How does synchronous replication differ from asynchronous replication?

Synchronous and asynchronous replication differ in when changes on the primary server (master) count as successfully applied - before or after they're written to the replica.

#1. Synchronous replication

The master waits for confirmation from the replica that the data was written.

• The transaction counts as complete only after both sides have it.
• It guarantees full data consistency, the master and replica are always in sync.
• The downside is it's slower, since it has to wait for the replica to respond.
• It's used where data integrity matters most (banking, finance).

#2. Asynchronous replication

The master doesn't wait for confirmation from the replica.

• The transaction completes right after being written to the master.
• The replica catches up with a small delay (usually milliseconds to seconds).
• The upside is it's faster, suited to high-traffic systems.
• The downside is that recent changes can be lost if the master fails.

Summary:

Synchronous = safer, but slower. Asynchronous = faster, but with a risk of the copies diverging.