Achieving high availability
The last thing we need to talk about is availability. At the most basic level, in order to make the application highly available we need to remove all single points of failure from the architecture. In order to do that, we need to treat every single component as unreliable and assume that it will fail sooner or later.
It is important to realize that the availability of the system as a whole can be defined as the product of the availability of its tightly coupled components:
AS = A1 * A2 * ...* An
For example, if we have a web server, an application server, and a database server, each of which is available 99% of the time, the expected availability of the system as a whole is only 97%:
0.99 * 0.99 * 0.99 = 0.970299 = 97% ...
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