A four-hour outage struck the Amazon cloud services which affected thousands of websites in the U.S. on Thursday afternoon. The Amazon Web Services provides back-end services to sites such as Netflix, Pinterest, Spotify and Buzzfeed.

150,000 Connected Sites

USA Today reports that a large portion of services S3 system went offline, causing Amazon cloud services to fail to update its service health dashboard in the first two hours when the outage hit it since the dashboard was hosted on the AWS. Almost 150,000 sites use the S3 system which has about 3 trillion to 4 trillion pieces of data stores on it, according to Dave Bartoletti, cloud analyst of Forrester.

Also affected by the outage were Trello, Quora, IFTT and Splitwise and websites built with Wix, a site-creation service. GroupMe had difficulty loading assets and Alexa struggled to stay online, The Verge reports.

The AWS provides cloud-based storage and web services for companies which free the companies from having to build their own server farms. It also allows AWS customers to rapidly deploy computing power without the need to invest in infrastructure. With the service, a company could store its videos, photos, or databases on the AWS server and access it using the Internet.

A Major Outage

Bartoletti says the Tuesday outage is a big one. He adds AWS is rarely hit by outages but when it happens, it becomes the talk of town, similar to the one that hit AWS in September 2015 for five hours.

Catchpoint Systems, a digital experience monitoring firm, recorded the start of the outage at 12:35 p.m. ET. Amazon could services fully recovered by 4:49 p.m. ET.

Lydia Leong, cloud analyst with Gartner, explains that the usual cause of that type of outage is related to software. It is either a bug in the code or human error, although the cause of Tuesday’s outage remains unidentified.