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Selenium Grid 4 lets a WebDriver test run in a browser hosted locally or on another machine, and it can run multiple sessions at once when the Grid has matching browser slots and enough capacity. For a first setup, start Grid in Standalone mode, connect a RemoteWebDriver to http://localhost:4444, then add browser capabilities and parallel test execution. Use Hub-and-Node or a fully distributed Grid when you need multiple machines or separately managed Grid components.
What Selenium Grid does—and what it does not do
Grid receives WebDriver commands and routes them to remote browser instances. It is useful when you need cross-browser or cross-platform coverage, or when you want independent tests to run concurrently on available browser sessions. The Grid overview and setup guide explain its capabilities at Selenium Grid documentation and Getting started.
Grid alone does not make a test suite parallel. Your test runner must schedule independent tests concurrently, and the Grid must have available slots that match their requested browser and platform capabilities. Browser startup time, application performance, shared test data, and machine resources can also limit speedup. Do not assume adding Nodes reduces elapsed time in a straight line.
Choose a Grid deployment mode
| Mode | When it fits | Machines and browser coverage | Operational considerations |
|---|---|---|---|
| Standalone | Local learning, development, or a small CI job. | Grid components and browser sessions run on one machine; coverage is limited to the browsers and drivers available there. | Fastest way to start. One endpoint, with no separate Hub/Node network configuration. |
| Hub and Node | A shared Grid for teams that need sessions distributed across machines. | A Hub provides the common entry point; Nodes can run on different machines and host different browsers or operating systems. | Configure and secure connectivity between Hub and Nodes, including the Event Bus ports and Node port. |
| Fully distributed | Deployments that need Grid components started and managed separately. | Components can be placed on separate machines or infrastructure according to deployment requirements. | More network, configuration, monitoring, and operations work. Follow the official component and architecture guidance for the release in use. |
Grid 4’s architecture includes a Router, New Session Queue, Distributor, Nodes, Session Map, and Event Bus. The Distributor matches requested capabilities to an available slot; the Router sends new-session requests through the queue and routes commands for established sessions to their hosting Node. See Grid components and Grid architecture.
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Prerequisites and download
- Java 11 or higher. This is the requirement in Selenium’s current getting-started documentation; check the guide for the release you install.
- The target browser or browsers. Install them on the machine that will host the browser sessions.
- Browser drivers. Configure compatible drivers, or use Selenium Manager as described in the official guide.
- Selenium Server JAR. Download the current release from the Selenium project, then use that JAR filename in the commands below. The filename includes its version; no particular release number is assumed here.
These requirements and the quick-start commands are documented in Selenium Grid: Getting started. Release requirements and defaults can change, so verify them against that guide when setting up a new environment.
Start a local Grid in Standalone mode
- Open a terminal in the directory containing the Selenium Server JAR.
- Start the single-process Grid, replacing the placeholder with the downloaded JAR’s actual filename:
java -jar selenium-server-<version>.jar standalone - Open http://localhost:4444 to view the Grid UI. You can also query the status endpoint:
curl http://localhost:4444/status - Leave this process running while your test client connects to the same machine’s Grid endpoint.
Standalone is a useful first step because it combines the Grid roles in one process. It does not provide browser or operating-system diversity that is not present on that machine.
Connect a RemoteWebDriver client
For example, the following Java test requests Chrome and connects to the default local Grid endpoint. Ensure the Java project includes the Selenium Java client dependency compatible with the Grid release, and that Chrome is installed and configured on the Grid host.
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 {
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();
}
}
}For a remote Grid, replace localhost with the address reachable from the test client. The se:name capability adds session metadata that can help identify the test in the Grid UI. Always call quit() so the session and its slot are released, including when a test fails.
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Request a browser and platform the Grid can actually provide
Capabilities express what the client needs. For example, use a browser-specific options class such as ChromeOptions or FirefoxOptions; set a platform capability when the test requires a particular operating system. The Distributor can start the session only if a registered Node has a compatible, available slot. A request for an unavailable browser or platform will remain unmatched or fail rather than creating that environment automatically.
Run independent tests concurrently
Parallelism is configured in the test framework or build system, not by changing the Grid URL. Set the runner’s worker or thread count to a value that the Grid can serve, and make sure each worker creates and closes its own WebDriver session. The exact configuration is framework-specific; consult the documentation for the runner used by your project.
- Parallelize tests that do not depend on shared mutable state or execution order.
- Give parallel tests isolated accounts, records, or other test data where necessary.
- Do not share one WebDriver instance across concurrent tests.
- Begin with a conservative worker count, then increase it while observing session creation, suite duration, CPU, memory, and failures.
Expand to Hub and Node
Use a Hub-and-Node setup when one machine is not enough or when browser and operating-system coverage must span different hosts. The getting-started guide uses hub and node subcommands; check its current command syntax and options for the release you are deploying.
- On the Hub host, start the Hub using the Selenium Server JAR and the
hubsubcommand. - On each browser host, start a Node using the same JAR and the
nodesubcommand, and configure it to register with the Hub. - Permit only the required network traffic between the client, Hub, and Nodes. The default Event Bus ports are
4442and4443, and the default Node port is5555; verify these defaults and any configured changes in the guide for your release. - Connect the client to the Hub’s Grid endpoint, then request capabilities matching browsers and platforms registered by its Nodes.
- Confirm registration and slot availability in the Grid UI or status endpoint before raising test-runner concurrency.
For separately deployed Router, Distributor, queue, Session Map, Event Bus, and Node roles, use the official component documentation and architecture documentation. Fully distributed mode provides deployment flexibility but requires you to arrange component addresses, ports, lifecycle, and monitoring; use the release-specific role defaults rather than assuming every deployment has the same network layout.
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Estimate capacity without treating guidance as a guarantee
Selenium’s getting-started guide offers about 1 GB of RAM per browser session as a planning reference. Its Node guidance relates default concurrent-session capacity to CPU count, with Safari limited to one session. These are starting points, not guarantees: actual safe concurrency depends on browser workload, operating system, application, memory pressure, and configuration.
Measure in the environment that will run the tests. Track how many sessions start successfully, elapsed suite time, browser and Grid errors, CPU and memory use, and whether the application under test becomes the bottleneck. Increase concurrency gradually; reduce it if sessions fail to launch, machines swap or exhaust memory, or the application becomes unstable. The equations in Selenium’s applicability guidance are illustrative and assume work can be divided across available Nodes, so do not use them as a promise of linear speedup.
Secure the Grid endpoint
Do not expose an unrestricted Grid to the public internet. Selenium warns that an exposed Grid can provide access to infrastructure, internal applications, and files, and can allow third parties to run binaries. Keep Grid endpoints on a trusted network, restrict inbound access with firewall rules or network controls, and expose only the ports required for your topology. Apply the same care to CI agents and cloud hosts as to developer machines.
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The browser session cannot start
Check that the requested browser is installed on the machine running the Node or Standalone process and that its driver is available and compatible. Review driver configuration or use Selenium Manager as supported by the release. A browser installed only on the client machine is not available to a remote Node.
The client cannot connect to Grid
Verify the Grid process is running, the URL and port are correct, and the client can reach the host. For Hub-and-Node or distributed setups, check network routes and firewall rules for the configured ports, including the default Event Bus ports 4442/4443 and Node port 5555 where those defaults apply.
A requested session is not created
Compare the client’s browser and platform capabilities with registered Node slots. An unavailable browser, platform, or occupied slot cannot satisfy the request. Check the Grid UI or /status, then add a matching Node, free a slot, or reduce the number of simultaneous tests.
Tests pass alone but fail in parallel
Look for shared accounts, fixed filenames, reused records, order dependencies, and a WebDriver instance shared between workers. Isolate test data and use a separate session per worker. If failures coincide with resource saturation, lower concurrency and measure again.
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Sessions remain after a test ends
Make sure every test calls driver.quit() in a cleanup path, including after assertion or navigation failures. An unreleased session continues to occupy a Grid slot until the session is terminated or otherwise expires under the configured setup.
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Frequently Asked Questions
Does Selenium Grid execute tests in parallel by itself?
No. The test runner must schedule concurrent tests, and Grid must have matching available slots.
Quick Recap
What is the default Selenium Grid URL for Standalone mode?
The documented default is http://localhost:4444.
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