The most commonly reported "503 Service Unavailable" error that you may see when accessing a website returns from PHP-FPM. An example of this would be:
[01-Jan-1970 00:00:00] WARNING: [pool domain_com] server reached max_children setting (5), consider raising it
[01-Jan-1970 00:00:00] WARNING: [pool domain_net] server reached max_children setting (5), consider raising it
[01-Jan-1970 00:00:00] WARNING: [pool domain_org] server reached max_children setting (5), consider raising it
The cause of the error likely stems from Apache and PHP-FPM becoming overloaded with requests. PHP-FPM, and occasionally Apache, will need adjustments to their limitations to get around the problem.
To start, we recommend attempting to adjust the PHP-FPM pool limits from within the WHM "MultiPHP Manager." To do so globally:
[1] Access WHM >> MultiPHP Manager
[2] Select "System PHP-FPM Configuration"
[3] Adjust the "Max Children" and "Max Requests" fields. We recommend incrementing in values of 5 to 10 to ensure that PHP-FPM does not get overloaded.
[4] Save your configuration settings
To perform these changes by domain:
[1] Access WHM >> MultiPHP Manager
[2] Scroll down and locate a domain that experiences the problem.
[3] Select "Edit PHP-FPM" for the domain in question.
[4] Adjust the "Max Children" and "Max Requests" fields.
[5] Save your configuration settings