Database Replication Lag, Split-Brain & Leader Election.

In Simple Words:

• Replication Lag: Time elapsed between a write committing to the Leader and replicating to Followers. Causes users to see stale data if they read from followers immediately after write.
• Split-Brain: When a network partition divides a cluster into two isolated subnets, and both subnets elect their own Leader and accept conflicting writes.
  • Fix: Strict Majority Quorum ($N/2 + 1$). A partition can only elect a leader if it contains more than half the total cluster nodes.