Apple: CloudKit Delta Sync, FoundationDB & Apple Push Notification (APNs)
How Apple powers cross-device synchronization for 2 billion active iOS and macOS devices using CloudKit change tokens, FoundationDB transactional metadata, and persistent APNs socket connections.
Apple synchronizes device state using deterministic delta tokens: instead of re-fetching entire records, clients request changes since their last known sync token. Server transactions are coordinated using distributed ACID FoundationDB clusters.
Server-Assisted Delta Synchronization with Change Tokens
2+ billion active devices synchronizing changes simultaneouslyKeeping photos, notes, and keychain data synchronized across multiple devices without transmitting redundant payload bytes over cellular networks.
Each client device persists an opaque server change token. When reconnecting, the client presents the token; CloudKit queries FoundationDB change logs and returns only modifications committed after that token timestamp.
Requires maintaining historical mutation logs on the server for a retention window (e.g. 30 days) vs minimal bandwidth consumption on battery-powered mobile devices.
Highlight the difference between snapshot sync and delta sync in mobile system design interviews.
Apple Engineering / SIGMOD • 2021
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