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Cloud Computing · Foundational

High Availability (HA)

Design goal of keeping a system operational and accessible for a high percentage of time by eliminating single points of failure.

Definition

High availability uses redundancy across fault domains, such as multiple instances across availability zones behind load balancers, replicated databases with automatic failover and health checks. Availability is often expressed in nines; 99.99 percent allows about 52.6 minutes of downtime per year.

Example

A web tier runs in an Auto Scaling group spanning three AZs with an RDS Multi-AZ database.

Exam tip

HA minimizes downtime through redundancy and quick recovery; fault tolerance aims for zero interruption. Distribute across AZs for HA.

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