The confusing 508 status code

Recently, UptimeRobot notified me that our site is down with HTTP status code 508 - Loop detected. The site recovered on its own after five minutes. On another occasion, a user reported seeing the message Resource Limit Is Reached when attempting to access our web app via a browser.

The standard definition of 508 says that it may be given in the context of the Web Distributed Authoring and Versioning (WebDAV) protocol and that it indicates that the server terminated an operation because it encountered an infinite loop. This was initially confusing, as I was not using any WebDAV and  the ini_set('max_execution_time', 300) in my PHP code would end hanging processes, which is consistent with the site recovering after 5 minutes.

Further research revealed that the 508 code could also signify that a Resource Limit Is Reached, a usage specific to CloudLinux environments, which deviates from the standard interpretation. In this case, Uptime Robot was applying the standard definition, whereas our hosting environment attributed a different meaning to the same code.

Ultimately, the issue was traced back to exceeding the maximum number of processes (NPROC) allocated to our web app. I am continuing to investigate the root cause.