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Reliability and High Availability

Understanding Availability​

Availability Metrics​

  • Five nines (99.999%): About 5 minutes of downtime per year
  • Four nines (99.99%): About 52 minutes of downtime per year
  • Three nines (99.9%): About 8.76 hours of downtime annually

Service Level Objectives (SLOs)​

Internal goals for system performance and availability that guide engineering decisions.

Service Level Agreements (SLAs)​

Formal contracts with users defining minimum service levels, often including compensation clauses.

Building Resilient Systems​

Redundancy Patterns​

Active-Passive​

  • Primary system handles all traffic
  • Backup system takes over on failure
  • Simple to implement but underutilizes resources

Active-Active​

  • Multiple systems handle traffic simultaneously
  • Load distributed across all systems
  • Better resource utilization but more complex

Failure Detection​

Health Checks​

  • Regular endpoint monitoring
  • Dependency health verification
  • Automated failover triggers

Monitoring and Alerting​

  • Real-time system metrics
  • Integration with communication platforms (Slack, PagerDuty)
  • Automated escalation procedures

Common Failure Modes​

Single Points of Failure​

  • Database servers
  • Load balancers
  • Network components
  • Authentication services

Cascading Failures​

  • Overload propagation
  • Resource exhaustion
  • Dependency chain failures

Best Practices​

Design for Failure​

"Building resilience into our system means expecting the unexpected."

  • Assume components will fail
  • Implement graceful degradation
  • Design for partial system failures

Testing Resilience​

  • Chaos engineering experiments
  • Failure injection testing
  • Disaster recovery drills

Operational Excellence​

  • Comprehensive logging
  • External log storage
  • Never debug in production

Implementation Checklist​

High Availability Requirements​

  • Redundant load balancers
  • Database replication
  • Geographic distribution
  • Automated failover
  • Health monitoring

Operational Procedures​

  • Incident response plans
  • Backup and recovery procedures
  • Monitoring dashboards
  • Alert configuration
  • Documentation updates

Key Takeaway: High availability is not achieved through individual components but through system-wide design patterns that embrace failure as a normal condition.