Which statement correctly describes SLA, SLO, and SLI in relation to mission-critical services?

• A. SLA is a formal agreement of service levels; SLO are target objectives; SLI are measurable metrics used to assess performance against SLOs.
• B. SLA is a design standard; SLO are metrics; SLIs are service cost metrics.
• C. SLA is optional; SLOs are guaranteed; SLIs are not measurable.
• D. SLA is an internal policy; SLOs are compliance checks; SLIs are staffing metrics.

Answer: (A)

In mission-critical services, the relationships among SLA, SLO, and SLI are tiered: the service level agreement is the formal contract with customers or stakeholders that states the expected level of service and what happens if it’s not met. The service level objectives are the specific targets defined within that agreement—like uptime, latency, or restore time—that the service aims to achieve. The service level indicators are the actual measurements used to gauge whether those targets are being met, such as the measured uptime over a window, percentile latency, or error rate. This structure provides clear accountability: SLAs set the commitments, SLOs define the targets, and SLIs provide the metrics to assess performance against those targets. The other descriptions mislabel these concepts—for example, treating SLAs as a design standard or optional, or claiming SLIs aren’t measurable—so they don’t align with how reliability is planned and monitored.