Explain the difference between an SLI, an SLO and an SLA, with an example.

What the interviewer wants to hear

• SLI is a measured indicator of service health
• SLO is a target for an SLI over a time window
• SLA is an external contract with consequences
• SLOs are usually stricter than SLAs
• Gives a concrete example such as availability or latency

Model answer

An SLI, or service level indicator, is a measurement of how the service behaves for users, for example the ratio of successful HTTP requests to total requests, or the fraction of requests faster than 300 ms. An SLO, or service level objective, is the target we set for that SLI over a window, such as 99.9% of requests succeeding over 30 days. An SLA, or service level agreement, is a contract with customers that includes consequences like refunds if we miss it, for example 99.5% monthly availability. We keep the SLO stricter than the SLA so we notice and fix problems before we break a contract. The SLO also defines the error budget, which is the amount of unreliability we can accept.

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