# MusicBrainz Server Integrations

## Cover Art Archive

### Overview

**Service:** Cover Art Archive (coverartarchive.org)  
**Storage:** Amazon S3 + Internet Archive  
**Purpose:** Store and serve album cover artwork

### Upload Process

**Method:** Signed POST to S3

**Authentication:** HMAC-SHA1 signed policy

**Configuration:**
```perl
# DBDefs.pm
sub COVER_ART_ARCHIVE_ACCESS_KEY { 'access_key' }
sub COVER_ART_ARCHIVE_SECRET_KEY { 'secret_key' }
sub COVER_ART_ARCHIVE_UPLOAD_PREFIXER { 'MB' }
sub COVER_ART_ARCHIVE_DOWNLOAD_PREFIX { 'https://coverartarchive.org' }
```

**Upload Flow:**
1. User uploads image via MusicBrainz interface
2. Server generates S3 policy document
3. Policy signed with HMAC-SHA1 using secret key
4. Browser POSTs directly to S3 with signed policy
5. S3 stores image and forwards to Internet Archive
6. Image becomes available at coverartarchive.org

**Policy Document:**
```json
{
  "expiration": "2024-12-31T23:59:59Z",
  "conditions": [
    {"bucket": "mbid-{release_mbid}"},
    {"acl": "public-read"},
    ["starts-with", "$key", "mbid-{release_mbid}/"],
    ["content-length-range", 0, 10485760]
  ]
}
```

**Signature:**
```perl
use Digest::SHA qw(hmac_sha1_base64);

my $policy_b64 = encode_base64($policy_json);
my $signature = hmac_sha1_base64($policy_b64, $secret_key);
$signature .= '=' while length($signature) % 4;  # Pad to multiple of 4
```

### Retrieval

**URL Pattern:** `https://coverartarchive.org/release/{mbid}/front`

**Image Types:**
- `front` - Front cover
- `back` - Back cover
- `{id}` - Specific image by ID

**Sizes:**
- Original (full resolution)
- `250` - 250px thumbnail
- `500` - 500px thumbnail
- `1200` - 1200px large

**Example:**
```
https://coverartarchive.org/release/76df3287-6cda-33eb-8e9a-044b5e15ffdd/front-250.jpg
```

## Wikipedia/Wikidata/Wikimedia Commons

### MediaWiki API Integration

**Purpose:** Fetch article extracts, images, and structured data

**Endpoints:**
- Wikipedia: `https://{lang}.wikipedia.org/w/api.php`
- Wikidata: `https://www.wikidata.org/w/api.php`
- Commons: `https://commons.wikimedia.org/w/api.php`

### Wikipedia Extracts

**API Action:** `query` with `prop=extracts`

**Request:**
```perl
my $url = "https://en.wikipedia.org/w/api.php?" . 
          "action=query&" .
          "prop=extracts&" .
          "exintro=1&" .
          "explaintext=1&" .
          "titles=" . uri_escape($artist_name) .
          "&format=json";

my $response = $ua->get($url);
my $data = decode_json($response->content);
```

**Caching:** 3 days for extracts

**Display:** Artist/release pages show Wikipedia extract in sidebar

### Language Links

**API Action:** `query` with `prop=langlinks`

**Purpose:** Find Wikipedia articles in different languages

**Request:**
```perl
my $url = "https://en.wikipedia.org/w/api.php?" .
          "action=query&" .
          "prop=langlinks&" .
          "titles=" . uri_escape($title) .
          "&lllimit=500&" .
          "&format=json";
```

**Caching:** 7 days for language links

**Usage:** Display Wikipedia links in user's preferred language

### Wikidata Integration

**Purpose:** Fetch structured data (birth dates, locations, etc.)

**API Action:** `wbgetentities`

**Request:**
```perl
my $url = "https://www.wikidata.org/w/api.php?" .
          "action=wbgetentities&" .
          "ids=Q{wikidata_id}&" .
          "format=json";
```

**Data Extracted:**
- Birth/death dates
- Birth/death places
- Occupations
- Genres
- Record labels
- Official websites

### Wikimedia Commons Images

**Purpose:** Fetch artist/band photos

**API Action:** `query` with `prop=imageinfo`

**Request:**
```perl
my $url = "https://commons.wikimedia.org/w/api.php?" .
          "action=query&" .
          "prop=imageinfo&" .
          "iiprop=url|size|mime&" .
          "titles=File:" . uri_escape($filename) .
          "&format=json";
```

**Display:** Artist pages show Commons images in sidebar

## CritiqueBrainz

### Overview

**Service:** CritiqueBrainz (critiquebrainz.org)  
**Purpose:** User-generated music reviews

### Integration

**Method:** URL linking

**Pattern:** `https://critiquebrainz.org/release/{mbid}`

**Display:** Release pages show link to CritiqueBrainz reviews

**Embedding:** Review count and average rating displayed on release pages

**API:** CritiqueBrainz API used to fetch review statistics

**Request:**
```perl
my $url = "https://critiquebrainz.org/ws/1/release/$mbid";
my $response = $ua->get($url);
my $data = decode_json($response->content);

my $review_count = $data->{review_count};
my $avg_rating = $data->{average_rating};
```

## Event Art Archive

### Overview

**Service:** Event Art Archive  
**Purpose:** Store event posters and promotional materials

**Architecture:** Similar to Cover Art Archive (S3 + Internet Archive)

**URL Pattern:** `https://eventartarchive.org/event/{mbid}`

## Discourse SSO

### Overview

**Service:** MusicBrainz Community Forum (community.metabrainz.org)  
**Protocol:** Discourse SSO (Single Sign-On)

### Authentication Flow

**Method:** HMAC-SHA256 signed payload

**Flow:**
1. User clicks "Log in" on Discourse
2. Discourse redirects to MusicBrainz with nonce
3. MusicBrainz authenticates user
4. MusicBrainz generates SSO payload
5. Payload signed with HMAC-SHA256
6. User redirected back to Discourse with signed payload
7. Discourse verifies signature and logs in user

**Configuration:**
```perl
# DBDefs.pm
sub DISCOURSE_SSO_SECRET { 'shared_secret' }
sub DISCOURSE_SERVER { 'https://community.metabrainz.org' }
```

**Payload Generation:**
```perl
use Digest::SHA qw(hmac_sha256_hex);
use MIME::Base64;

my $payload = encode_base64(
    "nonce=$nonce&" .
    "email=$email&" .
    "external_id=$user_id&" .
    "username=$username&" .
    "name=$name"
);

my $signature = hmac_sha256_hex($payload, $sso_secret);

my $redirect_url = "$discourse_server/session/sso_login?" .
                   "sso=" . uri_escape($payload) .
                   "&sig=$signature";
```

**User Data Synced:**
- Email address
- Username
- Display name
- User ID (external_id)
- Avatar URL (optional)
- Admin status (optional)
- Moderator status (optional)

## MetaBrainz OAuth

### Overview

**Service:** Centralized OAuth provider for MetaBrainz services  
**Protocol:** OAuth 2.0 with token introspection

### Token Introspection

**Endpoint:** `https://musicbrainz.org/oauth2/introspect`

**Method:** POST

**Request:**
```perl
my $response = $ua->post(
    'https://musicbrainz.org/oauth2/introspect',
    {
        token => $access_token,
        client_id => $client_id,
        client_secret => $client_secret,
    }
);

my $data = decode_json($response->content);
```

**Response:**
```json
{
  "active": true,
  "scope": "profile email tag rating collection",
  "client_id": "client_id",
  "username": "username",
  "token_type": "Bearer",
  "exp": 1609459200,
  "iat": 1609372800,
  "sub": "user_id"
}
```

**Usage:** Other MetaBrainz services (ListenBrainz, BookBrainz, etc.) validate tokens via introspection

### Services Using MetaBrainz OAuth

- ListenBrainz (listening history)
- BookBrainz (book metadata)
- CritiqueBrainz (music reviews)
- AcousticBrainz (audio analysis)
- Picard (music tagger)

## Replication System

### Overview

**Purpose:** Synchronize database changes from master to mirrors  
**Protocol:** dbmirror2 packet system

### Replication Modes

**RT_MASTER:**
- Generates replication packets
- Writes to `dbmirror_pending` and `dbmirror_pendingdata` tables
- Exports packets for mirrors

**RT_MIRROR:**
- Consumes replication packets
- Applies changes from master
- Read-only (no edits)

**RT_STANDALONE:**
- No replication
- Fully independent database

**Configuration:**
```perl
# DBDefs.pm
sub REPLICATION_TYPE { RT_MASTER }  # or RT_MIRROR or RT_STANDALONE
sub REPLICATION_ACCESS_TOKEN { 'secret_token' }
```

### Packet Structure

**Tables:**
- `dbmirror_pending` - Pending transactions
- `dbmirror_pendingdata` - Data changes (INSERT/UPDATE/DELETE)

**Packet Format:**
```
SeqId: 12345
TransactionId: 67890
Operation: i  # i=INSERT, u=UPDATE, d=DELETE
TableName: artist
Data: {"id":123,"gid":"...","name":"..."}
```

### Replication Flow

**Master Side:**
1. Edit applied to database
2. Triggers capture changes to `dbmirror_pending`
3. Export script generates replication packets
4. Packets uploaded to FTP server

**Mirror Side:**
1. Download replication packets from FTP
2. Apply packets in sequence order
3. Update replication state
4. Verify data integrity

**Packet Export:**
```bash
# On master
./admin/replication/ExportReplicationPackets

# Generates packets in replication/ directory
# Uploads to FTP server
```

**Packet Import:**
```bash
# On mirror
./admin/replication/LoadReplicationChanges

# Downloads packets from FTP
# Applies changes to database
```

### Replication Lag

**Monitoring:** Mirrors track replication lag (time behind master)

**Typical Lag:** Minutes to hours depending on packet size and network

**Status Endpoint:** `/replication-status` shows current replication state

## Redis Integration

### Architecture

**Connection:** Single Redis instance, 16 databases (0-15)

**Configuration:**
```perl
# DBDefs.pm
sub REDIS_SERVER { 'localhost:6379' }
sub REDIS_NAMESPACE { 'MB' }
```

### Use Cases

**Session Management (DB 1):**
- Store user sessions
- 10 hour absolute expiry
- 3 hour idle timeout

**Entity Cache (DB 0):**
- Cache entity lookups by MBID
- 1 hour TTL
- Invalidate on edit

**Search Cache (DB 2):**
- Cache search results
- 15 minute TTL

**Statistics Cache (DB 3):**
- Cache homepage statistics
- 1 hour TTL

**Rate Limiting (DB 4):**
- Track API request counts
- 1 second sliding window

**Pub/Sub (DB 5):**
- Real-time notifications
- Edit submission events
- Cache invalidation events

### Connection Pooling

**Library:** Redis.pm with connection pooling

**Pool Size:** 10 connections per worker

**Reconnection:** Automatic reconnection on connection loss

## HTTP Client

### LWP::UserAgent

**Purpose:** HTTP client for external service communication

**Configuration:**
```perl
use LWP::UserAgent;

my $ua = LWP::UserAgent->new(
    agent => 'MusicBrainz/1.0 (https://musicbrainz.org)',
    timeout => 30,
    max_redirect => 5,
);
```

**User-Agent:** Always identifies as MusicBrainz with contact URL

**Timeout:** 30 seconds default

**Redirects:** Follow up to 5 redirects

**SSL Verification:** Enabled by default

### Rate Limiting

**External Services:** Respect rate limits via delays

**Wikipedia API:** 1 request per second (recommended)

**Wikidata API:** 1 request per second (recommended)

**Implementation:**
```perl
use Time::HiRes qw(sleep);

my $last_request_time = 0;

sub rate_limited_request {
    my ($url) = @_;
    
    my $elapsed = time() - $last_request_time;
    if ($elapsed < 1) {
        sleep(1 - $elapsed);
    }
    
    my $response = $ua->get($url);
    $last_request_time = time();
    
    return $response;
}
```

### Error Handling

**Retry Logic:** Exponential backoff for transient errors

**Timeouts:** Fail gracefully on timeout

**Logging:** Log all external service errors to Sentry

**Example:**
```perl
use Try::Tiny;

my $response;
my $retries = 3;

for my $attempt (1..$retries) {
    try {
        $response = $ua->get($url);
        last if $response->is_success;
    } catch {
        warn "Request failed (attempt $attempt): $_";
        sleep(2 ** $attempt);  # Exponential backoff
    };
}
```