SLO definition

An SLO turns "is it reliable enough?" from an argument into a measurement. Its real purpose is not the number — it is the error budget, which converts reliability from a value everyone claims to hold into a quantity you spend.

Without a budget, every outage produces "we must be more careful" and every roadmap discussion pits features against reliability with no way to settle it. With one, the answer is arithmetic.

100% is the wrong target. It is unachievable, and pursuing it means shipping nothing. The right target is the point past which users cannot tell the difference.

1. Measure what the user experiences

An SLI must reflect something a user would notice. Server-side CPU does not; a failed checkout does.

Good indicators, expressed as a ratio of good events to valid events: