What is an SLO?
An SLO is an internal reliability target, such as 99.95% of checks succeeding over 30 days, usually set stricter than any SLA so problems show up before a promise is broken.
In more detail
An SLO is built on an SLI, a service level indicator: something measurable, such as the share of successful checks or the share of requests answered within 500 ms. The SLO is the target for that indicator over a rolling period.
The gap between 100% and the SLO is the error budget: how much failure is acceptable. While budget is left, a team can ship changes; when it runs out, reliability work comes first.
Worked example
An SLO of 99.95% availability over 30 days leaves an error budget of 0.05% of 43,200 minutes, which is 21.6 minutes. After a 15-minute outage, 6.6 minutes of budget remain for the rest of the period.
See also
Related terms: SLA (service level agreement) and Uptime. Free tool: Uptime calculator. How OwlHound helps: Website uptime monitoring. All terms: monitoring glossary.
Frequently asked questions
What is an error budget?
The amount of failure an SLO allows: 100% minus the target, applied to the period. A 99.9% monthly SLO has an error budget of about 43 minutes.
What is an SLI?
A service level indicator: the measurement an SLO is set on, such as the percentage of successful checks or of fast responses.
Does a small business need SLOs?
A simple one helps: decide what uptime you consider acceptable, then let monitoring tell you when you are drifting below it.
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