**6.4 Remote replication: synchronous**

Synchronous replication as shown in **Figure 12** provides near zero RPO where the replica is identical to the source at all times.

In synchronous replication, writes must be committed to the source and the remote replica (or target) prior to acknowledging "write complete" to the compute

**Figure 12.** *Remote Replication: Synchronous.*

### *Network Function Virtualization over Cloud-Cloud Computing as Business Continuity Solution DOI: http://dx.doi.org/10.5772/intechopen.97369*

system. Additional writes on the source cannot occur until each preceding write has been completed and acknowledged. This ensures that data is identical on the source and the replica at all times. Further, writes are transmitted to the remote site exactly in the order in which they are received at the source. Therefore, write ordering is maintained. The figure on the slide illustrates an example of synchronous remote replication. Data can be replicated synchronously across multiple sites. If the primary zone is unavailable due to disaster, then the service can be restarted immediately in other zone to meet the required SLA.

Application response time is increased with synchronous remote replication because writes must be committed on both the source and the target before sending the "write complete" acknowledgment to the compute system. The degree of impact on response time depends primarily on the distance and the network bandwidth between sites. If the bandwidth provided for synchronous remote replication is less than the maximum write workload, there will be times during the day when the response time might be excessively elongated, causing applications to time out. The distances over which synchronous replication can be deployed depend on the application's capability to tolerate the extensions in response time. Typically synchronous remote replication is deployed for distances less than 200 KM (125 miles) between the two sites.

#### **6.5 Remote replication: asynchronous**

It is important for a service provider to replicate data across geographical locations in order to mitigate the risk involved during disaster. If the data is replicated (synchronously) between zones and the disaster strikes, then there would be a chance that both the zones may be impacted. This leads to data loss and service outage. Replicating data across zones which are 1000s of KM apart would help service provider to face any disaster. If a disaster strikes at one of the regions then the data would still be available in another region and the service could move to the location.

In asynchronous remote replication as shown in **Figure 13**, a write from a computer system is committed to the source and immediately acknowledged to the compute system.

It enables replication of data over distances of up to several thousand kilometers between the primary zone and the secondary zones (remote locations). Asynchronous replication also mitigates the impact to the application's response time because the writes are acknowledged immediately to the compute system. In this case, the required bandwidth can be provisioned equal to or greater than the average write workload. Data can be buffered during times when the bandwidth is insufficient and moved later to the remote zones. Therefore, adequate buffer capacity should be provisioned. RPO depends on the size of the buffer, the available network

**Figure 13.** *Remote replication: asynchronous.*

bandwidth, and the write workload to the source. Asynchronous replication implementations can take advantage of locality of reference (repeated writes to the same location). If the same location is written multiple times in the buffer prior to transmission to the remote zones, only the final version of the data is transmitted. This feature conserves link bandwidth.
