# GraphBrainz Architecture

## Schema Construction Strategy

GraphBrainz employs a hybrid schema construction approach:

- **Core Schema**: Programmatic construction using GraphQL.js constructors
- **Extensions**: SDL (Schema Definition Language) strings merged via `extendSchema()`

This strategy provides type safety and runtime flexibility for the core while allowing extensions to use the more ergonomic SDL syntax.

### Why Programmatic Construction?

| Benefit | Description |
|---------|-------------|
| Type Safety | Compile-time validation of schema structure |
| Dynamic Fields | Runtime field generation based on configuration |
| AST Inspection | Direct access to GraphQL AST for resolver optimization |
| Extension Points | Programmatic hooks for schema modification |

## Entity Type System

GraphBrainz defines 17 entity types in `src/types/` (~2000 lines of code):

| Entity Type | File Path | Purpose |
|-------------|-----------|---------|
| Area | src/types/area.js | Geographic regions |
| Artist | src/types/artist.js | Musicians and groups |
| Collection | src/types/collection.js | User-curated lists |
| Disc | src/types/disc.js | Physical media |
| Event | src/types/event.js | Concerts and performances |
| Instrument | src/types/instrument.js | Musical instruments |
| Label | src/types/label.js | Record labels |
| Place | src/types/place.js | Venues and locations |
| Recording | src/types/recording.js | Audio recordings |
| Release | src/types/release.js | Album releases |
| ReleaseGroup | src/types/release-group.js | Release groupings |
| Series | src/types/series.js | Ordered collections |
| Tag | src/types/tag.js | User-generated tags |
| Track | src/types/track.js | Individual tracks |
| URL | src/types/url.js | External links |
| Work | src/types/work.js | Musical compositions |
| Relationships | src/types/relationships.js | Entity connections |

Each type file exports a GraphQL object type with field definitions, resolvers, and relationship mappings.

## Query Type Hierarchy

GraphBrainz exposes four primary query patterns:

### 1. Lookup Queries

Direct entity retrieval by MusicBrainz ID (MBID).

**Supported Entities**: 13 types

```
lookup {
  area(mbid: String!)
  artist(mbid: String!)
  collection(mbid: String!)
  event(mbid: String!)
  instrument(mbid: String!)
  label(mbid: String!)
  place(mbid: String!)
  recording(mbid: String!)
  release(mbid: String!)
  releaseGroup(mbid: String!)
  series(mbid: String!)
  url(mbid: String!)
  work(mbid: String!)
}
```

### 2. Browse Queries

Retrieve entities linked to a parent entity with cursor-based pagination.

**Supported Entities**: 9 types

```
browse {
  areas(collection: String, first: Int, after: String)
  artists(area: String, collection: String, recording: String, release: String, releaseGroup: String, work: String, first: Int, after: String)
  collections(area: String, artist: String, editor: String, event: String, label: String, place: String, recording: String, release: String, releaseGroup: String, work: String, first: Int, after: String)
  events(area: String, artist: String, collection: String, place: String, first: Int, after: String)
  labels(area: String, collection: String, release: String, first: Int, after: String)
  places(area: String, collection: String, first: Int, after: String)
  recordings(artist: String, collection: String, release: String, first: Int, after: String)
  releases(area: String, artist: String, collection: String, label: String, recording: String, releaseGroup: String, track: String, trackArtist: String, first: Int, after: String)
  releaseGroups(artist: String, collection: String, release: String, first: Int, after: String)
}
```

### 3. Search Queries

Lucene-based full-text search across entity types.

**Supported Entities**: 10 types

```
search {
  areas(query: String!, first: Int, after: String)
  artists(query: String!, first: Int, after: String)
  events(query: String!, first: Int, after: String)
  instruments(query: String!, first: Int, after: String)
  labels(query: String!, first: Int, after: String)
  places(query: String!, first: Int, after: String)
  recordings(query: String!, first: Int, after: String)
  releases(query: String!, first: Int, after: String)
  releaseGroups(query: String!, first: Int, after: String)
  works(query: String!, first: Int, after: String)
}
```

### 4. Node Query (Relay)

Global object identification via Relay-compliant node interface.

```
node(id: ID!)
```

## Resolver Architecture

GraphBrainz implements a three-tier resolver structure:

### Tier 1: Query Resolvers

Entry points for lookup, browse, search, and node queries. Responsibilities:

- Validate input parameters
- Construct MusicBrainz API URLs
- Delegate to DataLoader
- Return raw API responses

**Location**: `src/resolvers/query.js`

### Tier 2: Field Resolvers

Resolve individual fields on entity types. Responsibilities:

- Extract field values from parent object
- Trigger subqueries for related entities
- Apply field-level transformations
- Handle null/undefined cases

**Location**: `src/types/*.js` (per entity type)

### Tier 3: Subquery Resolvers

Handle nested entity relationships. Responsibilities:

- Inspect GraphQL AST for required fields
- Determine MusicBrainz `inc` parameters
- Batch related entity requests
- Resolve circular dependencies

**Location**: `src/resolvers/subquery.js`

## AST Inspection for Query Optimization

GraphBrainz resolvers inspect the GraphQL AST to determine which MusicBrainz `inc` parameters are needed. This eliminates over-fetching and under-fetching.

### Example

**GraphQL Query**:
```graphql
{
  lookup {
    artist(mbid: "5b11f4ce-a62d-471e-81fc-a69a8278c7da") {
      name
      releases {
        title
        date
      }
    }
  }
}
```

**AST Inspection Result**:
- Detects `releases` field in selection set
- Adds `inc=releases` to MusicBrainz API request
- Avoids fetching recordings, works, or other unneeded relationships

**MusicBrainz API Call**:
```
GET /ws/2/artist/5b11f4ce-a62d-471e-81fc-a69a8278c7da?inc=releases
```

### Implementation

AST inspection occurs in resolver functions via `info.fieldNodes`:

```javascript
function resolveArtist(parent, args, context, info) {
  const selections = info.fieldNodes[0].selectionSet.selections;
  const inc = [];
  
  for (const selection of selections) {
    if (selection.name.value === 'releases') {
      inc.push('releases');
    }
    if (selection.name.value === 'recordings') {
      inc.push('recordings');
    }
  }
  
  return context.loaders.artist.load({ mbid: args.mbid, inc });
}
```

## Extension System

Extensions modify the schema and context in two phases:

### Phase 1: Context Extension

Extensions add custom HTTP clients, DataLoaders, and caches to the GraphQL context.

**Interface**:
```javascript
{
  extendContext(context, options) {
    return {
      ...context,
      [extensionName]: {
        client: new ExtensionClient(options),
        loader: new DataLoader(batchFn),
        cache: new LRUCache(options)
      }
    };
  }
}
```

### Phase 2: Schema Extension

Extensions add fields to existing types or define new types via SDL.

**Interface**:
```javascript
{
  extendSchema(schema, options) {
    const typeDefs = `
      extend type Artist {
        fanArt: FanArtImages
      }
      
      type FanArtImages {
        backgrounds: [FanArtImage]
        logos: [FanArtImage]
      }
    `;
    
    const resolvers = {
      Artist: {
        fanArt(artist, args, context) {
          return context.fanart.loader.load(artist.id);
        }
      }
    };
    
    return extendSchema(schema, { typeDefs, resolvers });
  }
}
```

### Extension Loading

Extensions are loaded via environment variable or programmatic options:

**Environment Variable**:
```bash
GRAPHBRAINZ_EXTENSIONS="cover-art-archive,fanart,mediawiki,theaudiodb"
```

**Programmatic**:
```javascript
import { middleware } from 'graphbrainz';
import lastfm from 'graphbrainz-extension-lastfm';

app.use('/graphql', middleware({
  extensions: [lastfm]
}));
```

## DataLoader Integration

GraphBrainz uses DataLoader for request batching and deduplication.

### Per-Request Batching

Each GraphQL request receives a fresh DataLoader instance. This ensures:

- Requests within a single query are batched
- Duplicate requests are deduplicated
- Cache is scoped to request lifecycle

### Batch Functions

Each entity type has a batch function that:

1. Receives array of keys (MBIDs or query parameters)
2. Groups keys by API endpoint
3. Makes batched HTTP requests
4. Returns array of results in same order as keys

**Example**:
```javascript
async function batchArtists(keys) {
  const results = await Promise.all(
    keys.map(key => 
      got(`/ws/2/artist/${key.mbid}?inc=${key.inc.join(',')}`)
    )
  );
  return results.map(r => r.body);
}

const artistLoader = new DataLoader(batchArtists);
```

## LRU Cache Layer

Shared LRU cache sits above DataLoader for cross-request caching.

### Configuration

| Parameter | Environment Variable | Default |
|-----------|---------------------|---------|
| Size | GRAPHBRAINZ_CACHE_SIZE | 8192 items |
| TTL | GRAPHBRAINZ_CACHE_TTL | 86400000 ms (1 day) |

### Cache Key Strategy

Cache keys combine entity type, MBID, and `inc` parameters:

```
artist:5b11f4ce-a62d-471e-81fc-a69a8278c7da:releases,recordings
```

This ensures different queries for the same entity don't collide.

### Per-Extension Caches

Each extension maintains its own LRU cache with separate configuration:

- `FANART_CACHE_SIZE` / `FANART_CACHE_TTL`
- `THEAUDIODB_CACHE_SIZE` / `THEAUDIODB_CACHE_TTL`
- `COVERART_CACHE_SIZE` / `COVERART_CACHE_TTL`

## Rate Limiting

Custom priority queue implementation ensures API compliance.

### MusicBrainz Rate Limits

- **Limit**: 5 requests per 5.5 seconds
- **Strategy**: Token bucket with 5 tokens, refill rate 0.909 tokens/second
- **Concurrency**: 1 (sequential requests)

### Extension Rate Limits

- **Limit**: 10 requests per second (default)
- **Strategy**: Token bucket with 10 tokens, refill rate 10 tokens/second
- **Concurrency**: 5 (parallel requests)

### Priority Queue

Requests are queued with priority levels:

1. **High**: Lookup queries (direct MBID access)
2. **Medium**: Browse queries (relationship traversal)
3. **Low**: Search queries (full-text search)

Higher priority requests are processed first when rate limit is reached.

### Implementation

**Location**: `src/rate-limit.js`

```javascript
class RateLimiter {
  constructor(options) {
    this.tokens = options.limit;
    this.limit = options.limit;
    this.refillRate = options.limit / options.interval;
    this.queue = new PriorityQueue();
  }
  
  async acquire(priority = 'medium') {
    if (this.tokens > 0) {
      this.tokens--;
      return Promise.resolve();
    }
    
    return new Promise(resolve => {
      this.queue.enqueue({ resolve, priority });
    });
  }
  
  refill() {
    this.tokens = Math.min(this.limit, this.tokens + this.refillRate);
    while (this.tokens > 0 && this.queue.length > 0) {
      const { resolve } = this.queue.dequeue();
      this.tokens--;
      resolve();
    }
  }
}
```

## File Structure

```
src/
├── index.js                 # Entry point, start() function
├── schema.js                # Schema construction
├── context.js               # Context factory
├── types/                   # Entity type definitions
│   ├── area.js
│   ├── artist.js
│   ├── collection.js
│   ├── disc.js
│   ├── event.js
│   ├── instrument.js
│   ├── label.js
│   ├── place.js
│   ├── recording.js
│   ├── release.js
│   ├── release-group.js
│   ├── series.js
│   ├── tag.js
│   ├── track.js
│   ├── url.js
│   ├── work.js
│   └── relationships.js
├── resolvers/               # Resolver implementations
│   ├── query.js
│   └── subquery.js
├── loaders/                 # DataLoader batch functions
│   └── musicbrainz.js
├── rate-limit.js            # Rate limiter implementation
├── client.js                # Base HTTP client
└── extensions/              # Built-in extensions
    ├── cover-art-archive/
    ├── fanart/
    ├── mediawiki/
    └── theaudiodb/
```

## Relay Compliance

GraphBrainz implements the Relay specification for cursor-based pagination:

### Connection Pattern

All list fields return connection types:

```graphql
type ArtistConnection {
  edges: [ArtistEdge]
  nodes: [Artist]
  pageInfo: PageInfo!
  totalCount: Int
}

type ArtistEdge {
  node: Artist
  cursor: String!
}

type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}
```

### Pagination Arguments

- `first: Int` - Number of items to return
- `after: String` - Cursor for pagination
- `last: Int` - Number of items from end (not implemented)
- `before: String` - Cursor for reverse pagination (not implemented)

### Node Interface

Global object identification via `node(id: ID!)` query:

```graphql
interface Node {
  id: ID!
}
```

All entity types implement the Node interface with globally unique IDs.