iCloud Sync Architecture — CloudKit Under the Hood

Three database types

• Public: shared across all users of an app.
• Private: one per user account.
• Shared: subset shared with specific people.

Three database types

• Public: shared across all users of an app.
• Private: one per user account.
• Shared: subset shared with specific people.

FoundationDB backbone

Apple acquired FoundationDB in 2015. Now backs iCloud's structured data. ACID across shards. Insanely fast reliability testing.