File Integrity Monitoring and Change Audit is the two words I use the most in a week - and this blog explores Change Variance over at Tripwire: https://www.tripwire.com/state-of-security/change-variance-how-tiny-differences-can-impact-your-it-world

Service availability is the lifeblood of your IT operation. Change Variance can be like a sudden juggling act, where the balls are about to drop at any moment. Servers running different software versions or missing crucial files can spell trouble for service availability, and that risks disgruntled users, potential financial losses, and a few sleepless nights for your IT team.