A 99.9% SLO gives you 43.2 minutes a month. Then features stop.

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Your SLO is 99 point 9 percent. Sounds strict. It means you can be broken for 43 point 2 minutes a month.

30 days is 43,200 minutes. 99 point 9 percent leaves 0 point 1 percent to be bad. 43 point 2 minutes. That's your error budget.

The month starts. 43 point 2 minutes in the bank. Day 5: a 10 minute outage. 33 point 2 left. Day 12: a slow deploy. 15 minutes degraded. 18 point 2 left. Day 20: a third-party outage, 20 minutes. Minus 1 point 8.

Budget exceeded. And the action isn't a shrug. It's a feature freeze. Until the budget recovers.

A written policy decides. Over half the budget left: ship freely. Under half: warning. No risky deployments. Under 20 percent: critical. Feature work pauses. Exhausted: a director approves every deploy.

When you're reliable, you ship fast. When you're not, the focus shifts to reliability. Automatically.

Full breakdown in the lecture.