Cap Theorem hard
You request 'Strong Consistency' for a write in a globally distributed database (Nodes in US, EU, Asia). What is the theoretical minimum latency?
- A The execution time of the local leader's disk I/O, which is typically estimated to be approximately 5 to 10 milliseconds.
- B Zero milliseconds, provided that the system utilizes a high-speed Write-Behind cache to decouple the database from the network.
- C The speed-of-light Round-Trip Time (RTT) to the closest majority member in another region (potentially 100 milliseconds or more).
- D A constant value that is entirely independent of physical distance due to the use of high-bandwidth global fiber optic cabling.
Answer: C. The speed-of-light Round-Trip Time (RTT) to the closest majority member in another region (potentially 100 milliseconds or more).
Strong Consistency in a distributed cluster requires that a majority of nodes acknowledge a write before it is considered successful. In a globally distributed cluster, that majority literally exists on another continent. Even with high-speed fiber, the speed of light limits how fast a signal can travel. This 'Physics Floor' means Strong Consistency always carries a significant latency penalty for global applications compared to Eventual Consistency.
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