Service Level Objective (SLO)
A service level objective is a target for a reliability indicator over time, such as 99.9% success, that defines how reliable a service should be and drives engineering decisions.
A service level objective (SLO) is a target for the reliability of a service, expressed as a goal for a service level indicator over a window of time. For example, an SLO might state that 99.9 percent of requests should succeed over a rolling 30-day period.
How It Works
An SLO is built on a service level indicator (SLI), a precise measurement such as the fraction of successful requests or the proportion served within a latency threshold. The SLO sets the target the SLI should meet, for instance 99.9 percent over 28 days. The gap between perfection and the target defines the error budget: with a 99.9 percent target, 0.1 percent of requests may fail before the objective is breached. Teams track SLO compliance continuously and use breaches as a signal to act.
Why It Matters
SLOs make reliability an explicit, measurable goal rather than an unspoken expectation. They align engineering and business stakeholders on "how reliable is reliable enough," recognizing that 100 percent is neither achievable nor cost-effective. SLOs drive decisions: when the error budget is healthy, teams can ship faster; when it is exhausted, they prioritize stability. Well-chosen SLOs based on user experience are a cornerstone of site reliability engineering.
Related Terms
An SLO is defined in terms of a service level indicator and consumes an error budget. It guides incident management and depends on observability.