High availability engineering eliminates single points of failure so that systems remain accessible even when individual components fail.
Why it matters
- Modern businesses depend on 24/7 system availability.
- Downtime costs range from thousands to millions per hour.
- SLAs often require 99.9% or higher uptime guarantees.
- Customer experience suffers from even brief outages.
The "nines" of availability
- 99% (two nines): 3.65 days downtime/year
- 99.9% (three nines): 8.76 hours downtime/year
- 99.99% (four nines): 52.6 minutes downtime/year
- 99.999% (five nines): 5.26 minutes downtime/year
- 99.9999% (six nines): 31.5 seconds downtime/year
HA design principles
- Redundancy: Duplicate critical components (servers, storage, network paths).
- Failover: Automatic switching to standby systems when primary fails.
- Load balancing: Distribute traffic across multiple instances.
- Geographic distribution: Spread across data centers/regions.
- Health monitoring: Detect failures quickly to trigger failover.
Common HA patterns
- Active-passive: Standby takes over only when primary fails.
- Active-active: All nodes serve traffic simultaneously.
- N+1 redundancy: One extra instance beyond minimum required.
- 2N redundancy: Double the required capacity.
Implementation considerations
- Database replication and clustering.
- Stateless application design for easy scaling.
- Session management across instances.
- DNS failover or global load balancing.
- Chaos engineering to test failure scenarios.
- Monitoring and alerting for rapid incident response.
Trade-offs
- Higher complexity and operational overhead.
- Increased infrastructure costs.
- Potential for split-brain scenarios in distributed systems.
- Need for thorough testing of failover mechanisms.
Related Articles
View all articlesFormal Security Models Explained: Bell-LaPadula, Biba, Clark-Wilson, and Beyond
Master the formal security models that underpin all access control systems. This comprehensive guide covers Bell-LaPadula, Biba, Clark-Wilson, Brewer-Nash, lattice-based access control, and how to choose the right model for your organization.
Read article →Biometric Authentication: Understanding FAR, FRR, and CER for Security Professionals
Master the critical metrics behind biometric authentication systems including False Acceptance Rate (FAR), False Rejection Rate (FRR), and Crossover Error Rate (CER). Learn how to evaluate, tune, and deploy biometric systems across enterprise, consumer, and high-security environments.
Read article →Database Inference & Aggregation Attacks: The Complete Defense Guide
Learn how inference and aggregation attacks exploit aggregate queries and combined data to reveal protected information, and discover proven countermeasures including differential privacy, polyinstantiation, and query restriction controls.
Read article →NIST 800-88 Media Sanitization Complete Guide: Clear, Purge, and Destroy Methods Explained
Master NIST SP 800-88 Rev. 1 media sanitization methods including Clear, Purge, and Destroy. Covers SSD vs HDD sanitization, crypto erase, degaussing, regulatory compliance, and building a media sanitization program.
Read article →