Implement Playlists Skill

Plan and implement Stage 6 Playlists for NovaTune: CRUD endpoints, track management, stable ordering, and lifecycle integration.

Overview

Stage 6 implements playlist management with:

• GET /playlists - List playlists with search and cursor-based pagination
• POST /playlists - Create playlist with quota enforcement
• GET /playlists/{playlistId} - Get playlist with paginated tracks
• PATCH /playlists/{playlistId} - Update playlist metadata
• DELETE /playlists/{playlistId} - Hard delete playlist
• POST /playlists/{playlistId}/tracks - Add tracks at position
• DELETE /playlists/{playlistId}/tracks/{position} - Remove track
• POST /playlists/{playlistId}/reorder - Reorder tracks

Implementation Plan

Phase 1: Models and Configuration

1. Create Playlist Model (ApiService/Models/Playlist.cs)

   • PlaylistId (ULID)
   • UserId (owner)
   • Name, Description
   • Tracks (embedded List<PlaylistTrackEntry>)
   • TrackCount, TotalDuration (denormalized)
   • Visibility enum (Private, Unlisted, Public)
   • CreatedAt, UpdatedAt

2. Create PlaylistTrackEntry (ApiService/Models/PlaylistTrackEntry.cs)

   • Position (0-based index)
   • TrackId (ULID reference)
   • AddedAt

3. Add Configuration (ApiService/Configuration/PlaylistOptions.cs)

   • MaxPlaylistsPerUser (default: 200)
   • MaxTracksPerPlaylist (default: 10,000)
   • MaxTracksPerAddRequest (default: 100)
   • MaxMovesPerReorderRequest (default: 50)
   • MaxNameLength (default: 100)
   • MaxDescriptionLength (default: 500)
   • DefaultPageSize (default: 20)
   • MaxPageSize (default: 50)

4. Add DTOs (ApiService/Models/)

   • PlaylistListQuery, PlaylistDetailQuery
   • PlaylistListItem, PlaylistDetails, PlaylistTrackItem
   • CreatePlaylistRequest, UpdatePlaylistRequest
   • AddTracksRequest, ReorderRequest, MoveOperation

Phase 2: RavenDB Indexes

1. Playlists_ByUserForSearch (ApiService/Infrastructure/Indexes/)

   csharpCopy

   Map = playlists => from playlist in playlists
                      select new
                      {
                          playlist.UserId,
                          playlist.Name,
                          playlist.TrackCount,
                          playlist.CreatedAt,
                          playlist.UpdatedAt,
                          SearchText = playlist.Name
                      };
   Index("SearchText", FieldIndexing.Search);
   

2. Playlists_ByTrackReference (ApiService/Infrastructure/Indexes/)

   csharpCopy

   Map = playlists => from playlist in playlists
                      from track in playlist.Tracks
                      select new
                      {
                          UserId = playlist.UserId,
                          PlaylistId = playlist.PlaylistId,
                          TrackId = track.TrackId
                      };
   

Phase 3: Service Layer

1. IPlaylistService (ApiService/Services/)

   • ListPlaylistsAsync(userId, query, ct)
   • CreatePlaylistAsync(userId, request, ct)
   • GetPlaylistAsync(playlistId, userId, query, ct)
   • UpdatePlaylistAsync(playlistId, userId, request, ct)
   • DeletePlaylistAsync(playlistId, userId, ct)
   • AddTracksAsync(playlistId, userId, request, ct)
   • RemoveTrackAsync(playlistId, userId, position, ct)
   • ReorderTracksAsync(playlistId, userId, request, ct)
   • RemoveDeletedTrackReferencesAsync(trackId, userId, ct)

2. Custom Exceptions (ApiService/Infrastructure/Exceptions/)

   • PlaylistNotFoundException
   • PlaylistAccessDeniedException
   • PlaylistQuotaExceededException
   • PlaylistTrackLimitExceededException
   • PlaylistTrackNotFoundException
   • InvalidPositionException

Phase 4: API Endpoints

1. PlaylistEndpoints.cs (ApiService/Endpoints/)

   csharpCopy

   group.MapGet("/", HandleListPlaylists).RequireRateLimiting("playlist-list");
   group.MapPost("/", HandleCreatePlaylist).RequireRateLimiting("playlist-create");
   group.MapGet("/{playlistId}", HandleGetPlaylist);
   group.MapPatch("/{playlistId}", HandleUpdatePlaylist).RequireRateLimiting("playlist-update");
   group.MapDelete("/{playlistId}", HandleDeletePlaylist).RequireRateLimiting("playlist-delete");
   group.MapPost("/{playlistId}/tracks", HandleAddTracks).RequireRateLimiting("playlist-tracks-add");
   group.MapDelete("/{playlistId}/tracks/{position:int}", HandleRemoveTrack).RequireRateLimiting("playlist-tracks-remove");
   group.MapPost("/{playlistId}/reorder", HandleReorderTracks).RequireRateLimiting("playlist-reorder");
   

2. Rate Limiting Policies

   • playlist-list: 60 req/min
   • playlist-create: 20 req/min
   • playlist-update: 30 req/min
   • playlist-delete: 20 req/min
   • playlist-tracks-add: 30 req/min
   • playlist-tracks-remove: 60 req/min
   • playlist-reorder: 30 req/min

Phase 5: Track Validation

When adding tracks to playlists:

1. Verify track IDs are valid ULIDs
2. Verify tracks exist in RavenDB
3. Verify tracks are owned by the same user
4. Verify tracks are not deleted (Status != Deleted)
5. Verify playlist track limit not exceeded

var trackDocs = await _session.LoadAsync<Track>(
    request.TrackIds.Select(id => $"Tracks/{id}"), ct);

foreach (var (trackId, track) in trackDocs)
{
    if (track is null)
        throw new TrackNotFoundException(trackId);
    if (track.UserId != userId)
        throw new TrackAccessDeniedException(trackId);
    if (track.Status == TrackStatus.Deleted)
        throw new TrackDeletedException(trackId);
}

Phase 6: Position Management

Adding tracks:

var insertPosition = request.Position ?? playlist.Tracks.Count;

// Shift existing tracks
foreach (var entry in playlist.Tracks.Where(t => t.Position >= insertPosition))
    entry.Position += request.TrackIds.Count;

// Add new tracks
var newEntries = request.TrackIds.Select((id, i) => new PlaylistTrackEntry
{
    Position = insertPosition + i,
    TrackId = id,
    AddedAt = now
});
playlist.Tracks.AddRange(newEntries);

Removing tracks:

playlist.Tracks.Remove(trackToRemove);

// Reindex positions
foreach (var entry in playlist.Tracks.Where(t => t.Position > position))
    entry.Position--;

Reordering tracks:

foreach (var move in request.Moves)
{
    var track = tracks[move.From];
    tracks.RemoveAt(move.From);
    tracks.Insert(move.To, track);
}

// Reassign positions
for (var i = 0; i < tracks.Count; i++)
    tracks[i].Position = i;

Phase 7: Lifecycle Integration

Extend lifecycle worker to clean up playlist references when tracks are physically deleted:

1. Query Playlists_ByTrackReference index to find affected playlists
2. Remove all entries for the deleted track
3. Reindex positions
4. Update denormalized TrackCount and TotalDuration

Phase 8: Observability

1. Metrics (ApiService/Infrastructure/Observability/)

   • playlist_list_requests_total
   • playlist_create_requests_total
   • playlist_get_requests_total
   • playlist_update_requests_total
   • playlist_delete_requests_total
   • playlist_tracks_add_requests_total
   • playlist_tracks_remove_requests_total
   • playlist_reorder_requests_total
   • playlist_track_count (histogram)

2. Logging

   • Playlist operations with PlaylistId, UserId, CorrelationId
   • Track additions/removals with count and position

Phase 9: Testing

1. Unit Tests

   • PlaylistServiceTests
   • Position reindexing logic
   • Quota enforcement
   • Track validation

2. Integration Tests

   • End-to-end CRUD flow
   • Add/remove/reorder tracks
   • Track deletion cascade to playlists
   • Concurrent modification handling

Files to Create/Modify

New Files

Modified Files

Stage 6 Documentation

Detailed specifications are available in doc/implementation/stage-6/:

Related Skills

• add-api-endpoint - For endpoint structure
• add-cursor-pagination - For playlist list pagination
• add-ravendb-index - For creating RavenDB indexes
• add-rate-limiting - For rate limiting policies
• add-observability - For metrics and tracing
• add-playlist-reordering - For reorder implementation
• add-playlist-tracks - For track add/remove

Claude Agents

• playlist-api-implementer - Implement playlist service, endpoints, and models
• playlist-tester - Write unit and integration tests for playlists

Validation Checklist

• All CRUD endpoints return RFC 7807 problem details on error
• Rate limiting enforced on all mutation endpoints
• Playlist quota enforced (200 per user)
• Track limit enforced (10,000 per playlist)
• Track ownership verified before adding to playlist
• Deleted tracks not allowed in playlists
• Position indices maintained correctly
• Denormalized fields updated atomically
• Optimistic concurrency on updates
• Lifecycle worker removes deleted track references
• All operations logged with correlation ID