Final answer:
To achieve a guaranteed availability of 99.99%, it is recommended to deploy the critical application across a minimum of two availability zones. This level of availability, known as 'four nines,' ensures the app remains accessible even if one zone becomes unavailable, and assumes that the zones are truly independent and isolated from each other's failures.
Step-by-step explanation:
For a guaranteed availability of 99.99 percent, commonly referred to as the "four nines," it is recommended to deploy applications across multiple availability zones to ensure high availability and fault tolerance. Deploying across multiple availability zones allows an application to remain accessible even if one zone becomes unavailable.
In practice, using two availability zones is generally considered sufficient to reach 99.99% availability. This is because with two independent zones, the probability that both would fail simultaneously is very low, assuming that the availability zones are indeed separate with their own power, networking, and cooling and that they are isolated from each other's failures.
There is no absolute rule that defines the exact number of zones required for a given availability percentage, as this depends on the design of the zones themselves and the specific cloud provider. However, two zones are usually recommended at the minimum for 99.99% availability.