Service Level Indicator (SLI)

I. Formal Definition

A measured proportion of valid events that meet a defined success criterion, used as the operational signal against which a service-level objective is evaluated. An SLI is not a metric in the general sense; it is a precisely scoped fraction with a numerator, a denominator, and an event-validity rule. The federation requires that every Tier-1 service publish at least one availability SLI and one latency SLI, and that the event-validity rule be documented such that an external auditor can reproduce the figure under UFMS-001:2.4.

II. Etymology

Formalised by Google in the Site Reliability Engineering literature published in 2016; predecessor concepts under various names in the carrier-grade telecoms tradition.

III. Federation Usage

Federation members must publish SLI definitions in a structured format with the event source, the validity rule, and the success criterion. Free-text SLIs are non-conforming under MEV-Annex:3.2.

IV. Related Terms

• slo
• error-budget
• metrics
• observability

V. Authoritative Citations

• UFMS-001:2.4UFMS Article II - Indicator Standard
• MEV-Annex:3.2MEV Annex III - Definition Format

VI. Citation in BibTeX

@misc{ifo4_glossary_sli,
  title        = {{Service Level Indicator (SLI)}},
  author       = {{IFO4 Federation Editorial Board}},
  howpublished = {{IFO4 Federation Glossary, slug \texttt{sli}}},
  year         = {2026},
  url          = {https://ifo4.org/glossary/sli},
  note         = {Category: DevOps; key: ServiceLevelIndicatorSLI}
}

VII. Challenge This Definition

Federation members and accredited practitioners may challenge any entry under TGS-002:1.7. Filed challenges are routed to the editorial board, triaged into the revision register, and resolved in writing on the public docket. The slug remains stable through any revision.