What is an SLI, an SLO, and an SLA, and how do they relate? [Basic]
Answer
An SLI is the measurement, an SLO is the target, and an SLA is a contractual promise. For example, request availability is an SLI, 99.9 percent monthly availability is an SLO, and customer credits for missing 99.9 percent are part of an SLA.
Technical explanation
SLIs should measure user-visible reliability, not only infrastructure health.
SLOs are internal reliability objectives that guide engineering decisions and alerting.
SLAs are external agreements with legal or financial consequences, so they are often less aggressive than internal SLOs.
Hands-on example
Example: define successful checkout requests as HTTP 2xx/3xx completed within 500 ms. SLI = good requests / total eligible requests. SLO = 99.5 percent over 28 days. SLA = 99.0 percent availability with customer credits. Alert on burn rate against the SLO, not on raw CPU.
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