Part 2 · SLI / SLO / SLA
Measure → target → external promise

The reliability contract

Measurement becomes a target, and the target can become a customer promise.

SLI Quantitative indicator of what users experience.
→
SLO Internal reliability target tied to real engineering policy.
→
SLA External contractual promise with consequences.

Typical SLI shape

good events
/
valid events

The numerator and denominator must be defined clearly, or the number is not trustworthy.

Key memory line

  • SLI says what happened
  • SLO says what target is acceptable
  • SLA says what was promised externally
04 / 20
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