On APM Blind Spots

“We only test the backend, frontend is static anyway”
“We only look at the APM, there’s not much difference between the end user and APM”
It’s time to do some debunking!

But to prove these statements wrong, we need the tools! So I’ve instrumented JMeter’s Selenium plugin with Core Web Vitals and OpenTelemetry (yes, the browser). I also instrumented my Django Application with OpenTelemetry. No react, so the page is fairly static. Just a bit of Bootstrap 5 to prettify the presentation layer.
Results? I have all the measurements on a single page. All of that to show you the importance of validation of frontend during your load tests.

10 milliseconds on the application side vs 48 ms on the browser side vs 383ms on the rendering side. If you measure your tests from server side, you’ll only see 10ms. If you do a backend load test, you’ll see 48ms. If you measure the same from the browser – that’s almost 400ms.

What’s the next steps? I’m also measuring the web vitals so it’s time to validate how they change during an actual load test.