Chapter 9: The Reliability Pillar

• 1. C. 99.95 percent availability translates to about 22 minutes of downtime per month, or 4 hours and 23 minutes per year. 99.999 percent availability is less than 30 seconds of downtime per month, but the question calls for the minimum level of availability. 99 percent yields more than 7 hours of downtime per month, while 99.9 percent is more than 43 minutes of downtime per month.
• 2. A. The EC2 instances are redundant components, so to calculate their availability, you multiply the component failure rates and subtract the product from 100 percent. In this case, 100% – (1% × 1%) = 99.99%. Because the database represents a hard dependency, you multiply the availability of the EC2 instances by the availability of the RDS instance, which is 99 percent. In this case, 99.99% × 99% = 98.99%. A total availability of 99 percent may seem intuitive since it’s the availability of each component, but because the redundant EC2 instances have...