In an async Leader-Follower setup across regions (e.g., US to EU), what is the *minimum* replication lag dictated by physics?
- A 1 to 2 seconds
- B 70 to 150 milliseconds
- C 1 millisecond
- D 10 minutes
In a cross-region database setup (e.g., replicating from Ohio to Ireland), the speed of light in fiber optic cables is the ultimate deciding factor. A signal simply cannot traverse the distance any faster than the physical limits of the universe allow, which for an Atlantic crossing is roughly 70-80ms. When you add the overhead of database serialization and network routing, a 'replication lag' of ~100ms is the best achievable performance. This reality forces architects to make a 'CAP Theorem' choice: do you wait for this 100ms delay to ensure the data is safe in both regions (Synchronous), or do you allow the regions to get slightly out of sync to keep the primary database fast (Asynchronous)?
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