Availability

Availability is the percentage of time a system remains operational and accessible to users. In high-scale systems, "High Availability" (HA) is a core requirement.

Measuring Availability (The "Nines")

Availability is often expressed as a series of nines.

Availability %	Downtime per Year	Class
99% (Two Nines)	3.65 days	Standard
99.9% (Three Nines)	8.77 hours	High Availability
99.99% (Four Nines)	52.6 minutes	Very High Availability
99.999% (Five Nines)	5.26 minutes	Fault Tolerant (Gold Standard)

Improving Availability

Availability is improved by removing Single Points of Failure (SPOF).

1. Failover (Active-Passive)

One server is active, while another is on standby. If the active server fails, the passive one takes over.

Heartbeat: The standby server periodically pings the active server to check its status.

2. Replication (Active-Active)

Multiple servers are active simultaneously. A load balancer distributes traffic among them. If one server fails, the load balancer stops sending it traffic.

High Availability Architecture

graph TD
    Client[Client Browser] --> DNS[Global DNS]
    DNS --> LB[Load Balancer]
    subgraph "Region A"
        LB --> AS1[App Server 1]
        LB --> AS2[App Server 2]
    end
    subgraph "Region B (Failover)"
        LB -.-> AS3[App Server 3]
        LB -.-> AS4[App Server 4]
    end

Availability vs. Reliability

Availability: Is the system up? (Measured by uptime percentage).
Reliability: Does the system perform its intended function correctly? (Measured by Mean Time Between Failures - MTBF).

A system can be available but unreliable (e.g., the website loads, but the "Buy" button does nothing).

SLA vs. SLO vs. SLI

SLI (Indicator): The specific metric (e.g., Error Rate).
SLO (Objective): The target value for the metric (e.g., Error Rate < 0.1%).
SLA (Agreement): The legal contract with users (e.g., If Availability < 99.9%, we refund you).

Key Takeaway

High Availability is expensive. Achieving "Five Nines" requires massive redundancy across geographical regions. Always match your availability goals to your business impact.