Design Music Search, Playlists, and Playback Controls

System Design: Music Streaming Infrastructure

Design a backend architecture for a music streaming service that supports three primary pillars: a catalog search engine, persistent user-defined playlists, and synchronized playback controls across multiple devices.

Requirements

1. Catalog & Search

• Support search by artist, album, and song title.
• The catalog contains tens of millions of tracks. Updates to metadata should reflect in search results with low latency.

2. Playlists

• Users can create, update, and reorder songs within playlists.
• Playlists must support high concurrency (e.g., multiple users editing a shared playlist) and ensure consistency.

3. Playback Controls

• Users may control playback on one device (e.g., phone) while listening on another (e.g., smart speaker).
• The system must handle state synchronization (play, pause, skip, seek) while managing command ordering and potential latency.

Tasks

1. Data Model: Propose a storage strategy for the immutable music catalog versus the mutable playlist state. How do you distinguish between these two access patterns?
2. API Design: Define the interface for playback controls. How do you handle "out-of-order" commands (e.g., a 'play' command arriving after a 'pause' command due to network jitter)?
3. Consistency: How do you prevent race conditions when two users reorder the same playlist simultaneously?
4. Scalability: Sketch the service boundaries. How do you ensure that search traffic does not impact playback stability?

Constraints & Assumptions

• Assume 50 million monthly active users.
• Global availability is required; users may switch networks frequently.
• Playback state changes are latency-sensitive (sub-200ms target).