Advanced fault tolerant system: A more advanced system creates two active systems running simultaneously. In this active-active setup, any instance, or even an entire AZ can fail and the system will automatically be able to complete all its functions from another AZ that is pre-architected and ready to run on. RightScale says this architecture will cost more than double the cost of a single AZ setup, because all of the services form the single AZ not only have to be replicated, but there are data transfer costs that come with ensuring both systems are kept up-to-date in real time.

There are other options, too.