RUM vs synthetic: alerting vs root-causing

RUM vs synthetic: alerting vs root-causing
Real-user monitoring and synthetic checks answer different questions.
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RUM (real users): true p75/p95 across every device and network. Catches the regression nobody can reproduce.
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Synthetic (scheduled lab runs): identical environment every time, so a +150ms jump is signal, not noise. Great for pre-deploy gates.
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RUM tells you it broke for users; synthetic tells you which commit and which resource.
Run synthetic in CI to block regressions, RUM in prod to size them.
Takeaway: synthetic catches the diff, RUM sizes the p75 blast radius.