Foundations
Client-Side Telemetry: The Missing Piece of your Uptime Strategy
Learn how to capture errors that happen inside your users' browsers. Discover how Watch.dog integrates with client-side telemetry to detect broken frontends even when your server is fine.
By Watch Dog TeamPublished March 20, 202510 min read
The Blank Page Paradox
Symptom Log
client_error.js
window.onerror = function(msg, url, line) {
// Error: Uncaught ReferenceError: Stripe is not defined
// Impact: 100% of checkouts failing on the client.
sendToWatchDogTelemetry({ msg, url, line });
};A server-side heartbeat only tells you if the server is alive. It doesn't tell you if a JavaScript error is breaking the 'Buy' button or if a CSS file failed to load because of a CDN mismatch.
Client-Side Telemetry (RUM) captures real events from real users, providing the 'Ground Truth' that synthetic monitors might miss.
The Hybrid Approach
Combine Watch.dog Synthetic Pings (to detect infra issues) with Client-Side Telemetry (to detect deployment bugs). If your server is 'UP' but telemetry shows a spike in JS errors, Watch.dog will notify you instantly.
Fix Verification
telemetry_digest.log
[INFO] Watch.dog: Server Health: 100%.
[ALERT] Client-Side JS Errors spike detected in Chrome/Windows.
[REASON] ChunkLoadError: Loading chunk 7 failed.
[STATUS] Deployment rollback recommended.Synthetics vs. Reality
Synthetic pings are perfect for baseline reliability. Telemetry is perfect for edge cases (specific browsers, devices, or regions). Watch.dog gives you both in a single, unified view.
Visibility Comparison
| Failure Type | Synthetic Monitor | Client Telemetry |
|---|---|---|
| Server Down | Dectected (Instant) | Delayed / Silent |
| CSS/Asset Failure | Not Detected | Detected (High Signal) |
| JS Logical Bug | Limited | Detected (Granular) |
| Database Timeout | Detected | Detected (Indirectly) |
Your users are your best monitors. Start listening to their browsers.