- Availability means how much time a system is operational and accessible.
- Expressed as a percentage of total uptime vs total time.
- High availability ensures minimal downtime.
- Achieved through redundancy, failover systems, load balancing, and monitoring.
- Formula: Availability = (Uptime / Total Time) × 100
- Real-world examples:
- 99% → ~3.65 days downtime/year
- 99.9% → ~8.76 hours/year
- 99.99% → ~52.6 minutes/year
- 99.999% → ~5.26 minutes/year