“The service should be reliable” expresses an intention, but it leaves a team arguing about what counts as success. A service-level objective makes the intention testable by connecting a measure to a target over a defined period.
Google’s Site Reliability Engineering chapter on service-level objectives distinguishes the indicator, or SLI, from the objective, or SLO. The indicator is the measured aspect of service; the objective is the target or range it should meet. A contractual service-level agreement is a further question, not another name for every internal target.
Write the sentence that defines the measure
For a hypothetical online booking service, an indicator might be the proportion of valid booking requests that complete successfully. A team could set a target over a rolling 28-day window. The exact target should come from user needs, risks and achievable operation, not from choosing an impressive number.
Before debating the percentage, define “valid request” and “success.” Decide what happens to duplicate requests, planned maintenance and missing telemetry. If two analysts can calculate different answers from the same records, the metric is not ready to guide a consequential decision.
The measurement window matters too. One good hour does not establish a good month. A rolling period and a calendar month can produce different views near their boundaries. Label the dashboard so readers know which one they are seeing.
Agree what a miss will change
Name the owner who reviews performance and the route for investigating a breach. Will the team pause risky changes, examine a recurring failure or adjust capacity? A target that triggers no decision can become a decorative dashboard item.
Also check whether the indicator reflects the user’s experience. A server answering quickly is not enough if the booking is never recorded. Conversely, an internal diagnostic failure may not affect customers in the way a headline availability number implies.
Start with a small number of meaningful objectives and review their definitions with the people who operate and use the service. A clear, modest measure that prompts useful action is more valuable than a demanding target nobody can calculate consistently.