Troubleshooting PHP-FPM in virtualmin

I have virtualmin GPL installed on CentOS 7 . Yesterday sites stopped working with no apparent reason. I checked apache logs and found following errors :

  [proxy_fcgi:error] [pid 14139] [client 40.77.167.10:3566] AH01079: failed to make connection to backend: localhost

Turned out PHP-FPM was not running, I restarted it and sites got back. However, I am not sure how to troubleshoot and find the cause so that i won't repeat in future. Looking for pointers to start with ..

PS : This is first time I am dealing with php-fpm based server.