FIRST THERE WAS SYNC
Synchronous replication enables zero data loss disaster recovery by ensuring that at any given moment, the data stored at the secondary storage site is an exact, mirror image of the data at the primary data center. Each input/output update must be acknowledged and confirmed at both the primary and secondary sites before the application may continue production. In this way, the system ensures that the secondary site is always in sync with the primary data center, enabling the secondary site to take over production immediately following any type of disruption at the primary site.
Synchronous replication comes with its disadvantages as well. It requires expensive, high performance storage hardware, high performance replication software and high performance synchronous communication lines, which, all together, result in high costs, making these types of solutions affordable for only the largest of enterprises.
Distance is also a limitation of synchronous replication. The distance between the primary and backup data centers is limited by its use of fiber channels. While fiber can be extended to as far as 200 km (124 miles), latency quickly becomes a problem as propagation delays lengthen with increased distance. This decreases the distance for synchronous replication to approximately 35-50kms or 20-30 miles which is not enough distance for both sites to withstand wide-area disasters.