11.2The SRE Vocabulary

Concept	Definition	Example
SLI (Service Level Indicator)	A quantitative measure of a service attribute	p99 latency of the checkout API
SLO (Service Level Objective)	A target value or range for an SLI	p99 latency < 500ms, 99.9% of the time
SLA (Service Level Agreement)	A business contract with financial consequences	99.9% uptime or credits issued
Error Budget	The allowed failure margin: 100% - SLO	0.1% = ~43 minutes of downtime/month

The Relationship Between Them

SLI = The measurement (what you observe)
SLO = The target (what you commit to internally)
SLA = The contract (what you promise customers, with penalties)
Error Budget = SLO's tolerance (how much failure is acceptable)

Always: SLA <= SLO (your internal target should be stricter than your customer promise)