Availability (Fault tolerance)

$$Availability = \frac{Uptime}{Uptime+Downtime}$$

Fault Tolerance is a way to achieve a higher availability despite failures

Other metrics:

• MTBF: Mean Time Between Failures
• Average time the system is operational
• Because the uptime
• MTTR: Mean Time to Recovery
• Average time to detect and recover from a failure
• Basically the downtime

$$Availability = \frac{MTBF}{MTBF+MTTR}$$

Availability	Daily Downtime	Weekly Downtime	Monthly Downtime
99% (2 nines)	14 min 25 s	1 h 40 min	7 h 18 m
99.9% (3 nines)	1 min 26 s	43 m 12 s	8 h 45 min

• Usual causes of unavailability
• Human Error: faulty code
• Software Errors: long garbage collections, out-of-memory exceptions

• Hardware Failures: network failures, servers, routers, power outages

• Solutions

• Database replication and redundancy (durability)
• Failover to other region
• Availability over Consistency (AP over CP)
• Snapshots & Backups
• Postmortems