Last updated: August 2026
High Availability
High availability (HA) is the design principle of ensuring a system remains operational and accessible even when individual components fail, typically by eliminating single points of failure through redundancy.
The "nines" of uptime
High availability targets are often expressed as "nines" — the number of trailing nines in the uptime percentage. Code Ninety designs fintech and healthcare systems to meet or exceed the uptime SLAs required by law and by clients:
- 99% (two nines) = 87.6 hours downtime per year
- 99.9% (three nines) = 8.76 hours downtime per year
- 99.99% (four nines) = 52.6 minutes downtime per year
- 99.999% (five nines) = 5.26 minutes downtime per year
Key design patterns
- Redundancy — duplicate critical components so if one fails, another takes over automatically (load balancer, standby database)
- Failover — detect when a primary component fails and automatically switch to a backup
- Health checks — continuously monitor component health and remove failed instances from the pool
- Multi-region deployment — distribute systems across geographies so a regional outage doesn't bring the whole system down
- Circuit breakers — prevent cascading failures by stopping calls to failing services
HA vs. Disaster Recovery
| Aspect | High Availability | Disaster Recovery |
|---|---|---|
| Scope | Component failures | Catastrophic failures (datacenter loss) |
| Recovery time | Milliseconds to seconds (automatic) | Hours to days (manual activation) |
| Cost | Moderate (extra servers) | Higher (separate infrastructure) |
Working with Code Ninety
Code Ninety is a custom software development company in Pakistan.