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SLOs and SLIs for APIs: availability, latency, and error budgets with CORS in scope
Include browser-facing error budgets for CORS denials when your product depends on cross-origin calls.
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sresloapi
Choosing SLIs
Measure successful preflight ratio separately from API business success—both matter to end users.
Use histograms for end-to-end latency including TLS and CDN rather than only app server timers.
Error budgets
Spend budget deliberately: freeze features if CORS regressions consume budget faster than availability dips.
Postmortems should cite header diffs and deployment diffs, not only application exceptions.