Availability and Reliability

Reliability vs Availability

A system can be available but unreliable (up but returning wrong data) or reliable but unavailable (down for maintenance). You want both.

---

The Nines of Availability

Availability	Downtime/year	Downtime/month
99%	3.65 days	7.2 hours
99.9%	8.7 hours	43.8 minutes
99.99%	52.6 minutes	4.4 minutes
99.999%	5.3 minutes	26 seconds

---

Eliminating Single Points of Failure

---

Active-Passive vs Active-Active Failover

---

Circuit Breaker Pattern

---

Health Check Flow

---

MTTR and MTBF

---

Key Takeaway

High availability requires all of these together:

1. Redundancy at every layer — no SPOFs
2. Automatic failover when components fail
3. Health checks to detect failures fast
4. Circuit breakers to prevent cascade failures
5. Graceful degradation — serve partial functionality under failure