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For a local or small CI suite, start Selenium Grid in Standalone mode and point your WebDriver client at http://localhost:4444. When you need browsers on multiple machines or operating systems, use Hub/Node; split Grid into separate components only when you need that additional operational control. Grid routes WebDriver commands to remote browser instances, making parallel and cross-platform testing possible.
Choose a Grid deployment that fits your tests
| Mode | Use it when | What to expect |
|---|---|---|
| Standalone | You want to run tests on one machine, such as for local debugging or a small CI job. | All Grid components run in one process. The client endpoint is typically http://localhost:4444. |
| Hub/Node | You need browser or operating-system combinations on multiple machines behind one client entry point. | The Hub accepts client traffic; Nodes register browser slots and execute sessions. You can add or remove Nodes as capacity needs change. |
| Fully distributed | You need Grid roles deployed and operated separately. | Grid components run independently, so addresses, ports, routing, and operations require more coordination. |
Selenium’s deployment guidance describes small grids as Standalone or Hub/Node with up to five Nodes, middle-sized grids as Hub/Node with 6–60 Nodes, and large grids as Hub/Node with 60–100 Nodes or distributed above 100. These are rough estimates, not fixed scaling limits. Choose based on the number of machines and browser/OS combinations, your parallel-session target and available resources, isolation needs, and which network zones can reach Grid. See Selenium’s getting-started guidance.
Prepare the server and browsers
- Install Java 11 or higher.
- Install the browser or browsers you intend to test.
- Obtain the Selenium Server JAR for the release your project uses. The release number is not fixed here; use your chosen, pinned version rather than assuming a particular release is current.
- Make browser drivers available. Selenium Manager can configure drivers automatically when enabled with
--selenium-manager true; otherwise install the required drivers and put them onPATH.
These prerequisites and setup options are described in the Selenium Grid quick start.
Start a local Standalone Grid
- Open a terminal on the machine that will run the tests.
- Start the server, replacing
<version>with the JAR version you obtained:java -jar selenium-server-<version>.jar standalone - Set your test client’s RemoteWebDriver URL to
http://localhost:4444. - Open
http://localhost:4444for the Grid UI, or checkhttp://localhost:4444/statusto inspect Grid status.
Standalone is the shortest way to get a Grid running because one process hosts the components on one machine. It is a practical first step for local debugging and small CI jobs; it does not provide multiple machines’ browser and operating-system coverage.
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Point a WebDriver test at Grid
The client must use RemoteWebDriver and the URL of the Grid entry point. This Java example uses Selenium’s Java client and requests a Chrome session; ensure the matching browser is installed on the machine running Grid.
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeOptions;
import org.openqa.selenium.remote.RemoteWebDriver;
import java.net.URL;
public class GridSmokeTest {
public static void main(String[] args) throws Exception {
URL gridUrl = new URL("http://localhost:4444");
ChromeOptions options = new ChromeOptions();
WebDriver driver = new RemoteWebDriver(gridUrl, options);
try {
driver.get("https://example.com");
System.out.println(driver.getTitle());
} finally {
driver.quit();
}
}
}Replace the URL with the Hub address in Hub/Node mode or the Router address in distributed mode. The default port is 4444. Always end a test session with quit() so its browser slot is released for another test.
Expand to a Hub and Nodes
Start the Hub
On the machine that will serve as the central entry point, run:
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java -jar selenium-server-<version>.jar hubClients send their RemoteWebDriver requests to the Hub, normally at http://<hub-ip>:4444.
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On the same host as the Hub, a Node can register with:
java -jar selenium-server-<version>.jar nodeOn a different machine, specify the reachable Hub address:
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java -jar selenium-server-<version>.jar node --hub http://<hub-ip>:4444Nodes advertise browser slots and execute sessions. Add Nodes on machines with the operating systems or browser versions your suite needs. If you run several Nodes on one machine, give them distinct ports, for example 5555 and 6666, and verify the installed release’s configuration help for the applicable port options.
Allow the required internal connections
When Hub and Node run on separate machines, allow the Hub event bus ports 4442 and 4443, as well as the Node’s port, through the relevant internal network controls. If the Hub uses non-default event bus ports, configure matching publish and subscribe event addresses on the Node. Use reachable hostnames and addresses for every component.
Do not expose Grid endpoints to the public internet. Selenium warns that an exposed Grid could provide access to internal web applications and files or allow third parties to run custom binaries. Restrict access to trusted test infrastructure and administrators. See Selenium’s Grid security and setup guidance.
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When to separate Grid components
Fully distributed mode separates the roles that a Hub/Node deployment keeps more closely grouped:
- Event Bus: carries internal component messages.
- Session Queue: holds new session requests.
- Distributor: matches queued requests to Nodes.
- Session Map: tracks session IDs and their Nodes.
- Router: accepts client traffic and routes requests.
- Node: provides browser slots and runs sessions.
Selenium’s example defaults are Event Bus 4442/4443/5557, Session Queue 5559, Session Map 5556, Distributor 5553, Router 4444, and Node 5555. These are configuration examples, not guaranteed to fit your network. The client connects to the Router. Check the installed server’s component --help and global --config-help output before deploying. The Selenium distributed-mode guide explains the component layout.
Configure capacity and concurrency
Grid capacity is limited by the browser slots on registered Nodes and the resources available to run those browsers. Selenium’s guide gives these as starting references, not universal capacity guarantees:
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- For Chromium-based browsers and Firefox, the default Node capacity is one concurrent session per CPU.
- For Safari, the default is one concurrent session.
- Around 1 GB of RAM per browser session is an approximate sizing reference.
- Distributor session-creation concurrency depends on available processors.
Real throughput varies with browser, test workload, memory pressure, and host configuration. Measure your own tests continuously before setting a dependable parallel-session target. Selenium recommends smaller Nodes for process isolation and notes that Docker can help achieve that approach. See Selenium’s sizing guidance.
Configuration can be passed with CLI flags or stored in a TOML file. Selenium recommends TOML for readability and source control; flags can be combined with it. Node session caps and driver implementations are among the configurable items. Docker-backed sessions can be configured on Standalone or a Node, with an appropriate image-to-capability mapping and Docker daemon connectivity. Because available flags can vary by release, confirm them using the installed server’s --help and --config-help commands. See CLI options and TOML configuration options.
Verify Grid and diagnose session failures
Use the Grid UI at http://localhost:4444 or request GET http://localhost:4444/status on the relevant Grid endpoint. Status reports registered Node availability, sessions, and slots. The appropriate client URL is the Standalone server in Standalone mode, the Hub in Hub/Node mode, and the Router in distributed mode. See Selenium Grid endpoints.
| Symptom | What to check | Next step |
|---|---|---|
| Client cannot reach Grid | Wrong endpoint, port, hostname, firewall rule, or inaccessible network route. | Use the mode’s actual entry point and verify that the client can reach it. In distributed mode, use the Router; in Hub/Node mode, use the Hub. |
| No Node appears or it is unavailable | Node registration address, Hub event bus ports, and Node port. | Confirm the Hub address is reachable; allow ports 4442 and 4443 and the Node port between separate machines. Match event bus publish/subscribe addresses if using non-default ports. |
| Requested session does not start | Browser and driver availability, requested capabilities, registered slots, and Node health. | Install or enable driver management for the browser; align the requested browser capabilities with a Node’s slots; inspect status for availability. |
| Options or flags are rejected | Configuration may differ in the selected Selenium Server release. | Check the installed component’s --help and server --config-help; compare with the matching CLI or TOML documentation. |
| Tests slow down as parallelism rises | CPU saturation, RAM pressure, browser startup costs, or test workload. | Measure the workload, reduce concurrent sessions or add capacity, and reassess Node size rather than assuming the documented defaults guarantee throughput. |
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