What is a Hub in Selenium Grid?

Hub is the central point (server) in Selenium Grid that:

1. ✓Receives test execution requests from test scripts
2. ✓Manages registrations from Nodes
3. ✓Distributes test commands to the appropriate Node
4. ✓Tracks the status of all connected Nodes

Role of the Hub:

Test Script → [HUB: port 4444] → routes to → [Node with matching capabilities]

Starting the Hub:

# Selenium 3 — standalone jar
java -jar selenium-server-standalone-3.141.59.jar -role hub

# Access console at: http://localhost:4444/grid/console

Selenium 4 Hub:

java -jar selenium-server-4.x.x.jar hub

# Access at: http://localhost:4444

What the Hub does:

1. ✓Listens on port 4444 (default) for incoming test requests
2. ✓Accepts node registrations from machines that want to be test executors
3. ✓Matches capabilities — when a test requests Chrome on Windows, Hub routes to a Node with those capabilities
4. ✓Monitors node health — removes dead nodes automatically
5. ✓Returns results to the test script

Hub configuration example:

{
  "port": 4444,
  "host": "localhost",
  "maxSession": 50,
  "browserTimeout": 60
}

How test connects to Hub:

// Test points to Hub URL, not a local driver
WebDriver driver = new RemoteWebDriver(
    new URL("http://hub-server:4444/wd/hub"),
    new ChromeOptions()  // Hub routes to a Chrome node
);

Hub in context:

[TestNG Suite]
     ↓
[RemoteWebDriver → http://hub:4444/wd/hub]
     ↓
[HUB] ←—— registered ——— [Node 1: Chrome/Windows]
 |                         [Node 2: Firefox/Mac]
 |—routes based on caps——→ [Node 3: Safari/Mac]

Key definition: A Hub is a server or a central point that controls the test executions on different machines. It does not execute tests itself — it delegates to Nodes.