Intermediate
Open
Free
Availability Nines
A payment processing service has a monthly availability SLA of 99.95%.
- How many minutes of downtime is that per month (assume a 30-day month)?
- If the service is composed of two independent components each with 99.99% availability, what is the combined availability?
Solution
1. Downtime per month: 30 days × 24 hours × 60 minutes = 43,200 minutes/month. Downtime = (1 - 0.9995) × 43,200 = 0.0005 × 43,200 = 21.6 minutes/month.
2. Combined availability of two independent components: When two components are in series (both must be up for the system to work), the combined availability is the product of each: 0.9999 × 0.9999 = 0.99980001 ≈ 99.98%.
This is why multi-component systems have lower availability than any single component, and why reducing the number of sequential dependencies improves overall availability.