Explain the differences between SLIs, SLOs, and SLAs. How do they work together to define service reliability?

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Accepted Answer

SLIs, SLOs, and SLAs form a hierarchy that defines and measures service reliability. Understanding each component and how they interact is fundamental to Site Reliability Engineering.

A Service Level Indicator, or SLI, is a quantitative measure of a specific aspect of service performance. It is the raw metric that tells you how your service is actually behaving. Common SLIs include availability, which measures the proportion of successful requests. Latency measures the time taken to serve a request, often expressed as percentiles like the 99th percentile. Error rate measures the percentage of requests that result in errors. Throughput measures the number of requests processed per unit of time.

For example, an availability SLI might be calculated as the number of successful HTTP requests divided by the total HTTP requests, expressed as a percentage.

A Service Level Objective, or SLO, is a target value or range for an SLI. It defines what "good enough" looks like for your service. An SLO sets the threshold that you aim to meet or exceed. For example, an SLO might state that 99.9 percent of requests should complete successfully, or that the 99th percentile latency should be below 200 milliseconds.

SLOs should be based on user expectations and business requirements, not just on what is currently achievable. They represent a commitment to a certain level of service quality.

A Service Level Agreement, or SLA, is a formal contract between a service provider and its customers that specifies the expected level of service. It typically includes the SLOs that the provider commits to meet, along with consequences if those objectives are not met. Consequences might include service credits, refunds, or contract termination rights.

These three concepts work together in practice. The SLI is measured continuously to generate real data about service performance. The SLO defines the target that the SLI should meet. The SLA formalizes the SLO commitment with customers and defines what happens when targets are missed.

For internal services, you might have SLIs and SLOs without a formal SLA. For customer-facing services, the SLA provides accountability and often includes financial implications.