Selenium Grid 4 lets WebDriver clients run tests against browser instances on remote machines, including different browser and operating-system combinations. For a first setup, run Grid in Standalone mode and connect a RemoteWebDriver to http://localhost:4444. Grid can distribute work, but parallel execution depends on your tests being independent and on having available browser slots and enough machine resources.
What Selenium Grid does—and what it does not do
Grid accepts WebDriver requests and routes them to browser sessions that match the requested capabilities. That makes it useful for running tests remotely, covering multiple browsers or platforms, and distributing independent tests across available machines. The Selenium Project describes these uses in its Grid overview.
Grid is not a test runner or a source of automatic speedup. Your test framework must schedule independent tests concurrently, and Grid needs matching, available browser slots to serve those requests. Actual throughput also depends on browser startup time, test behavior, network conditions and host capacity.
Choose a Grid deployment mode
| Mode | How it works | When it fits | Trade-offs |
|---|---|---|---|
| Standalone | Grid components run together in one process on one machine. | Local learning, development, or a small CI job using one host. | Simple to start; browser diversity and capacity are limited to what that host can run. |
| Hub and Node | A Hub provides a shared entry point; one or more Nodes host browser sessions. | A shared Grid across machines, especially when Nodes provide different browsers or operating systems. | Requires network connectivity between the Hub and Nodes and attention to reachable ports and host security. |
| Fully distributed | Grid components are started and deployed separately rather than bundled as a Hub or Standalone process. | Deployments that need individual component placement or operational control. | More configuration and operational work; use the role-specific setup in the official guide. |
These modes are alternatives, not a ranking. Choose according to your browser and OS matrix, expected concurrent sessions, number of machines, available resources, and how you will monitor and secure the service. The Selenium Project describes the roles in Grid components.
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Prerequisites for the quick start
- Java 11 or higher. This is the minimum listed in the current Selenium getting-started documentation; verify the version requirement for the Selenium Server release you download.
- The browser or browsers you intend to test. Install them on the machine that will host the sessions.
- Browser drivers. Configure drivers for your browsers, or use Selenium Manager as described by Selenium’s getting-started guide.
- The Selenium Server JAR. Download a current release from Selenium’s official downloads and use its actual filename in the command below. The filename is versioned; no particular release number is assumed here.
Start a local Standalone Grid
- Open a terminal in the directory containing the Selenium Server JAR.
- Start the server, replacing
<version>with the version in your downloaded filename:java -jar selenium-server-<version>.jar standalone - Leave that process running. Open
http://localhost:4444to view the Grid UI, or requesthttp://localhost:4444/statusto inspect status. - Configure a client to use
http://localhost:4444as its RemoteWebDriver endpoint.
These are the default local Standalone instructions in Selenium’s getting-started guide. Release-specific commands and defaults can change, so consult that guide for the exact server version you use.
Connect a Java test with RemoteWebDriver
Add the Selenium Java client dependency appropriate for your build and Selenium Server release. The example below creates a remote Chrome session, names it in the Grid UI, opens a page, and quits the session even if navigation fails. The Grid host must have a Chrome slot available, with a usable browser and driver configuration.
import java.net.URL;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeOptions;
import org.openqa.selenium.remote.RemoteWebDriver;
public class GridSmokeTest {
public static void main(String[] args) throws Exception {
ChromeOptions options = new ChromeOptions();
options.setCapability("se:name", "Grid smoke test");
WebDriver driver = new RemoteWebDriver(
new URL("http://localhost:4444"), options);
try {
driver.get("https://example.com");
System.out.println(driver.getTitle());
} finally {
driver.quit();
}
}
}se:name is session metadata that can make a session easier to identify in the Grid UI. Keep the requested capabilities aligned with what the Grid’s browser slots provide. For example, request Firefox options for Firefox rather than Chrome options; add platform requirements only when your Nodes actually provide the requested platform.
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Run tests concurrently across browsers
Make tests safe to run at the same time
Parallelize independent test cases, not steps that share a browser session or mutable test data. Give concurrent cases isolated accounts, records, files, or other state where needed. A test suite that serializes access to shared data may become flaky when moved to Grid even if browser sessions are separate.
Request the browser and platform each test needs
Create a separate remote session with the appropriate browser options for each test or worker. Grid’s Distributor matches a new-session request to an available slot based on its requested capabilities. If no slot matches, the session cannot be assigned; ensure the requested browser and platform are registered on a Node and that capacity is free.
Let the test runner control parallelism
Configure your test framework or CI job to run multiple independent tests at once. Grid supplies remote sessions; it does not decide how many test cases your framework schedules. Start with a conservative worker count, then increase it only while session creation succeeds and measured run time and resource use remain acceptable.
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Scale to Hub and Node
Use Hub and Node when one machine is insufficient or when browser and OS coverage must span machines. Start the Hub on the machine that clients will reach, then start a Node on each browser host, following the commands for your Selenium release in the official guide.
The getting-started documentation lists Event Bus ports 4442 and 4443 by default and Node port 5555 by default. These are defaults, not universal values: check your release and configuration. Make the required ports reachable between the Hub and Nodes, and the Grid endpoint reachable from authorized test clients. Use the guide’s fully distributed instructions if you deploy individual roles separately; do not assume Hub-and-Node port instructions cover every distributed role.
How Grid 4 routes sessions
Grid 4 separates responsibilities across the Router, New Session Queue, Distributor, Node, Session Map, and Event Bus. The Router accepts client requests, directing new sessions through the queue and Distributor or routing commands for an existing session to the Node hosting it. The Distributor selects an available slot whose capabilities match the request; the Node runs the browser session. The Session Map tracks sessions, while the Event Bus carries internal events between Grid components. The Selenium Project explains the roles in its components guide and architecture overview.
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Estimate capacity by measuring your own workload
Selenium’s getting-started guide offers about 1 GB of RAM per browser session as a planning reference, not a guarantee. Node defaults relate concurrent sessions to CPU count, and Safari is limited to one session in the documented default guidance. Actual safe concurrency depends on browser, page weight, test activity, host resources, and configuration; the components guide also cautions that these are configurable behaviors.
- Begin with a small number of concurrent sessions on representative tests.
- Record session throughput, run duration, failed session starts, CPU and memory use, and browser stability.
- Increase concurrency in measured increments, watching for resource pressure or a rise in failures.
- Adjust Node configuration or add machines only after identifying the limiting resource or browser slot.
Do not plan around a linear speedup. The Selenium applicability page’s distribution equations are illustrative and assume work can be divided across available Nodes; real test suites have setup, dependencies, contention, and other overhead.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Secure the Grid endpoint
Do not expose an unauthenticated Grid to the public internet. Selenium warns that an exposed Grid can provide access to infrastructure, internal applications and files, and may allow third parties to run binaries. Restrict access with private networking, firewall rules, or other controls appropriate to your environment. Expose the endpoint only to authorized clients and Nodes; do not treat the default port as a security boundary. See the warning in the getting-started guide.
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Troubleshooting common setup failures
| Symptom | Likely cause | What to check |
|---|---|---|
| Server does not start | Java is missing or too old, the JAR path or filename is wrong, or the command does not match the release. | Confirm Java 11 or higher, check the JAR filename, and follow the current release’s getting-started instructions. |
Client cannot connect to localhost:4444 |
Grid is not running, the client is on another machine, or the endpoint/port is not reachable. | Check the server terminal, open the Grid UI or /status from the client host, and use the Hub address rather than localhost for remote clients. |
| New session request fails or waits | No free slot matches the requested browser or platform, or the Node is unavailable. | Inspect registered Nodes and available slots in the UI; align requested capabilities with installed browsers and configured slots. |
| Hub does not see a Node | Node cannot reach the Hub or required Event Bus/Node ports are blocked or configured differently. | Check hostnames, firewall rules and the release’s port settings; verify Event Bus ports 4442/4443 and Node port 5555 only if using their defaults. |
| Browser fails to launch on a Node | Browser or driver is missing, incompatible, or not configured for that environment. | Install the target browser and configure its driver, or verify Selenium Manager behavior for your setup. |
| More parallel workers make tests slower or flaky | The host may be resource constrained, or tests may contend for shared data or services. | Reduce worker count, isolate test data, and measure resource use and failures before increasing capacity again. |
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Frequently Asked Questions
Can Selenium Grid run tests on different operating systems?
Yes, when Nodes running those operating systems are registered and provide slots matching the session capabilities your tests request.
Does Grid require a specific test framework?
No framework is specified by Grid itself; the client needs to use a Selenium WebDriver binding and the framework must schedule any desired parallel test execution.
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