# AcoustID API Reference

## API Overview

The AcoustID API provides fingerprint-based music identification services. The API is RESTful, supports multiple response formats (JSON, XML, JSONP), and requires API key authentication for most operations.

**Base URL**: `https://api.acoustid.org`  
**Protocol**: HTTPS only  
**Authentication**: API key (application key + user key for submissions)  
**Rate Limiting**: Multi-tier (global, application, IP-based)

## Public API Endpoints

### Fingerprint Lookup

Identify recordings by audio fingerprint.

#### `/v2/lookup`

**Methods**: GET, POST  
**Authentication**: Required (client key)  
**Rate Limit**: 3 requests/second (IP), 10 requests/second (application)

**Required Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |
| `duration` | integer | Track duration in seconds (if using fingerprint) |
| `trackid` | string | AcoustID track ID (alternative to fingerprint) |

**Optional Parameters**:

| Parameter | Type | Description | Default |
|-----------|------|-------------|---------|
| `fingerprint` | string | Chromaprint fingerprint (base64 or compressed) | - |
| `format` | string | Response format: `json`, `xml`, `jsonp` | `json` |
| `jsoncallback` | string | JSONP callback function name | - |
| `meta` | string | Metadata to include (see below) | - |

**Metadata Options** (comma-separated):

- `recordings`: Include MusicBrainz recording metadata
- `recordingids`: Include only recording MBIDs (faster)
- `releases`: Include release metadata
- `releaseids`: Include only release MBIDs
- `releasegroups`: Include release group metadata
- `releasegroupids`: Include only release group MBIDs
- `tracks`: Include track metadata
- `compress`: Compress response with gzip
- `usermeta`: Include user-submitted metadata
- `sources`: Include submission source information

**Batch Lookup**:

Submit multiple fingerprints in a single request using indexed parameters:

```
duration.0=240&fingerprint.0=AQADtN...
duration.1=180&fingerprint.1=AQABtK...
```

**Limits**:
- Maximum 20 fingerprints per batch request
- Maximum 100 track IDs per request

**Example Request** (GET):
```
GET /v2/lookup?client=8XaBELgH&duration=240&fingerprint=AQADtNGiJE...&meta=recordings
```

**Example Request** (POST):
```
POST /v2/lookup
Content-Type: application/x-www-form-urlencoded

client=8XaBELgH&duration=240&fingerprint=AQADtNGiJE...&meta=recordings
```

**Example Response** (JSON):
```json
{
  "status": "ok",
  "results": [
    {
      "id": "7e8b1234-5678-90ab-cdef-1234567890ab",
      "score": 0.95,
      "recordings": [
        {
          "id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
          "title": "Example Song",
          "duration": 240,
          "artists": [
            {
              "id": "12345678-90ab-cdef-1234-567890abcdef",
              "name": "Example Artist"
            }
          ],
          "releases": [
            {
              "id": "abcdef12-3456-7890-abcd-ef1234567890",
              "title": "Example Album",
              "country": "US",
              "date": {
                "year": 2020,
                "month": 5,
                "day": 15
              },
              "track_count": 12,
              "medium_count": 1
            }
          ]
        }
      ]
    }
  ]
}
```

**Response Fields**:

| Field | Type | Description |
|-------|------|-------------|
| `status` | string | `ok` or `error` |
| `results` | array | Array of match results |
| `results[].id` | string | AcoustID track ID |
| `results[].score` | float | Match confidence (0.0-1.0) |
| `results[].recordings` | array | MusicBrainz recordings (if requested) |

### Fingerprint Submission

Submit audio fingerprints with optional metadata.

#### `/v2/submit`

**Method**: POST  
**Authentication**: Required (client key + user key)  
**Rate Limit**: 3 requests/second (IP), 10 requests/second (application)

**Required Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |
| `user` | string | User API key |
| `duration.#` | integer | Track duration in seconds |
| `fingerprint.#` | string | Chromaprint fingerprint |

**Optional Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `clientversion` | string | Client application version |
| `bitrate.#` | integer | Audio bitrate in kbps |
| `fileformat.#` | string | Audio file format (mp3, flac, etc.) |
| `mbid.#` | string | MusicBrainz recording MBID |
| `track.#` | string | Track title |
| `artist.#` | string | Artist name |
| `album.#` | string | Album title |
| `albumartist.#` | string | Album artist name |
| `year.#` | integer | Release year |
| `trackno.#` | integer | Track number |
| `discno.#` | integer | Disc number |

**Batch Submission**:

Use indexed parameters (`.0`, `.1`, `.2`, etc.) to submit multiple fingerprints:

```
duration.0=240&fingerprint.0=AQADtN...&mbid.0=a1b2c3d4...
duration.1=180&fingerprint.1=AQABtK...&mbid.1=e5f67890...
```

**Example Request**:
```
POST /v2/submit
Content-Type: application/x-www-form-urlencoded

client=8XaBELgH&user=AbCdEfGh&duration.0=240&fingerprint.0=AQADtNGiJE...&mbid.0=a1b2c3d4-e5f6-7890-abcd-ef1234567890
```

**Example Response**:
```json
{
  "status": "ok",
  "submissions": [
    {
      "id": 12345678,
      "status": "pending"
    }
  ]
}
```

**Response Fields**:

| Field | Type | Description |
|-------|------|-------------|
| `status` | string | `ok` or `error` |
| `submissions` | array | Array of submission results |
| `submissions[].id` | integer | Submission ID |
| `submissions[].status` | string | `pending`, `imported`, or `error` |

### Submission Status

Check the processing status of submitted fingerprints.

#### `/v2/submission_status`

**Method**: GET  
**Authentication**: Required (client key)

**Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |
| `id` | integer | Submission ID (from submit response) |
| `format` | string | Response format: `json`, `xml`, `jsonp` |

**Example Request**:
```
GET /v2/submission_status?client=8XaBELgH&id=12345678
```

**Example Response**:
```json
{
  "status": "ok",
  "submission": {
    "id": 12345678,
    "status": "imported",
    "result": {
      "id": "7e8b1234-5678-90ab-cdef-1234567890ab"
    }
  }
}
```

**Status Values**:
- `pending`: Queued for processing
- `imported`: Successfully processed
- `error`: Processing failed

### Fingerprint Retrieval

Retrieve stored fingerprint data.

#### `/v2/fingerprint`

**Method**: GET  
**Authentication**: Required (client key)

**Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |
| `id` | string | AcoustID track ID |
| `format` | string | Response format: `json`, `xml`, `jsonp` |

**Example Request**:
```
GET /v2/fingerprint?client=8XaBELgH&id=7e8b1234-5678-90ab-cdef-1234567890ab
```

**Example Response**:
```json
{
  "status": "ok",
  "fingerprints": [
    {
      "id": 987654321,
      "fingerprint": "AQADtNGiJE...",
      "duration": 240,
      "submission_count": 5
    }
  ]
}
```

### Track Listing by MBID

List AcoustID tracks linked to a MusicBrainz recording.

#### `/v2/track/list_by_mbid`

**Method**: GET  
**Authentication**: Required (client key)

**Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |
| `mbid` | string | MusicBrainz recording MBID |
| `format` | string | Response format: `json`, `xml`, `jsonp` |

**Example Request**:
```
GET /v2/track/list_by_mbid?client=8XaBELgH&mbid=a1b2c3d4-e5f6-7890-abcd-ef1234567890
```

**Example Response**:
```json
{
  "status": "ok",
  "tracks": [
    {
      "id": "7e8b1234-5678-90ab-cdef-1234567890ab",
      "disabled": false
    }
  ]
}
```

### Track Listing by PUID

List AcoustID tracks linked to a MusicIP PUID (legacy).

#### `/v2/track/list_by_puid`

**Method**: GET  
**Authentication**: Required (client key)

**Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |
| `puid` | string | MusicIP PUID |
| `format` | string | Response format: `json`, `xml`, `jsonp` |

### User Management

#### `/v2/user/lookup`

Lookup user API key by MusicBrainz account.

**Method**: POST  
**Authentication**: Required (client key)

**Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |
| `musicbrainz_id` | string | MusicBrainz username |

#### `/v2/user/create_anonymous`

Create anonymous user API key.

**Method**: POST  
**Authentication**: Required (client key)

**Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |

**Example Response**:
```json
{
  "status": "ok",
  "user": {
    "apikey": "AbCdEfGh"
  }
}
```

#### `/v2/user/create_musicbrainz`

Create user API key linked to MusicBrainz account.

**Method**: POST  
**Authentication**: Required (client key)

**Parameters**:

| Parameter | Type | Description |
|-----------|------|-------------|
| `client` | string | Application API key |
| `access_token` | string | MusicBrainz OAuth access token |

## Legacy API Endpoints

### `/lookup`

Legacy lookup endpoint (API v1).

**Status**: Deprecated, use `/v2/lookup` instead  
**Differences**: Limited metadata options, different response format

### `/submit`

Legacy submit endpoint (API v1).

**Status**: Deprecated, use `/v2/submit` instead  
**Differences**: Synchronous processing, no batch support

## Health Check Endpoints

### `/_health`

Full health check with database write test.

**Method**: GET  
**Authentication**: None

**Response**:
```json
{
  "status": "ok"
}
```

**Status Codes**:
- `200`: All systems operational
- `503`: Service unavailable

### `/_health_ro`

Read-only health check (database read test only).

**Method**: GET  
**Authentication**: None

### `/_health_docker`

Docker-specific health check (minimal checks).

**Method**: GET  
**Authentication**: None

## Internal API Endpoints

These endpoints are for administrative use only and require special authentication.

### `/v2/internal/update_lookup_stats`

Trigger lookup statistics update.

**Method**: POST  
**Authentication**: Internal only

### `/v2/internal/update_user_agent_stats`

Trigger user agent statistics update.

**Method**: POST  
**Authentication**: Internal only

### `/v2/internal/lookup_stats`

Retrieve lookup statistics.

**Method**: GET  
**Authentication**: Internal only

### `/v2/internal/create_account`

Create new user account.

**Method**: POST  
**Authentication**: Internal only

### `/v2/internal/create_application`

Create new API application.

**Method**: POST  
**Authentication**: Internal only

### `/v2/internal/update_application_status`

Update application status (active/inactive).

**Method**: POST  
**Authentication**: Internal only

### `/v2/internal/check_application`

Check application validity.

**Method**: GET  
**Authentication**: Internal only

## Index API Endpoints

The fingerprint index service exposes its own HTTP API (separate from the main API).

**Base URL**: `http://index:6081` (internal)  
**Protocol**: HTTP  
**Format**: MessagePack

### `PUT /:index`

Create new index.

**Parameters**:
- `:index`: Index name

### `GET /:index`

Get index information.

**Response**:
```json
{
  "name": "fingerprints",
  "doc_count": 1234567,
  "segment_count": 42,
  "memory_segment_size": 1048576
}
```

### `DELETE /:index`

Delete index.

### `POST /:index/_search`

Search for fingerprints.

**Request Body** (MessagePack):
```python
{
    "query": [term1, term2, term3, ...],
    "limit": 10,
    "min_score": 0.5
}
```

**Response** (MessagePack):
```python
{
    "results": [
        {"id": fpid1, "score": 0.95},
        {"id": fpid2, "score": 0.87}
    ]
}
```

### `POST /:index/_update`

Batch update fingerprints.

**Request Body** (MessagePack):
```python
{
    "updates": [
        {"id": fpid1, "terms": [term1, term2, ...]},
        {"id": fpid2, "terms": [term3, term4, ...]}
    ]
}
```

### `GET /:index/_segments`

List index segments.

**Response**:
```json
{
  "segments": [
    {
      "id": 0,
      "type": "memory",
      "doc_count": 1024,
      "size_bytes": 1048576
    },
    {
      "id": 1,
      "type": "file",
      "doc_count": 100000,
      "size_bytes": 52428800
    }
  ]
}
```

### `GET /:index/_snapshot`

Create index snapshot.

**Response**:
```json
{
  "snapshot_id": "snapshot_20250428_120000",
  "path": "/var/lib/acoustid-index/snapshots/snapshot_20250428_120000"
}
```

### `PUT /:index/:fpid`

Insert or update fingerprint.

**Parameters**:
- `:index`: Index name
- `:fpid`: Fingerprint ID

**Request Body** (MessagePack):
```python
{
    "terms": [term1, term2, term3, ...]
}
```

### `GET /:index/:fpid`

Retrieve fingerprint.

**Response** (MessagePack):
```python
{
    "id": fpid,
    "terms": [term1, term2, term3, ...]
}
```

### `DELETE /:index/:fpid`

Delete fingerprint.

### `GET /_health`

Index health check.

**Response**:
```json
{
  "status": "ok"
}
```

### `GET /_metrics`

Prometheus metrics.

**Response** (Prometheus text format):
```
# HELP fpindex_search_duration_seconds Search duration
# TYPE fpindex_search_duration_seconds histogram
fpindex_search_duration_seconds_bucket{le="0.005"} 1234
fpindex_search_duration_seconds_bucket{le="0.01"} 5678
...
```

## Rate Limiting

### Rate Limit Tiers

AcoustID implements a three-tier rate limiting system:

| Tier | Scope | Default Limit | Override |
|------|-------|---------------|----------|
| Global | All requests | 3 req/s | Config: `cluster.rate_limiter.global_limit` |
| Application | Per API key | 10 req/s | Database: `application.rate_limit` |
| IP Address | Per client IP | 3 req/s | Config: `cluster.rate_limiter.ip_limit` |

### Rate Limit Algorithm

**Implementation**: Redis-based sliding window

**Window Configuration**:
- Window duration: 20 seconds
- Window steps: 4 (5-second buckets)
- Cleanup: Automatic expiration (25-second TTL)

**Redis Keys**:
```
rl:bucket:global:{timestamp}
rl:bucket:app:{api_key}:{timestamp}
rl:bucket:ip:{ip_address}:{timestamp}
```

### Rate Limit Headers

Responses include rate limit information:

```
X-RateLimit-Limit: 10
X-RateLimit-Remaining: 7
X-RateLimit-Reset: 1714305600
```

### Rate Limit Exceeded Response

**Status Code**: 429 Too Many Requests

**Response**:
```json
{
  "status": "error",
  "error": {
    "code": 5,
    "message": "Rate limit exceeded"
  }
}
```

## Error Handling

### Error Response Format

All errors return a consistent structure:

```json
{
  "status": "error",
  "error": {
    "code": 1,
    "message": "Invalid API key"
  }
}
```

### Error Codes

| Code | Message | Description |
|------|---------|-------------|
| 1 | Invalid API key | Client or user key is invalid |
| 2 | Missing required parameter | Required parameter not provided |
| 3 | Invalid fingerprint | Fingerprint format is invalid |
| 4 | Internal error | Server-side error occurred |
| 5 | Rate limit exceeded | Too many requests |
| 6 | Invalid format | Unsupported response format |
| 7 | Fingerprint not found | Requested fingerprint doesn't exist |
| 8 | Too many requests | Batch size exceeds limit |

### HTTP Status Codes

| Code | Meaning | Usage |
|------|---------|-------|
| 200 | OK | Successful request |
| 400 | Bad Request | Invalid parameters |
| 401 | Unauthorized | Missing or invalid API key |
| 403 | Forbidden | API key lacks permission |
| 404 | Not Found | Resource not found |
| 429 | Too Many Requests | Rate limit exceeded |
| 500 | Internal Server Error | Server error |
| 503 | Service Unavailable | Service down or degraded |

## Authentication

### API Key Types

1. **Application Key** (`client` parameter):
   - Identifies the client application
   - Required for all API calls
   - Obtain from https://acoustid.org/new-application

2. **User Key** (`user` parameter):
   - Identifies the end user
   - Required for submissions
   - Created via `/v2/user/create_*` endpoints

3. **Demo Key**:
   - Limited functionality
   - For testing only
   - Key: `8XaBELgH`

### Key Management

**Application Keys**:
- Created via web UI or internal API
- Can be active or inactive
- Rate limits configurable per key
- Usage statistics tracked

**User Keys**:
- Anonymous or MusicBrainz-linked
- Created programmatically
- Tied to application key
- Submission history tracked

## Best Practices

### Lookup Optimization

1. **Use batch lookups** for multiple files (up to 20 per request)
2. **Request only needed metadata** (use specific `meta` flags)
3. **Cache results** to avoid redundant lookups
4. **Handle rate limits** with exponential backoff

### Submission Guidelines

1. **Include MBIDs** when known (improves accuracy)
2. **Provide metadata** (artist, album, track) for better matching
3. **Use batch submissions** for efficiency
4. **Poll submission status** asynchronously

### Error Handling

1. **Retry on 5xx errors** with exponential backoff
2. **Respect rate limits** (check headers)
3. **Validate fingerprints** before submission
4. **Log errors** for debugging

### Performance

1. **Use POST** for large requests (avoid URL length limits)
2. **Enable compression** (`meta=compress`)
3. **Reuse connections** (HTTP keep-alive)
4. **Implement timeouts** (30-60 seconds recommended)