What is high availability or four 9's and why is it important

Understand the impact of moving from 99.9% to 99.99% availability

What is high availability or four 9's and why is it important

Three 9’s… Four 9's… Five 9’s… and Sold!

Ever wondered why AWS, Azure, or GCP advertise so much about how they can ensure 99.99% or 99.999% availability of a service? On the surface, it doesn’t seem like this additional 9 could make much difference but it could cost your business a lot of money and time.

Remember how we on the internet go crazy when we find out that our favorite app is down and for the next few minutes, we will not be able to text someone or upload our new selfie. These few minutes cost the businesses a lot of money and unhappy customers.

This is where these 9’s come into the picture. As we know most businesses today are migrating to cloud technologies, so it becomes very important to find a service they can rely on, one that promises high availability and guaranteed uptime.

What is high availability?

In simple words, it means “Always on”. High availability is a characteristic of a system that ensures uninterrupted continuity of services to the users.

Availability is expressed as a percentage of uptime in a year. What we generally see is availability measured between 99% to 100%.

This means service with 100% uptime will work 24/7 throughout the year and the system will never fail. Now, that is pretty rare. On the other side, anything less than 99% should be a red flag and you should check what is restricting the provider to reach industry standards.

What do these percentages mean?

Cloud service providers calculate the availability percentage of a service using the following equation. This percentage is calculated over a period of time, such as a month, year.

Source — AWS Availability

Let’s take an example where a provider guarantees that a service will be up 99.9% (Three 9’s) over a year, and calculate how much downtime your business can face.

There are 8,760 hours in a year when multiplied by the availability percentage i.e. 99.9% we get an uptime of 8,751.24 hours.
This means a downtime of 8760 — 8751.24 = 8.76 hours

So even with 99.9% availability, your business can be unavailable for 8.76 hours in a year, and depending on the scale of your business this can incur some heavy losses.

For reference,
In March 2015, a 12-hour Apple store outage cost the company $25 million.
In March 2019, a 14-hour outage cost facebook an estimated $90 million.

This is why cloud providers provide options for a higher number of nines so that they can reduce downtime. Check the following table below to understand how downtime reduces with an increase in the number of nines.

As we move from three 9’s to four 9’s or five 9’s the downtime per year significantly decreases which ensures that your business can run smoothly.

As we increase these nines to make our system more reliable, the cost charged by cloud providers also increases. So we should balance these nines according to our use case.

Here is a table of common application availability design goals that contains examples of the types of applications we commonly see at each availability tier.

Source — AWS Availability

Whenever you choose a provider that guarantees these availability percentages make sure you protect your investment by Service Level Agreements (SLA). The SLA is a legal contract between the service provider and your business to define the expectations of the platform. SLA consists of agreements on availability and performance metrics to compare with your business expectations.


So, now we have seen the objective meaning of the “number of nines” that cloud providers advertise and how they can impact businesses. So, if you are looking to build a business on top of a reliable cloud provider make sure to be considered of appropriate availabilities tier for your use case.