SLI: pick a user-impact signal
Latency p99 of the critical user request. Availability of the homepage. Error rate on checkout. Not: 'CPU utilization', 'queue depth', 'cache hit rate' — those are causes, not user impact.
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SLO: pick a number you can defend
99.9% availability = 43 min/month downtime. 99.95% = 21 min. The number should match user expectation and competitive landscape, not be aspirational. Higher SLO = exponentially more engineering cost.
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Error budget = (1 - SLO) × time
99.9% SLO = 0.1% × 30 days = 43 minutes/month error budget. Spend it on planned maintenance, risky deploys, novel features. Budget exhausted = freeze feature work and fix reliability. The mechanism only works if leadership respects the freeze.