Last but not least is the multisite active-active approach. In this scenario, there is more than one production site in more than one region. All of the production sites are able to receive traffic and respond to requests at any time and can be balanced to ensure that the application never experiences any downtime. The real trick in this scenario is not the implementation nor the strategy for recovery; rather, it is the correct approach to data replication across regions. Replicating data to another region can be difficult to do in a consistent manner, due to the distances across regions. Replicating to another region can also be costly. On top of cost, you need to consider that sometimes replicating out-of-country could carry legal consequences as well. However, multisite active-active is the best solution to ensure zero downtime for your application.
Figure 6.2 compares the backup strategizes from the point of view of RPO/RTO, cost, and downtime.
FIGURE 6.2 Comparison of backup strategies
You are bound to have at least one question concerning the backup strategies. Often exam takers select the most powerful backup strategy; however, you should always consider that the lower the RPO/RTO, the higher the cost will be.
Answer these questions. The answers follow the last question. If you cannot answer these questions correctly, consider reading this section again until you can.
1. You have been asked to operate an existing application with a pilot light backup/DR strategy. Production is in us-west-2, and the pilot light is deployed in eu-west-1. Your complete customer base resides in the continental US. The application has an RPO of 90 minutes and RTO of 1 hour. The budget for the application regularly breaches 90 percent of allocated cost before the end of month. You do not know much else about the application. Based on the information provided, which of the following changes should be implemented in your opinion?
A. The RPO can be improved by using warm standby.
B. The backup/DR region should be changed.
C. The cost can be improved by moving to backup/restore.
D. Leave the application as is.
2. You have been asked to select a backup/DR strategy for a mission-critical application. The RPO for the application is defined as 5 minutes, and the RTO is defined as 10 minutes. You have already set up an RDS read replica that shows a lag of less than 45 seconds in a secondary region. The design must meet the requirements and be cost-effective. What approach would you use to deploy the application components?
A. Deploy a multisite active-active design to minimize the disruption of the mission-critical application.
B. Deploy a warm standby to fail over the mission-critical application in case of a disaster.
C. Deploy a pilot light scenario with powered-off copies of the production EC2 instances. Powering them on always meets or exceeds the RTO of the mission-critical application.
D. Deploy a pilot light scenario with the same AMIs being used by the production EC2 instances. Deploying the AMIs into EC2 always meets or exceeds the RTO of the mission-critical application.
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