What is an error budget, and how should it be used?

Summary

The error budget is the amount of unreliability an SLO allows: with a 99.9% availability SLO over 30 days, 0.1% of requests (or about 43 minutes of full downtime) may fail. While budget remains, teams can ship and take risks; when it is spent, the agreed policy shifts effort to reliability.

Longer Answer

The budget turns the reliability-versus-velocity argument into data: both product and engineering agree on the SLO, so spending the budget on launches is legitimate, and exhausting it triggers a pre-agreed response rather than a negotiation.

An error budget policy typically says: if the budget is exhausted over the window, freeze non-critical releases (except fixes), prioritise postmortem action items, and require extra review for risky changes until the service is back within SLO. It also says what happens when a single incident consumes a large share (for example 20%): mandatory postmortem. A budget that is never spent can mean the SLO is too loose or the team is too cautious.

Key Points

• Budget = 1 - SLO.
• Spend it on velocity; protect it with a policy.
• Policy agreed in advance.
• Unspent budget is a signal too.

What interviewers look for

The candidate explains the policy and the cultural point: the budget aligns product and engineering.

Follow-up questions

• What would you do if product wants to ship during a budget freeze?