High Availability (HA) refers to systems designed to remain operational and accessible for extended periods, typically measured as uptime percentages (99.9%, 99.99%, etc.). HA is crucial because:

• Business Continuity: Minimizes revenue loss from downtime
• User Experience: Ensures consistent service availability
• Reputation: Prevents damage from service outages
• Compliance: Meets regulatory requirements for uptime

The goal is to eliminate single points of failure through redundancy, failover mechanisms, and robust system design. For example, 99.9% availability means approximately 8.77 hours of downtime per year, while 99.99% allows only 52.6 minutes annually.

RTO (Recovery Time Objective) and RPO (Recovery Point Objective) are critical disaster recovery metrics:

• RTO: Maximum acceptable time to restore service after a failure. This is about how fast you can recover.
• RPO: Maximum acceptable amount of data loss measured in time. This is about how much data you can afford to lose.

Example:
If a database fails at 2:00 PM:

• RTO of 1 hour means service must be restored by 3:00 PM
• RPO of 15 minutes means you can only lose data from 1:45 PM onward

These metrics drive backup frequency, infrastructure investment, and recovery strategy decisions.

Availability levels are expressed as "nines":

• 99% (Two 9s): 3.65 days downtime/year - Basic consumer services
• 99.9% (Three 9s): 8.77 hours downtime/year - Small business applications
• 99.99% (Four 9s): 52.6 minutes downtime/year - Enterprise applications
• 99.999% (Five 9s): 5.26 minutes downtime/year - Critical systems (telecom, financial)
• 99.9999% (Six 9s): 31.5 seconds downtime/year - Ultra-critical systems

Each additional "9" exponentially increases complexity and cost. Achieving five 9s requires redundant everything: power, network, hardware, software, and often geographically distributed infrastructure.

A Single Point of Failure (SPOF) is any component whose failure would cause the entire system to fail. Common SPOFs include:

• Hardware: Single server, network switch, power supply
• Software: Single database instance, application server
• Network: Single internet connection, DNS server
• Human: Single administrator with exclusive knowledge

Elimination strategies:

• Redundancy: Multiple instances of critical components
• Load Balancing: Distribute traffic across multiple nodes
• Clustering: Group servers to act as one logical unit
• Geographic Distribution: Multiple data centers
• Documentation: Ensure knowledge is shared among team members

Example: Instead of one web server, deploy three servers behind a load balancer with health checks.