Frontend Monitoring: Real User Monitoring vs Synthetic Testing
Your backend can be 100% healthy while your users stare at a blank screen. JavaScript errors, slow Core Web Vitals, and CDN failures all live in the frontend — and they are invisible to server-side monitoring.
Backend uptime checks miss the browser. Real user monitoring shows you what actual users experience — slow renders, JavaScript errors, and failed resource loads that your API monitors never see.
RUM vs Synthetic: the difference
Real User Monitoring (RUM) — Collects performance data from actual user browsers. Shows real-world experience across devices, browsers, and geographies. Has gaps where users haven't visited yet.
Synthetic monitoring — Scripted browser sessions run from fixed locations at fixed intervals. Deterministic, always-on, but misses the long tail of real-world conditions.
Use both: synthetic for always-on alerting, RUM for post-incident analysis.
Core Web Vitals you must track
LCP (Largest Contentful Paint) — Time until the main page content is visible. Target: <2.5s.
CLS (Cumulative Layout Shift) — Measure of visual stability. Target: <0.1.
FID/INP (Interaction to Next Paint) — Responsiveness to user input. Target: <200ms.
A degradation in any of these is directly correlated to conversion rate drops.
JavaScript error monitoring
Uncaught JavaScript errors are silent from the server side. Instrument your frontend: ```javascript window.addEventListener('error', (e) => { fetch('/api/log-error', { method: 'POST', body: JSON.stringify({ message: e.message, stack: e.error?.stack }) }); }); ```
Alert when JavaScript error rate exceeds 1% of page views.
Synthetic monitoring with AlertsDock
Create an HTTP monitor for your critical frontend pages: - Home page: check for expected content (title, key element) - Login page: verify form renders - Checkout page: verify cart components load
Pair each monitor with a 1-minute check interval and multi-region verification.
CDN and static asset monitoring
CDN failures are frontend outages that backend health checks miss. Monitor your CDN endpoints directly: - Check your JS bundle URL returns 200 - Check your CSS URL returns 200 with correct Content-Type - Alert if response time from your CDN exceeds 500ms
Feature Guide
Uptime Monitoring
AlertsDock gives teams uptime monitoring for websites, APIs, TCP checks, DNS checks, SSL expiry, and fast alert routing without enterprise overhead.
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UptimeRobot Alternative
Compare AlertsDock with UptimeRobot for teams that want uptime monitoring plus heartbeat monitoring, status pages, webhook inspection, and per-resource alert routing.
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