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Replication is the transfer and synchronization of data between computing or storage resources, and typically between multiple regions or data centers, as illustrated in Figure 1.16. For disaster recovery purposes and data security, your data must be transferred, or replicated, between data centers. Remote copies of data have traditionally been implemented with storage backup applications. However, with the virtualization of servers in the cloud, you can easily replicate complete VM instances, which allows you to replicate complete server instances, with all of the applications, service packs, and content, to a remote facility.

FIGURE 1.16 Site-to-site replication of data

Applications such as databases have built-in replication processes that can be utilized based on your requirements. Also, many cloud service offerings can include data replication as a built-in feature or as a chargeable option.

Synchronous replication is the process of replicating data in real time from the primary storage system to a remote facility, as shown in Figure 1.17. Synchronous replication allows you to store current data at a remote location from the primary data center that can be brought online with a short recovery time and limited loss of data. Relational database systems offer synchronous replication along with automatic failover to achieve high availability.

FIGURE 1.17 Synchronous replication

With asynchronous replication, the data is first written to the primary storage system in the primary storage facility or cloud location. After the data is stored, it is then copied to a remote location on a delayed or scheduled basis, as shown in Figure 1.18.

FIGURE 1.18 Asynchronous replication

One common use case for asynchronous replication involves taking scheduled snapshots of VM storage volumes and storing those snapshots offline. The snapshots may also be replicated to a remote location for safekeeping. If you ever need to restore the VM, you can do so from the snapshot.

Another example of asynchronous replication is the creation of database read replicas. When an organization needs to run intensive, complex reports against a database, it can tax the database server and slow it down. Rather than taxing the primary database server, which might be performing critical business functions, you can asynchronously replicate the data to a read replica and then run your reports against the replica.

Asynchronous replication can be more cost effective than implementing a synchronous replication offering. Cloud providers often charge for data transfer between regions or availability zones. Because asynchronous replication is not in real time, there's typically less data to transfer.