Five-nines availability means keeping an application available 99.999% of the measurement period. That allows only 6 seconds of downtime per week or 26 seconds per month, so accepting the target without examining its consequences is a serious architecture mistake. section 13.2
The network-blip test
A five-nines target can be consumed by a simple 6-second network blip. Meeting it may require costly, geographically dispersed redundant infrastructure and redundant private links; a stand-alone Azure server was described as having only a 95% monthly uptime SLA at the time of writing. section 13.2
Use the target as a requirements and cost decision, not as an automatic promise:
- Estimate tangible and intangible downtime costs, then compare them with high-availability and disaster-recovery costs. section 13.2
- Do not let the application owner define availability in isolation; validate the requirement with architecture and analysis. section 13.2
