Dropbox: Block Storage Sync & Magic Pocket Architecture

Dropbox synchronizes hundreds of millions of files across user laptops, desktops, and mobile devices with seamless background execution.

The desktop synchronization engine relies on a carefully tuned local architecture:

1. OS File System Watcher: Uses native operating system event hooks (fsevents on macOS, inotify on Linux, ReadDirectoryChangesW on Windows) to detect file additions, modifications, renames, and deletions in the synced folder.
2. Local SQLite Database: The client maintains a local SQLite database that records file paths, directory trees, file modification timestamps, and the cryptographic SHA-256 hashes of all 4MB chunks.
3. Delta Synchronization (Chunking):
   • Files are split into contiguous 4MB chunks.
   • If a user modifies a 1GB Photoshop file by changing a few brush strokes, the sync engine re-computes the SHA-256 hashes of all 250 chunks.
   • It discovers that 249 chunks are identical and only 1 chunk (4MB) has changed.
   • The client uploads only that single 4MB chunk over the network, saving 99.6\% of network bandwidth and battery life.