Error budget

The amount of unreliability an SLO allows over a period, used to decide when a team ships new features and when it stops to fix reliability.

If the SLO is 99.9% of requests succeeding in a month, the other 0.1% is the budget. While budget remains, the team can take risks: ship, experiment, migrate. When it is spent, new releases pause and the work goes to reliability until the budget recovers. It turns the argument between product and operations into a number both agreed on in advance.

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Related terms

SLO and SLI: An SLI is a measured indicator of service quality; an SLO is the internal target for it, stricter than any SLA you sign.
SLA (service level agreement): A contract that defines availability, response and fix times per priority, and what happens when they are missed.
Canary release: Sending a new version to a small share of traffic first, watching error rates and latency, and widening the rollout only if it holds.
Feature flags: Switches in code that turn a feature on or off for some or all users without a new deploy.

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