Relationship between SLI, SLO, and SLA

WHAT THIS TESTS This checks command of the foundational reliability vocabulary every SRE conversation builds on, and whether you understand how the three layers relate rather than just reciting definitions.

A GOOD ANSWER COVERS An SLI, the indicator, is a concrete measurement of some aspect of service quality, expressed as a ratio or percentage, such as the proportion of HTTP requests that return a non-error status, or the proportion served under three hundred milliseconds. An SLO, the objective, is the target value or range you commit to internally for that SLI over a window, for example availability of at least 99.9 percent measured over a rolling thirty days. An SLA, the agreement, is an external contract with customers that promises a level of service and specifies consequences, such as service credits, when it is not met. The key relationship: the SLI is what you measure, the SLO is the goal you hold yourselves to, and the SLA is the weaker promise you make to the outside world, deliberately set below the SLO so you have internal headroom before contractual breach.

COMMON WRONG ANSWERS Treating the three as synonyms, or defining the SLO without an associated SLI to measure it. A serious error is setting the SLA equal to or tighter than the SLO, which removes any buffer and means an internal miss is instantly a contract breach.

LIKELY FOLLOW-UPS How do you choose a good SLI? What is an error budget and how does it derive from the SLO? Why keep the SLA looser than the SLO? Over what window should you measure?

ONE CONCRETE EXAMPLE For a web service, the SLI is the ratio of successful requests to total requests. The SLO states this ratio must be at least 99.9 percent over a rolling twenty-eight-day window, and the team alerts and prioritizes work to hold that line. The SLA, written into the customer contract, promises only 99.5 percent monthly availability, with service credits if missed. The gap between the 99.9 percent internal target and the 99.5 percent external promise is the safety margin that lets the team absorb a bad day without breaching the contract.