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Service Level Objectives (SLOs) provide a framework for defining and measuring reliability from the user's perspective. Rather than chasing 100% uptime (which is impossible and infinitely expensive), SLOs set explicit reliability targets that balance user happiness with engineering velocity.
| Term | Definition | Example |
|---|---|---|
| SLI (Service Level Indicator) | A quantitative measure of service behaviour | 99.2% of requests complete in under 500ms |
| SLO (Service Level Objective) | A target value or range for an SLI | 99.5% of requests should complete in under 500ms |
| SLA (Service Level Agreement) | A contract with consequences for missing an SLO | "99.9% uptime or we refund 10% of your bill" |
| Error Budget | The allowed amount of unreliability (1 - SLO) | 0.5% of requests can fail over 30 days |
SLI (measurement) → SLO (target) → SLA (contract) → Error Budget (remaining allowance)
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