Synchronous vs Asynchronous vs Semi-Synchronous Database Replication | System Design

Differences between synchronous, asynchronous, and semi-synchronous are as follows: 

Replication Strategy	Synchronous Replication	Asynchronous Replication	Replication Semi-Synchronous Replication
Data Consistency	Ensures data consistency between source and target databases by waiting for target to confirm the receipt of data before the write operation is considered complete	Does not ensure data consistency as the write operation is considered complete before the data is copied to the target database	Balances performance and data consistency by waiting for a certain time period or amount of data before copying it to the target database
Write Operations Performance	Can slow down write operations as the source database has to wait for the target database to confirm the receipt of data before the write operation is considered complete	Can increase write operations performance as the write operation is considered complete before the data is copied to the target database	Balances write operations performance with data consistency
Failover Readiness	High failover readiness as the target database always has the latest data	Low failover readiness as there might be a delay between the source and target databases	Moderate failover readiness as data is copied to the target database after a certain time period or amount of data
Conflict Resolution	Rarely requires conflict resolution mechanisms as data consistency is ensured	Requires conflict resolution mechanisms as data consistency is not ensured	Requires conflict resolution mechanisms as data consistency is not ensured
Suitable Use Cases	Suitable for applications where data consistency is critical, such as financial systems	Suitable for applications where performance is critical and data consistency can be achieved through conflict resolution mechanisms, such as content management systems	Suitable for applications that require a balance between performance and data consistency, such as e-commerce websites

Conclusion: In summary, the application-specific requirements, such as data consistency, performance, and availability, determine the replication mode to be used. Applications, where data consistency is essential, should use synchronous replication, those where performance is crucial but data consistency can be achieved using conflict resolution techniques should use asynchronous replication, and those that need to strike a balance between performance and data consistency should use semi-synchronous replication.