Glossary · Monitoring

SLO (Service Level Objective)

A service level objective is an internal reliability target, such as 99.95% of requests succeeding over 30 days, that guides priorities and alerting.

Updated October 5, 2026

A service level objective (SLO) is a target value for a service level indicator (SLI), the metric you measure, over a defined period. The SLI might be the fraction of successful requests or the fraction served under 500 ms; the SLO says what level you aim to hit.

Why it matters

An SLO turns "be reliable" into a number the team can act on. Subtracting it from 100% gives the error budget: the amount of failure you can tolerate. While budget remains you can ship changes; when it is spent, you focus on stability. The SLO is normally stricter than the SLA you promise customers, so you notice trouble before you owe credits.

Example

SLI: successful checks / total checks (HTTP 200, body contains "Add to cart")
SLO: 99.9% over a rolling 30 days
Error budget: 0.1% = about 43 minutes of failed checks per 30 days

If an incident consumed 30 minutes, roughly 70% of the month's budget is gone.

Common mistakes

  • Setting 100%. It is unachievable and leaves no room for releases.
  • Choosing an SLI users do not feel, like server CPU, rather than a request or page outcome.
  • Setting an SLO and never alerting on budget burn, so you learn about it at the end of the month.
  • Using one SLO for everything. Checkout and a marketing page deserve different targets.

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