Logging & Observability interview question · Most asked
What are SLIs, SLOs and error budgets?
Summary
An SLI is a measured indicator of service quality (e.g., % of requests under 300 ms), an SLO is the target for it (99.9% over 30 days), and the error budget is the allowed failure (0.1%) that teams can 'spend' on releases and experiments.
Longer Answer
SLOs align engineering with user experience. When the error budget is being burned too fast, teams slow down releases and prioritize reliability. Alert on burn rate (how fast the budget is being consumed, using multi-window rules) rather than on every spike, which reduces noisy pages. An SLA is a contractual promise to customers, usually looser than the internal SLO.
Key Points
- SLI = measurement
- SLO = target
- Error budget = allowed unreliability
- Burn-rate alerts reduce noise
- SLA = external contract
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