What is Selenium Grid? Hub-Node Architecture
What Is Selenium Grid?
Selenium Grid is a tool that lets you run automated tests across multiple machines, browsers, and operating systems simultaneously, dramatically cutting down total test execution time for large test suites.
The Hub-Node Architecture
Selenium Grid works on a central Hub that receives test requests and distributes them to registered Nodes, which are the actual machines running the browsers where tests execute.
- Hub - the central point that receives and routes test requests
- Node - a machine registered with the hub that executes tests on a specific browser/OS combination
- Each node reports its available browsers and capabilities to the hub
How a Test Request Flows
| Step | What Happens |
|---|---|
| 1. Test Starts | A script requests a browser session matching desired capabilities |
| 2. Hub Matches | The Hub finds an available Node matching those capabilities |
| 3. Node Executes | The matched Node runs the test in its browser instance |
| 4. Results Return | Results flow back through the Hub to the test runner |
Why Teams Use Grid
Without Grid, cross-browser testing means running the same suite repeatedly and sequentially. Grid lets teams run those same tests in parallel across different environments, turning hours of execution into minutes.
Understanding the Hub-Node architecture is the first step - next, let's actually set up Selenium Grid for parallel testing.
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