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Multi-AZ everything (four nines)

intermediate

Scenario & brief

A revenue-critical service must reach 99.99% availability ("four nines"). Today it runs two instances in two AZs with a single database — good enough for three-and-a-half nines, but not four.

Targets: 99.99% availability, critical durability, p99 ≤ 160 ms, under $1,600/mo.

4,000 rps peakp99 ≤ 160ms99.99% availdurability: criticalbudget $1,600/mo

App Servers

Compute

2instances

System health

Erupting · SLA breach

28

/ 100

Score

SLA not met yet

Monthly cost

$653

Budget $1,600/mo · within budget

Metrics

Capacity43
Availability90
Durability25
Cost efficiency84

Requirements

  • Peak capacity — 3600 rps compute · 1700 rps db (need ≥ 4000 rps)
  • p99 latency — ~194 ms (need ≤ 160 ms)
  • Availability — 99.95% (need ≥ 99.99%)
  • Durability — at risk (need redundancy + backups)
  • Budget — $653/mo (need ≤ $1600/mo)

Advisor

  • Compute tops out at ~3600 rps but peak demand is 4000 rps — requests will queue.
  • The database is saturated at peak — add a cache to shed read load, or scale it up.
  • Enable automated backups to satisfy the durability requirement.

Discussion

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For learning purposes only. Costs and capacities are illustrative, not live AWS prices. Not affiliated with or endorsed by Amazon Web Services.