Okay, let's talk about disaster recovery. So a company typically decides on an acceptable business continuity plan based on the financial impact to the business when systems are unavailable. So the company determines the financial impact by considering many factors, such as the loss of business and the damage to its reputation due to downtime or the lack of systems availability. Now the common metrics for business continuity commonly refer to Recovery Time Objective and the Recovery Point Objective.

So let's just delve into these two concepts. So Recovery Time Objective, or the RTO, is the time it takes after a disruption to restore a business process to its service level, as was defined by the operational level agreement So for example, if a disaster occurs at 12 o'clock lunchtime, and the RTO is eight hours, then the disaster recovery process should restore the business process to the acceptable service level by 8:00 p.m. Now, the Recovery Point Objective, or RPO, is the acceptable amount of data loss measured in time. So, just to confuse everyone, it is also a time value But it's a slightly different one. The two are quite different concepts. So the acceptable amount of data loss measured in time. So for example, if the disaster occurred at 12 o'clock, around lunchtime, and the RPO is one hour, the system should recover all data that was in the system before 11:00 a.m. So the data loss will span only one hour between 11:00 a.m. and 12:00 p.m. So there's a number of different scenarios that we can apply in AWS to help meet the RPOs and RTOs. And the first one is what we call Backup & Restore.

Now with Backup & Restore, data is stored as a virtual tape library, using AWS storage gateway or another network appliance of a similar nature. We can use AWS import and export to shift large archives or in setting up archives for a Backup & Restore scenario. Then, in the disaster, archives are recovered from Amazon S3, and restored as if we were using a virtual tape.

All right, so the second potential design is what we call Pilot Light. And in Pilot Light, data is mirrored, and the environment is scripted as a template, which can be built out and scaled in the unlikely event of a disaster. So what that looks like in our recovery phase, in the unlikely event of a disaster, to recover the remainder of our environment around our Pilot Light, we can start our systems from the Amazon machine images on the appropriate instance types. And for our dynamic data service, we can resize them to handle production volumes as needed, or add capacity accordingly.

Okay, so the third scenario we can implement is what we call Warm Standby. And our key steps for preparation in a Warm Standby which is, as it says, essentially ready to go with all key services running in the most minimal possible way, so we're essentially running a smaller version of our full production environment. Then during our recovery phase, in the case of failure of the production system, the standby environment will be scaled up for production load, and the DNS records will be changed to route all traffic to the AWS environment.

Now our fourth potential scenario is what we call Multi-Site. With Multi-Site, we set up our AWS environment to duplicate our production environment. So essentially we've got a mirror of reproduction running in AWS. Now, in our recovery phase, traffic is cut over to the AWS infrastructure by updating the DNS record in route 53. And all traffic and supporting data queries are supported by the AWS infrastructure. Our Multi-Site scenario is usually the preferred one, and where time is a priority, recovery time and Recovery Point Time are priorities, and costs are not the main constraint, then that would be the ideal scenario.

Okay, so one key thing to ensure when we are running any of our scenarios is to ensure that we test the recovered data. So once we've restored our primary site to a working state, we then need to restore to a normal service, which is often referred to as a fail back process. So depending on your DR strategy, this typically means reversing the flow of data replication, so that any data updates received while the primary site was down can be replicated back without loss of data. Right, very good. Let's do a quick summary of the four options we have for disaster recovery so you're well-prepped for your exam. So Backup & Restore, like AWS as a virtual tape library. It's likely to have the highest Recovery Time Objective, because we need to factor in the time it would take us to access or download backup archives. Our Recovery Point Objective is most likely to be quite high as well, because if we're only doing daily backups, it could be up to 24 hours. With Pilot Light, we've got that minimal version of our environment running on AWS, which can be expanded to full size when needed.

We've got potentially a lower Recovery Time Objective than we would for Backup & Restore, but we need to factor in that we still may need to install applications, or patches onto our AMIs before we have a fully operational system. Our Recovery Point Objective is going to be since the last snapshot, so it's going to be reasonably low. For Warm Standby, we've got that scaled down version of a fully functional environment always running. So our Recovery Time Objective is likely to be lower than Pilot Light, as some of our services are always running. Our Recovery Point Objective is ideally going to be quite low, since it will be since our last data write, if it's master slave multi-AZ database. Even if it's asynchronous only, it's still going to give us quite a good Recovery Point Objective. And the benefit of having a Warm Standby environment is we can actually use it for dev tests, for one-off projects, or for skunkworks, et cetera.

And Multi Site is that fully operational version of our environment, running off-site or in another region. And it's likely to give us our lowest Recovery Time Objective, if we're using active / active fail over, it could be a matter of seconds. With our Recovery Point Objective likewise. It depends on the choice of data replication that we choose, but it's gonna be since our last asynchronous or synchronous DB write. And using route 53 as an active / active failover, it's going to give us a very very aggressive short Recovery Point Objective and Recovery Time Objective. The considerations with that is the cost is going to be reasonably higher, proportionately, than the other three options we have, and we need to factor in that there will be some ongoing maintenance required to keep that kind of environment running. The benefit is that we have a way of regularly testing our DR strategy. We also have a way of doing blue-green deployments, and it gives us a lot more diversity in our IT infrastructure.