Breadcrumbs

Load Testing - Selenium (better than hp loadrunner..)

Description

Selenium offers a compelling alternative to traditional load testing tools for teams looking to get a more realistic picture of how their web application performs under load.

The key difference: Selenium simulates the browser, while a tool like HP LoadRunner simulates traffic by sending raw HTTP requests directly to the server. That distinction matters more than it sounds.

Why the distinction matters

HP LoadRunner (and most classic load-testing tools) work at the protocol level: they record the HTTP calls a browser would make, then replay thousands of copies of those calls concurrently. This is efficient and can generate huge request volumes cheaply, but it skips everything that happens inside the browser:

  • JavaScript execution and rendering time

  • DOM manipulation and reflows

  • Client-side frameworks (React, Angular, etc.) doing their own work

  • Real browser network behavior (connection reuse, caching, async asset loading)

  • CSS/asset loading order and its effect on perceived load time

For a modern, JavaScript-heavy web app, a lot of the actual user-perceived latency lives in that browser-side work — not just in the server response. A protocol-level tool can tell you your server responded in 200ms and still miss that the page wasn't usable for another 3 seconds.

Selenium drives real, actual browsers (Chrome, Firefox, etc.), so it captures that full end-to-end experience: server response and client-side rendering. That gives a more realistic picture of what users actually experience under load.

Getting started

Typical setup at a high level:

  1. Write (or record) a Selenium script that walks through a realistic user journey in your app.

  2. Run multiple instances of that script in parallel — via Selenium Grid, a cloud browser provider, or containerized browsers — to simulate concurrent users.

  3. Capture timing metrics from the browser side (page load, time to interactive, specific element render times) alongside your usual server-side metrics.

  4. Compare results against your target concurrency and acceptable latency thresholds.

Injecting GermainUX-RUM JS monitoring into the load test

Because Selenium drives a real browser instance rather than firing raw HTTP requests, you can install any browser extension before a test run — which means you can test your application both with and without an extension-delivered app in place, and compare the results.

One example our clients use: they install the GermainUX-RUM Extension, which in turn enables Real-time User Experience Monitoring (GermainUX-RUM JS) on the load-tested app — exactly the same way it works for end users who run the app in their own browsers and are monitored by our platform. This injection serves two purposes: it lets you load test GermainUX-RUM JS itself under realistic concurrency, and it lets you monitor your automated test runs so the captured sessions are available for later review.

Selenium supports loading a browser extension at session startup: