Availability
Assurance that systems and data are accessible when authorized users need them.
SLAs commonly measure availability as “nines” of uptime — 99.9% (three nines) allows roughly 8.8 hours of downtime per year, while 99.999% (five nines) allows about five minutes. Availability controls include redundancy, load balancing, failover clustering, and off-site backups; the chief threats are DoS/DDoS attacks, hardware failure, and ransomware.
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