Database Replication: Master-Slave vs Multi-Master

Replication means keeping copies of your data on multiple servers.

Master-Slave (Primary-Replica)

• 1 Master: Handles all Writes.
• N Slaves: Handle Reads. Replicates data from Master.

Use Case: Your app has 90% reads (viewing profiles) and 10% writes (updating profile). You can add more slaves to scale reads indefinitely.

Failover: If Master dies, a Slave is promoted to be the new Master.

Multi-Master

• Multiple Masters: All servers accept Writes and Reads.
• Complexity: Conflict resolution. What if User A updates a row on Server 1, and User B updates the same row on Server 2 at the same time?

Async vs Sync Replication

• Synchronous: Write is confirmed only after ALL replicas acknowledge. (Safe but slow).
• Asynchronous: Write is confirmed immediately. Replicas catch up later. (Fast but risk of data loss on crash).