SLO Definition Vocabulary
5 exercises — Practice the language for writing and discussing SLO definitions: SLI selection, measurement windows, P99 latency targets, and target-setting conversations.
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An SRE is helping a team define their first SLO. They say: "We need to choose a good service level indicator before we can set any target." Which statement best explains what makes an SLI well-chosen?
SLI selection is user-centric: the indicator should measure what users actually experience, not what is easiest to instrument.
The four common SLI categories from Google's SRE book are:
• Availability — proportion of requests that succeeded (e.g., non-5xx responses)
• Latency — proportion of requests faster than a threshold (e.g., P99 < 300 ms)
• Quality — proportion of requests served with full quality (not degraded mode)
• Freshness — proportion of reads that returned data updated within a threshold
CPU and memory are resource metrics, not user-experience metrics — they are useful for capacity planning but not directly tied to what the user experiences. The SLI formula is always: good events / valid events, expressed as a percentage.
Key vocabulary:
• SLI (Service Level Indicator) — a quantitative measure of the service as experienced by the user
• good event — a request or interaction that met the quality bar
• valid event — any request that a correct service should be able to serve
• request-based SLI — measures individual requests rather than time-based uptime checks
The four common SLI categories from Google's SRE book are:
• Availability — proportion of requests that succeeded (e.g., non-5xx responses)
• Latency — proportion of requests faster than a threshold (e.g., P99 < 300 ms)
• Quality — proportion of requests served with full quality (not degraded mode)
• Freshness — proportion of reads that returned data updated within a threshold
CPU and memory are resource metrics, not user-experience metrics — they are useful for capacity planning but not directly tied to what the user experiences. The SLI formula is always: good events / valid events, expressed as a percentage.
Key vocabulary:
• SLI (Service Level Indicator) — a quantitative measure of the service as experienced by the user
• good event — a request or interaction that met the quality bar
• valid event — any request that a correct service should be able to serve
• request-based SLI — measures individual requests rather than time-based uptime checks