What Is the Leader and Follower Pattern in Distributed Systems?

In distributed systems, data is often replicated across multiple servers for fault tolerance and high availability. However, ensuring consistency across replicas can be tricky—especially if using quorum-based approaches, which require a majority of servers to be available. Quorum ensures some consistency but may reduce availability and still allow rare inconsistencies.

The Leader and Follower Pattern: A Better Approach

To solve this, distributed systems often use the Leader and Follower pattern. Here's how it works:

Key Concepts

• Leader: A single server that handles all write operations, coordinates with followers, and ensures data is replicated properly.
• Followers: Servers that replicate data from the leader and act as backups. They may also handle read requests for load balancing.

How It Works

1. One server is elected as the leader.
2. The leader receives client requests (especially writes).
3. It replicates data to all followers.
4. Followers only accept writes from the leader.
5. If the leader fails, a new leader is elected from the followers.

Benefits

• Improved consistency: Only one source of truth for writes.
• Simplified coordination: Leader manages all replication.
• Fault recovery: Followers are ready to take over as leader.
• Load balancing: Followers can serve read requests.

The Leader and Follower pattern is a cornerstone in systems like Apache Kafka, ZooKeeper, and Raft-based consensus protocols, where strong coordination and consistency are required.