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Glossary

SLI

Service level indicator: a carefully defined quantitative measure of some aspect of service, such as the share of requests served successfully.

SLO
Service level objective: a target value or range for an SLI, such as 99.9% of requests succeeding over 30 days.
SLA
Service level agreement: a contract with users that includes consequences for meeting or missing the SLOs it contains.
Error budget
The amount of unreliability an SLO allows over a period: with a 99.9% target, 0.1% of requests may fail

Sources

Google SRE book, ch. 4: Service Level Objectives

Updated October 22, 2026.

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