Tools / Monitoring and Observability Interview Questions
What is a Service Level Indicator (SLI) and how does it differ from an SLO and SLA?
An SLI (Service Level Indicator) is a specific, measurable signal that reflects user experience — typically a ratio or rate. Common SLIs include availability (percentage of successful HTTP requests), latency (fraction of requests served under 200 ms), and error rate (5xx responses divided by total requests).
An SLO (Service Level Objective) is a target set on top of an SLI. For example: "99.9% of requests must succeed over a rolling 28-day window." The SLO is an internal agreement between engineering and product; it defines what "good enough" looks like and drives the error budget concept.
An SLA (Service Level Agreement) is a contractual commitment made to customers, usually with financial penalties for breach. SLAs are typically looser than SLOs — the SLO is the engineering guardrail that keeps the team well inside the SLA boundary.
The hierarchy is: SLI → SLO → SLA. You measure with SLIs, aim for SLOs, and promise SLAs. Getting this order wrong — alerting directly on SLA thresholds — leaves no runway to detect and fix issues before customers are impacted.
| Term | What it is | Audience | Example |
|---|---|---|---|
| SLI | Measured signal | Engineering | 99.95% success rate (last 7 days) |
| SLO | Internal target | Eng + Product | ≥ 99.9% success rate over 28 days |
| SLA | Customer contract | Customers / Legal | ≥ 99.5% uptime or credits issued |
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