The Replication Dilemma: Speed vs Consistency

Now comes the million-dollar question: How does data get from your master database to the replicas? This choice will define your entire system's behavior.

Option 1: Synchronous Replication (The Perfectionist)

• Write happens on master AND replica before responding to user
• Perfect consistency - both databases always match
• Slower response times - user waits for both writes
• Like writing the same note in two notebooks before calling it done

Option 2: Asynchronous Replication (The Pragmatist)

• Write happens on master, responds to user immediately
• Replica catches up later by periodically asking: "What's new?"
• Faster response times - user doesn't wait
• Brief moments where databases don't match (replica lag)

The Real-World Choice: Almost every production system uses asynchronous replication because speed usually matters more than perfect consistency for reads.

Key Insight: In system design, everything is a trade-off. You can have perfect consistency or better performance, but rarely both simultaneously.