Understanding of High Availability of nonfunctional requirements is not easy, it’s an 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 on servers and how much your acceptance on downtime? These are essential things before committing to the availability of your application to the stakeholder.
High availability requires less human intervention to restore operation for complex systems, the reason for this being, the most common cause of outages is human error.
1. High Availability Percentage and Downtime in a Year
High 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 availability 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 the formula.
2. Quiz on High Availability
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
Answer: 3 days.
3. Reference
Happy learning 🙂
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