Building Carousel Part I: How We Made Our Networked Mobile App Feel Fast and Local
Article Summary
Dropbox's Carousel team faced a brutal truth: their photo app was slower than local galleries, and users noticed. No one wants to wait for network requests just to delete a photo.
This deep dive from Dropbox Engineering explains how they rebuilt their mobile photo app architecture from the ground up. The goal was ambitious: make a cloud-backed gallery feel as fast as native device storage.
Key Takeaways
- Humans perceive delays at 100ms, so blocking network calls were eliminated entirely
- Built eventually consistent system using optimistic replication and operation queues
- LUIDs (locally unique IDs) let users modify photos before upload completes
- HTTP long polling plus delta API keeps client synced with server changes
- SQLite stores three data sources: server photos, pending uploads, and user operations
Critical Insight
By treating local and remote photos as equivalent objects and replaying pending operations on cached server state, Carousel eliminated all user-facing network waits.
