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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallSelenium Grid 4 lets WebDriver clients run browser tests on remote machines and in parallel. For a first setup, run Selenium Server in Standalone mode and point your test client to http://localhost:4444. Add Hub/Node roles when you need multiple machines, operating systems, or browser versions. Before increasing concurrency, check that the Grid has matching browser slots and enough measured CPU and memory.
What Selenium Grid does
Selenium Grid routes WebDriver commands from a client to remote browser instances. A test can request a browser and platform through capabilities; Grid assigns the request to an available matching slot. This is useful when you need to run tests in parallel or cover browser and operating-system combinations that are not available on one machine. See the official Selenium Grid overview.
Grid distributes browser execution; it does not make an incompatible capability or unavailable browser appear. The requested capabilities must match a browser slot registered with the Grid.
Choose a Grid deployment
| Mode | When it fits | What to plan for |
|---|---|---|
| Standalone | A simple setup on one machine, useful for getting started or running a local Grid. | The machine provides the browsers and capacity. It is not a substitute for distributing sessions across multiple machines. |
| Hub/Node | A Grid that combines browser capacity from multiple machines or environments. | The Hub is the single entry point; Nodes register with it and provide browser slots. More components mean more operational coordination. |
| Separate components or dynamic provisioning | More distributed deployments, including container-based or Kubernetes environments. | Configuration and support depend on the deployed Grid version and environment. Validate the options for your version. |
Choose based on the browser and operating-system combinations your tests require, the number of simultaneous sessions, and the resources available to run them. There is no universally best deployment mode.
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Prerequisites
The Selenium quick start lists Java 11 or higher, installed browsers, browser drivers, and the Selenium Server JAR as prerequisites. Selenium Manager can configure drivers when enabled with --selenium-manager true. Check the official getting-started guide for current instructions and release-specific details.
- Install a Java runtime that meets the documented requirement.
- Install the browser or browsers the tests will request.
- Download the Selenium Server JAR for the version you intend to run.
- Either provide compatible browser drivers or enable Selenium Manager as appropriate for your setup.
Start a local Standalone Grid
- Open a terminal in the directory containing the Selenium Server JAR.
- Start Standalone mode:
java -jar selenium-server-<version>.jar standalone
Replace<version>with the version in your downloaded JAR filename. - Configure the WebDriver client to connect to
http://localhost:4444. - Open
http://localhost:4444in a browser to view the Grid UI and check the local Grid.
This is the documented minimal local start, not a guarantee that every browser is ready: the Grid still needs an installed browser and a usable driver configuration. If you want Selenium Manager to configure drivers, use the documented flag: java -jar selenium-server-<version>.jar standalone --selenium-manager true. Refer to the Selenium quick start for the exact current command-line behavior.
Connect a WebDriver client and request parallel sessions
Point remote WebDriver clients at the Grid endpoint rather than launching a local browser. For a Selenium 4 Java client, the essential pattern is:
ChromeOptions options = new ChromeOptions();
WebDriver driver = new RemoteWebDriver(
new URL("http://localhost:4444"), options);
try {
driver.get("https://example.com");
System.out.println(driver.getTitle());
} finally {
driver.quit();
}Import the Selenium Java classes for ChromeOptions, WebDriver, RemoteWebDriver, and URL in your project. Replace the URL with your Grid’s reachable address when the client runs on another machine. Start multiple independent test workers or clients to request simultaneous sessions; Grid can run them concurrently only when it has available matching slots and sufficient resources.
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For cross-browser or platform coverage, request capabilities that correspond to actual registered browsers and platforms. A capability mismatch can leave a request waiting because the Distributor cannot assign it to a matching slot.
Set up Hub and Nodes across machines
- Choose a machine to run the Hub and start it with the Selenium Server JAR in Hub mode:
java -jar selenium-server-<version>.jar hub - On each browser machine, install the intended browser and configure its driver, then start a Node registered to the Hub:
java -jar selenium-server-<version>.jar node --hub http://<hub-host>:4444 - Allow the Hub and Nodes to communicate over the network using the addresses and ports required by your deployment.
- Point WebDriver clients at the Hub endpoint and request capabilities matching the Nodes’ browsers and platforms.
- Check the Grid UI and logs to confirm that Nodes register and expose the slots you expect before running the full test workload.
These commands illustrate the Hub/Node role split described in Selenium’s getting-started guidance. Check the current official guide for version-specific command options. Selenium also documents running Grid components separately for deployments that need a more distributed arrangement.
How Grid assigns sessions
A new session request enters the New Session Queue through the Router. The Distributor tracks available slots and assigns the request to a slot whose capabilities match. A Node runs the WebDriver session; the Session Map records which Node owns the session so later commands can be routed there. The names describe the flow, not separate steps your test code must orchestrate. See the Grid architecture documentation.
When a request waits or cannot start, check both sides of the match: the capabilities requested by the client and the browser/platform slots actually registered. Then check whether those slots are already occupied.
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Plan capacity by measuring your workload
Selenium’s setup guide offers planning guidance, not a performance guarantee: expect around 1 GB of RAM per browser session; a Node’s default maximum concurrent sessions is based on its CPU count; and Safari is limited to one concurrent session per Node. The guide’s page does not state a publication date, and these values are defaults or recommendations rather than independently measured benchmarks. Selenium recommends measuring continuously because the defaults may not suit a particular environment.
- Begin with a small number of concurrent sessions and observe resource use under representative tests.
- Increase concurrency only while the machines remain stable and test outcomes remain reliable.
- Consider smaller Nodes to isolate failures; Selenium identifies Docker as a useful way to achieve this.
- Account for browser mix: the Safari per-Node limit differs from the CPU-based default concurrency guidance.
The Distributor’s ability to create sessions concurrently also relies on its processors, so capacity planning applies to the Grid control components as well as browser Nodes. Avoid treating CPU count or the RAM-per-session figure as a sizing formula without workload measurements.
Containers, Kubernetes, and version-specific behavior
Selenium recommends Docker as one way to run smaller Nodes and isolate failures. Its CLI reference describes Docker and Kubernetes mappings from image names to browser stereotypes; the page cautions that options can change before its documentation is updated.
A Selenium release article dated February 22, 2026, for Grid 4.41.0 describes Dynamic Grid support in Kubernetes: browser Pods can be created for session requests and removed when sessions close. Treat that as functionality described for that release, not a promise about every version or deployment. Verify the configuration and support for the specific version you plan to run in the Selenium 4.41.0 release notes.
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Protect the Grid and operate it safely
Selenium warns that an externally accessible Grid can expose internal web applications and files or allow third parties to run custom binaries. Keep the endpoint restricted to its intended users with network controls appropriate to your environment; do not expose it publicly by default. The Selenium getting-started guide covers this security warning.
When sessions fail, use the logs and observability information from the components involved to trace the failure. Selenium’s observability documentation explains how observability can help operators understand and debug Grid internals. The relevant diagnostics depend on which components you deployed and what your tests were doing.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshoot common setup problems
| Symptom | Likely cause | What to check |
|---|---|---|
| Client cannot connect to the Grid | The endpoint is wrong, the Grid is not running, or the client cannot reach the host over the network. | Confirm the server started, use the correct host and port (the local default in the quick start is http://localhost:4444), and check network access between client and Grid. |
| Session request waits or fails to be assigned | No available slot matches the requested capabilities, or matching slots are occupied. | Compare requested browser/platform capabilities with registered slots in the Grid UI; check Node registration and current session load. |
| Browser does not launch | The browser is missing, or the driver is unavailable or incompatible with the setup. | Install the requested browser and configure its driver, or use Selenium Manager when enabled and supported by your setup. |
| Hub shows no Nodes | Nodes have not started, cannot reach the Hub, or were started with an incorrect Hub address. | Check each Node’s startup output, Hub hostname and port, and network communication between machines. |
| Tests become unstable as concurrency rises | CPU, RAM, browser constraints, or workload contention may be limiting the deployment. | Reduce simultaneous sessions, inspect component and machine diagnostics, then increase capacity based on repeated measurements rather than defaults alone. |
| Grid endpoint is reachable by unintended users | Network access is broader than intended. | Restrict access before continuing operation; treat an exposed Grid as a security risk, not merely a test reliability issue. |
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cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com -o shot.webpSee the ScreenshotNeo API documentation for parameters and response details. It removes cookie/consent banners, newsletter popups, and chat widgets before capture; bot checks, blank pages, failed loads, timeouts, and cache hits are not billed. Its MCP server lets AI agents use screenshot tools. The free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000 shots.
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Frequently Asked Questions
Can I use Selenium Grid on one machine?
Yes. Standalone mode is the documented simple single-machine arrangement; Hub/Node is for combining browser capacity across machines.
Does Selenium Grid run tests in parallel automatically?
Grid can assign simultaneous session requests to available matching slots, but your test runner or clients must submit concurrent work.
Does ScreenshotNeo replace Selenium Grid for browser testing?
No. ScreenshotNeo captures webpages as images or PDFs; Selenium Grid provides remote WebDriver sessions for browser automation and testing.
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