Selenium Grid uses a hub-node concept. Hub is a machine where you only run tests against it, but the execution will be done by different machines called nodes.
Selenium Grid Hub:
- The hub is the central point where QA Engineers load tests into.
- There should be only one hub in a grid.
- The hub is launched only on a single machine.
- On the machine containing the hub, the tests will run, and you will see the browser information on the node.
Selenium Grid Node:
- Nodes are Selenium instances that will execute the tests loaded on the hub.
- There can be one or more nodes in a grid.
- Nodes can be launched on multiple machines with different browsers and Operating system. Nodes can be the same computer.
- The machines running the nodes need not be the same platform as that of the hub.