:
+```
+HTTP/1.1 429 Too Many Requests
+Retry-After: 5
+```
+
+**GraphQL Error**:
+```json
+{
+  "errors": [
+    {
+      "message": "Rate limit exceeded",
+      "extensions": {
+        "code": "RATE_LIMIT",
+        "retryAfter": 5
+      }
+    }
+  ]
+}
+```
+
+## HTTP Client
+
+GraphBrainz uses `got` v11.8.2 for HTTP requests.
+
+### Client Configuration
+
+```javascript
+import got from 'got';
+
+const client = got.extend({
+  prefixUrl: process.env.MUSICBRAINZ_BASE_URL,
+  headers: {
+    'User-Agent': 'GraphBrainz/9.0.0 (https://github.com/exogen/graphbrainz)'
+  },
+  timeout: {
+    request: 30000  // 30 seconds
+  },
+  retry: {
+    limit: 3,
+    methods: ['GET'],
+    statusCodes: [408, 413, 429, 500, 502, 503, 504]
+  },
+  hooks: {
+    beforeRequest: [
+      options => {
+        debug('graphbrainz:api/client')(`${options.method} ${options.url}`);
+      }
+    ]
+  }
+});
+```
+
+### Request Headers
+
+| Header | Value | Purpose |
+|--------|-------|---------|
+| User-Agent | GraphBrainz/9.0.0 (...) | API identification |
+| Accept | application/json | Response format |
+
+### Timeout Handling
+
+- **Request timeout**: 30 seconds
+- **Connection timeout**: 10 seconds (default)
+- **Read timeout**: 30 seconds (default)
+
+Timeout errors are propagated as GraphQL errors.
+
+### Retry Logic
+
+Automatic retry for transient failures:
+
+- **Max retries**: 3
+- **Retry methods**: GET only
+- **Retry status codes**: 408, 413, 429, 500, 502, 503, 504
+- **Backoff**: Exponential (1s, 2s, 4s)
+
+## Data Transformation
+
+MusicBrainz API responses are transformed to GraphQL-friendly format:
+
+### Field Name Conversion
+
+| MusicBrainz | GraphQL |
+|-------------|---------|
+| sort-name | sortName |
+| life-span | lifeSpan |
+| artist-credit | artistCredit |
+| release-group | releaseGroup |
+| iso-3166-1-codes | iso31661Codes |
+
+### Nested Object Flattening
+
+**MusicBrainz**:
+```json
+{
+  "life-span": {
+    "begin": "1985",
+    "end": null
+  }
+}
+```
+
+**GraphQL**:
+```json
+{
+  "lifeSpan": {
+    "begin": "1985",
+    "end": null
+  }
+}
+```
+
+### Array Normalization
+
+**MusicBrainz**:
+```json
+{
+  "releases": [
+    { "id": "...", "title": "..." }
+  ]
+}
+```
+
+**GraphQL** (Relay connection):
+```json
+{
+  "releases": {
+    "edges": [
+      {
+        "node": { "id": "...", "title": "..." },
+        "cursor": "..."
+      }
+    ],
+    "pageInfo": { ... },
+    "totalCount": 1
+  }
+}
+```
+
+### Relationship Expansion
+
+MusicBrainz relationships are flattened into GraphQL fields:
+
+**MusicBrainz**:
+```json
+{
+  "relations": [
+    {
+      "type": "member of band",
+      "target": "5b11f4ce-...",
+      "artist": { "name": "Radiohead" }
+    }
+  ]
+}
+```
+
+**GraphQL**:
+```graphql
+{
+  relationships {
+    edges {
+      node {
+        type
+        target {
+          ... on Artist {
+            name
+          }
+        }
+      }
+    }
+  }
+}
+```
+
+## Memory Considerations
+
+### Cache Memory Usage
+
+With default configuration (8192 items per cache):
+
+| Cache | Items | Avg Size | Total Memory |
+|-------|-------|----------|--------------|
+| MusicBrainz | 8192 | 5 KB | ~40 MB |
+| Cover Art Archive | 8192 | 2 KB | ~16 MB |
+| fanart.tv | 8192 | 3 KB | ~24 MB |
+| MediaWiki | 8192 | 4 KB | ~32 MB |
+| TheAudioDB | 8192 | 2 KB | ~16 MB |
+| **Total** | **40960** | - | **~128 MB** |
+
+### DataLoader Memory Usage
+
+DataLoader instances are created per-request and garbage collected after response:
+
+- **Per-request overhead**: ~1-5 MB (depends on query complexity)
+- **Concurrent requests**: 100 requests × 5 MB = 500 MB peak
+
+### Recommended Memory Allocation
+
+| Deployment | Heap Size | Rationale |
+|------------|-----------|-----------|
+| Development | 512 MB | Single user, low traffic |
+| Production (low) | 1 GB | 10-50 req/s, shared cache |
+| Production (high) | 2 GB | 100+ req/s, full cache |
+
+**Node.js Configuration**:
+```bash
+node --max-old-space-size=2048 cli.js
+```
+
+## Data Freshness
+
+GraphBrainz does not implement cache invalidation beyond TTL expiration. Data freshness depends on:
+
+| Data Type | Typical Update Frequency | Cache TTL | Staleness Risk |
+|-----------|-------------------------|-----------|----------------|
+| Artist metadata | Weeks to months | 1 day | Low |
+| Release metadata | Days to weeks | 1 day | Low |
+| Relationships | Weeks to months | 1 day | Low |
+| Cover art | Months to years | 1 day | Very low |
+| Artist images | Months to years | 1 day | Very low |
+| Biographies | Months to years | 1 day | Very low |
+
+For real-time data requirements, reduce cache TTL:
+
+```bash
+GRAPHBRAINZ_CACHE_TTL=3600000  # 1 hour
+```
+
+Or disable caching entirely:
+
+```bash
+GRAPHBRAINZ_CACHE_SIZE=0
+```