# Understanding of High Availability NFR (Non Functional Requirement)

Understanding of HA ( High Availability ) of non functional requirement is not easy, it’s art with science. Being an architect, it is very important to understand cost vs acceptable downtime, i mean to say how much cost you are going to invest of servers and your acceptance downtime before committing to the stakeholder of your application.

High availability requires less human intervention to restore operation in complex systems, the reason for this being that the most common cause for outages is human error.

## Availability Percentage and Downtime in a Year

Availability is usually expressed as a percentage of uptime in a given year. On high level following are the facts which

Availability % downtime in a year
90% ("one nine") 36.5 days
99% (two nines) 3.65 days
99.9% (three nines) ~ 9 hours (8.76 hours)
99.99% (four nines) 52.56 minutes
99.999% (five nines) 5.26 minutes
99.9999% (six nines) 31.5 seconds
99.99999% (seven nines) 3.15 seconds
99.999999% (eight nines) 315.569 milliseconds
99.9999999% (nine nines) 31.5569 milliseconds
~99.9998 % 1 min
~99.9886 % 1 hour
~99.7260 % 1 day

if you go through the above tabular calculation data you can understand how much availabity you are required in your application and how much redundancy and cost have to pay for this Non-Functional requirement.
In general, the number of nines is not often used by a network engineer when modeling and measuring availability because it is hard to apply in formula.

## Quiz

1. In a year, what is the maximum number of days of downtime you can have and still claim >99% uptime?

a. less than 1 days
b. 2 days
c. 3 days
d. 4 days
e. 5 days

## 7 thoughts on “Understanding of High Availability NFR (Non Functional Requirement)”

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