System Design Interview

If we choose consistency over availability (CP system), we must block all write operations to n1 and n2 to avoid data inconsistency among these three servers, which makes the system unavailable. Bank systems usually have extremely high consistent requirements. For example, it is crucial for a bank system to display the most up-to-date balance info. If inconsistency occurs due to a network partition, the bank system returns an error before the inconsistency is resolved. However, if we choose availability over consistency (AP system), the system keeps accepting reads, even though it might return stale data. For writes, n1 and n2 will keep accepting writes, and data will be synced to n3 when the network partition is resolved. Choosing the right CAP guarantees that fit your use case is an important step in building a distributed key-value store.