Three nines or 99.9% availability

Luca Spiller
Head of Engineering
January 10, 2024

What is three nines?

Availability or uptime is commonly expressed as a percentage, indicating the duration during which a service remains operational within a specific timeframe. When we refer to three nines availability, we are talking about achieving an uptime of 99.9%, which translates to experiencing downtime of less than 8 hours, 41 minutes and 38 seconds per year. Attaining 100% uptime would mean the service remains accessible 24/7 without any interruptions.

What is an SLA?

Service providers commonly establish service-level agreements (SLAs) to specify the expected level of service. These agreements often include a clause detailing the required availability, typically expressed as a specific number of nines. If the provider's downtime exceeds this predefined threshold, the SLA usually requires compensation for the customer, often in the form of a credit applicable to future services.

100% Availability Goal

The objective of a high-availability system is to attain 100% availability, yet practical challenges may impede this aspiration. When a provider presents a Service Level Agreement (SLA), it signifies their confidence in the platform's reliability and the expected level of uptime. The importance of ensuring high availability is underscored by the potential for significant financial losses, as illustrated by the impact of even a brief 5-minute downtime in the stock market.

How to prevent downtime?

Avoiding downtime hinges on the implementation of effective monitoring systems. WatchSumo, specifically designed for this purpose, diligently observes your website. It identifies irregularities, such as heightened response times, which could signify impending issues and potential downtime. In essence, WatchSumo serves as a vigilant guardian, providing timely alerts to address concerns before they escalate into website downtime.

Calculating Downtime

99.9% in a day = 1 minute and 26 seconds of downtime

99.9% in a week = 10 minutes and 4 seconds of downtime

99.9% in a month = 43 minutes and 28 seconds of downtime

99.9% in a quarter = 2 hours, 10 minutes and 24 seconds of downtime

99.9% in a year = 8 hours, 41 minutes and 38 seconds of downtime

Other percentiles

Three nines of uptime is just one amount, other common 'nines' are as follows: