Log Replication and Majority Commit in Distributed Systems

Introduction to Log Replication

Log replication is a technique used in distributed systems to ensure that data written to one node is safely replicated to other nodes. This is crucial for maintaining consistency and durability of data across multiple servers.

How Log Replication Works

1. Leader and Followers: In a typical setup, there is one leader node and multiple follower nodes. The leader is responsible for processing client requests and replicating logs to followers.
2. Client Request: When a client sends a write request (e.g., SET X = 10), the leader appends this entry to its log.
3. Replication to Followers: The leader then sends this log entry to all follower nodes for replication.

Majority Commit

• Majority Acknowledgment: For a write operation to be considered committed, it must be acknowledged by a majority of nodes. This is often referred to as a quorum.
• Example with 3 Nodes:
  • Nodes: A (Leader), B (Follower), C (Follower)
  • Majority needed: 2 out of 3 nodes.
  • If A and B acknowledge the entry, the write is committed (X = 10). Even if C is lagging, the operation is considered successful.

Importance of Majority

• Fault Tolerance: Majority commit ensures that even if some nodes fail or lag, the system can still operate correctly.
• Durability: Once a majority acknowledges a write, it is durable and will not be lost, even if the leader crashes.

Raft Consensus Algorithm

• The Raft algorithm is a popular method for achieving consensus in distributed systems. It emphasizes simplicity and understandability while ensuring that log replication and majority commit work effectively.

Conclusion

Log replication and majority commit are essential for building reliable and fault-tolerant distributed systems. Understanding these concepts is crucial for software engineers working on high-availability systems.