Synchronous writes the data at the same time across source and target(s), simultaneously. It is a single transaction, and all-or-nothing. Asynchronous writes to source then propagates the changes to the target(s) at regular intervals. It is on a schedule, so there is a lag between local commit and replication to remote nodes. This is most commonly used in cloud backup situations.

Breaking it down to the host-storage relationship:

Synchronous

1. Source Host sends write request to Source Storage
2. Source Storage writes data and sends to Target Storage
3. Target Storage writes data and sends acknowledgement to Source Storage
4. Source Storage acknowledgement to Source Host

Asynchronous

1. Source Host sends write request to Source Storage
2. Source Storage writes data and sends acknowledgement to Source Host
3. The update is held in queue until a specified time, at which the Source Storage sends the update to Target Storage
4. Target Storage writes data and sends acknowledgement to Source Storage.

The key difference in the chain is where/when the acknowledgement is sent. In Synchronous, the write-to-target action must complete successfully before a Source Host receives acknowledgement. In Asynchronous, the acknowledgement is sent upon writing to source, without confirming an immediate write to target.

Primary concerns between these two methods are data integrity and performance. Synchronous may guarantee no data loss but it consumes much more bandwidth and cost than an Asynchronous solution. Asynchronous tends to be more cost-effective and uses less resources, and is more resilient by design; however, data loss at write is more likely. If data must match real-time across nodes, Synchronous Replication is preferable, as even a small delay between local and remote write in Asynchronous Replication may not be allowable.

References

Connolly, T., & Begg, C. (2015).  Database Systems – Database Systems: A Practical Approach to Design, Implementation, and Management (6th ed.). London, UK: Pearson.

Vembu. (2016). Synchronous (vs) Asynchronous Replication. Retrieved from https://www.vembu.com/blog/synchronous-vs-asynchronous-replication/

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