Spotify Modernizes Client-Side Architecture to Accelerate Service on All Devices

Spotify lifted the 10,000 Liked Songs limit. But that simple feature request triggered a year-long architecture overhaul affecting 100,000 lines of code.

Engineer Carl Engström shares how his team modernized Spotify's client-side architecture to serve users on older devices and slower networks. What started as removing a limit became a fundamental rethink of how the app handles data at startup.

Key Takeaways

• Modified 100,000 lines of shared client code across iOS and Android
• Metadata now downloads in batches and pre-sorts on disk, not in RAM
• Startup times improved dramatically on less powerful smartphones
• Gradual rollout from 30 employees to 50% of users to catch edge cases
• Users with thousands of likes typically only listen to a few hundred

The team had to identify dozens of ways users can 'like' a song and map each one to the new architecture while keeping the plane in flight.