An enterprise application consists of a 2 node active-active application server cluster connected to a 2 node active-passive database (DB) cluster. Both tiers need to be working for the system to be available. Over a long period of time it has been observed that an application server node fails every 100 days and a DB server node fails every 50 days. A passive DB node takes 12 hours to take over from the failed active node. Answer the following questions.

a) What is the overall MTTF of the 2-tier system?

b) Assume only a single failure at any time, either in the App tier or in the DB tier, and an equal probability of an App or a DB node failure. What is your estimate of the availability of the 2-tier system?