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4.3Availability and disaster preparedness
Covers judging which redundant configuration to choose from a target availability, the HA cluster, which makes multiple servers behave as one system, DR (disaster recovery) and BCP (business continuity plan) for large-scale disasters, and RTO (recovery time objective)/RPO (recovery point objective), which quantify recovery targets.
The previous section covered redundancy within a single data center (series/parallel availability calculation), but in practice, an architect must also prepare for the loss of an entire data center due to a large-scale disaster (earthquake, fire, or wide-area power outage), which redundancy alone cannot address. What matters here is quantitatively setting two recovery targets—"how quickly to recover" and "how much data loss is acceptable"—according to the criticality of the business. This section builds the judgment to design the level of availability and disaster preparedness by weighing it against cost, grounded in HA clusters for higher availability, the DR/BCP mindset, and the two metrics RTO and RPO.
4.3.1Configuration judgment from a target availability
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