What is the difference between synchronous and asynchronous replication, and in which cases is each of them more suitable?

The main difference is when the transaction is considered complete for the sender.

Synchronous replication:

• The transaction is considered complete only after the data is successfully written to at least one replica.
• Ensures that data will be available on the replica immediately after confirmation to the sender.
• High data consistency between the master and replicas.
• Lower performance due to waiting for confirmation from replicas.
• Suitable for systems where even minimal data loss is unacceptable (e.g., financial transactions).

Asynchronous replication:

• The transaction is considered complete immediately after writing data to the master server.
• Data is replicated to replicas with some delay.
• Data loss on the replica is possible in case of master failure before data is replicated.
• Higher performance as there is no waiting for confirmation from replicas.
• Suitable for systems where high performance is more important than strict data consistency (e.g., caching, analytical databases).

Comparison table:

Characteristic	Synchronous replication	Asynchronous replication
Transaction completion	After writing to M + R	After writing to M
Data consistency	High	Low (with delay)
Performance	Lower	Higher
Potential data loss	Low	Higher
Application	Financial systems, critical data	Caching, logging, analytics