Distinguish SLI, SLO, and SLA with an example.
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Quick Answer
SLI is a measured indicator (e.g., availability = 99.95%), SLO is the internal target on it (e.g., 99.9% uptime), and SLA is the external contract with the customer (looser, with a penalty if breached). SLI = measurement, SLO = target, SLA = promise.
Detailed Answer
They nest: you measure an SLI (success rate, latency), set an SLO above what you promise to keep margin, and the SLA is the commercial agreement with consequences. Example: SLI current availability 99.95%; SLO target 99.9%; SLA promises 99.5% or credits apply. The gap between the SLO and 100% is the error budget that governs release risk.
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Interview Tip
Use the mnemonic SLI=measurement, SLO=target, SLA=promise, and note SLA is looser than SLO.
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