Glossary · Reliability metrics
Service level objective
A service level objective (SLO) is an internal target for a service’s reliability, such as 99.95% of requests succeeding over 30 days, measured with a service level indicator (SLI).
An SLO turns “reliable enough” into a number the team can track. It is set from what users need, not from what the system happens to do today, and is usually stricter than any SLA, so there is room to react before customers are owed credits.
The gap between the SLO and 100% is the error budget. While budget is left, the team can ship changes and take risks; when it runs out, work shifts to reliability until the window recovers. That makes reliability a shared decision between product and engineering.
Good SLOs are few, tied to what users notice, such as the availability of checkout or the latency of the API, and measured over a rolling window, often 28 or 30 days.
Error budget = (100% − SLO) × window
Example: A 99.9% SLO over 30 days leaves an error budget of 43 minutes and 12 seconds; 99.95% leaves 21 minutes and 36 seconds.
Uptimecheck measures two common SLIs from outside your network, availability and response time, as often as every minute. Reports compare each monitor’s uptime with a target from 99% to 99.99% over a calendar month or the last 7, 30 or 90 days, and response time charts show the median and the 95th percentile.
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Guides 7 min read
An SLI measures how a service behaves, an SLO is the target for that measurement and an SLA is the promise to customers. Definitions, downtime tables and how to set each one.
ReadThe downtime or failed requests an SLO allows: 100% minus the target, over the window. At 99.9% over 30 days it is 43 minutes and 12 seconds.
A few: one per thing users notice, usually availability and latency of the main journeys. More SLOs spread attention thin.
No. 100% leaves no error budget, so every change becomes a risk nobody may take, and users can’t tell 99.99% from 100% through their own networks.
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