From wishes to numbers
A service level objective is a target value for a reliability metric over a window of time. It turns a wish like keep the service reliable into something you can measure and argue about, such as ninety nine point nine percent of requests succeed over thirty days.
The building blocks
- A service level indicator is the measured metric, for example the fraction of good requests.
- A service level objective is the target for that indicator over a window.
- A service level agreement is a contract with consequences when the objective is missed.
What makes an SLO good
- It reflects what users actually feel, not internal convenience metrics.
- It is achievable yet not perfect, because chasing one hundred percent is ruinously expensive.
- It has a clear measurement window and a precise definition of a good event.
A common trap is setting the bar too high. An SLO of one hundred percent leaves no room for deploys, experiments, or planned maintenance.
Key idea
A good SLO is a user centered, achievable target that makes reliability measurable rather than aspirational.