How important are the number of "nines" in your site's uptime? In a world where getting things right 99 percent of the time is normally a win, 99 percent uptime is widely perceived as a failure. Case in point: In June Twitter experienced serious availability problems due to a combination of network configuration problems and record traffic driven by the World Cup and NBA Finals.
For all the headlines about outages, what was Twitter's uptime for the month? It was 99.17 percent, according to Pingdom, with a total of 5 hours, 43 minutes of downtime. In a 30-day month, 99.9 percent uptime works out to about 43 minutes of downtime, while "four nines" equates to a little more than 4 minutes.
Over the course of a year, here's what the various "nines" look like:
- Two nines: 5,256 minutes or 3 days, 15 hours, 36 minutes
- Three nines: 525 minutes or 8 hours, 46 minutes
- Four nines: 52 minutes or 0.8 hours
- Five nines: 5.3 minutes or 0.09 hours
Uptime monitoring is important for businesses with a service level agreement (SLA) that stipulates the level of reliability a hosting provider or data center service will provide. Although some services "guarantee" uptime of 100 percent or 99.99 percent, they can't guarantee that there won't be outages. The SLA's function is to outline compensation to customers in the event of downtime (Hamlesh Motah has more on this). As a free consumer application, Twitter doesn't offer an SLA. While users may complain about Twitter outages, the service's traffic indicates that the downtime is not a barrier to growth.