SRE (Site Reliability Engineering)

Site Reliability Engineering (SRE) is an approach, started at Google, that applies software engineering to operations: keeping services reliable by automating manual work, measuring reliability and planning for failure.

Reliability is set with SLIs, SLOs and SLAs. An SLI (indicator) is a measurement such as the share of successful requests; an SLO (objective) is the target, such as 99.9% over 30 days; an SLA (agreement) is the promise to customers, with penalties if missed. The gap below 100% is the error budget: while it lasts teams can ship quickly, and when it runs out they focus on stability.