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To get Selenium Grid running quickly, start a Standalone server on a machine with Java, a browser, and its driver, then point your test’s RemoteWebDriver at http://localhost:4444. When you need different operating systems, browser versions, or more machines, expand to a Hub/Node setup. Use Distributed mode only when you need to operate Grid components separately.
Selenium Grid routes WebDriver commands to remote browser instances, enabling parallel runs and cross-platform coverage. The commands below follow the Selenium Project’s current Grid guide; its Downloads page reported Selenium Server 4.49.0 on September 9, 2026. Check the official downloads page for the current stable release before installing.
When would you use a Selenium Grid?
Use Grid when a test suite needs to run against browser instances beyond the one available on the machine running the test: for example, multiple browsers or versions, different operating systems, or concurrent sessions. The Selenium Project describes Grid as routing client commands to remote browser instances (Grid overview).
A Grid is not automatically faster or simpler than running tests locally. It adds machines, network paths, session allocation, and operational work. Choose the smallest topology that provides the environments and concurrency your suite actually requires.
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Choose a Grid topology
| Topology | Use it when | What it involves |
|---|---|---|
| Standalone | Development, debugging, or a small CI run on one machine | One process combines the Grid components and browser slots. |
| Hub/Node | You need a shared entry point backed by multiple machines, browsers, versions, or operating systems | A Hub routes requests and allocates sessions to registered Nodes; Nodes run browser sessions. |
| Distributed | You need to deploy or operate Grid components separately | Event Bus, queue, map, Distributor, Router, and Nodes run as separate components and need coordinated network configuration. |
These topologies are self-managed options. Selenium also documents Docker-backed browser sessions and configuration that relays commands to external WebDriver services, including cloud providers or Appium; those are integration paths, not endorsements of any provider’s quality or terms (TOML configuration options).
Start a Standalone Grid
Prerequisites
- Java 11 or higher.
- The browser you intend to test.
- A compatible browser driver, or Selenium Manager configured as described below.
- The Selenium Server JAR from the official downloads page.
Launch the server
- Download the current stable Selenium Server JAR and note its actual filename.
- In a terminal in the directory containing the JAR, run
java -jar selenium-server-<version>.jar standalone, replacing<version>with the version in that filename. For example, do not assume 4.49.0 remains current after the release date noted above. - Leave the process running. Open
http://localhost:4444for the Grid UI, or requesthttp://localhost:4444/statusto inspect status.
The documented quick start lists browser drivers among its prerequisites. Selenium 4.6 and later can use Selenium Manager to obtain a suitable driver; the Grid guide shows the --selenium-manager true option. For a predictable setup, especially in controlled CI environments, install and verify the driver yourself if automatic management cannot reach or resolve the required driver. See Selenium’s driver location guidance and Grid getting-started guide.
Connect a test with RemoteWebDriver
Your test creates a remote session by sending browser options to the Grid URL. The requested browser and platform must match a slot advertised by a Node; the Distributor assigns compatible requests. This Python example uses Selenium’s Python binding and requests Chrome by name. Install the binding with python -m pip install selenium.
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from selenium import webdriver
from selenium.webdriver.chrome.options import Options
options = Options()
options.set_capability("browserName", "chrome")
# Optional: request a specific version or platform if a matching slot exists.
# options.set_capability("browserVersion", "..." )
# options.set_capability("platformName", "..." )
options.set_capability("se:name", "cross-browser-smoke-test")
driver = webdriver.Remote(
command_executor="http://localhost:4444",
options=options,
)
try:
driver.get("https://example.com")
print(driver.title)
finally:
driver.quit()Replace the optional capability values with values actually advertised by your Nodes; requesting an unavailable combination leaves the session without a compatible slot. The se:name metadata is useful for identifying sessions in the Grid UI. Always call quit() so the remote session and its browser resources are released. Capability and session-routing details are documented in the Grid guide and Grid components guide.
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Expand to Hub and Nodes
Use Hub/Node when one machine cannot provide the browser, operating-system, or capacity coverage you need. A Hub is the common endpoint; each Node registers browser slots it can run. Nodes can use different operating systems from the Hub and from one another.
Start the Hub
On the machine that will accept client sessions, run the server JAR in Hub mode:
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java -jar selenium-server-<version>.jar hubUse the Hub’s reachable address—not localhost—in tests running on other machines.
Register a Node
On each browser machine, run a Node and point it to the Hub. For a Hub using its documented default address and ports:
java -jar selenium-server-<version>.jar node --hub http://<hub-host>:4444The Node detects browser drivers on its PATH by default. If Hub and Node are on different machines, the Node must reach the Hub’s Event Bus, and the Hub must be able to reach the Node’s HTTP port. The guide documents Event Bus ports 4442 and 4443 by default; the Node port must also be reachable. Open only the required paths in firewalls. If the Hub uses non-default ports, configure the publish and subscribe Event Bus addresses explicitly using the current CLI options documented by Selenium (Grid CLI options).
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What the Hub and Node do
- Router: entry point for client requests.
- New Session Queue: holds session requests until they can be assigned.
- Distributor: matches requests to compatible Node slots.
- Session Map: associates session IDs with the Nodes running them.
- Event Bus: coordinates internal component communication.
- Node: runs sessions using browser slots available on its machine.
These roles explain common connection failures: a test may reach the Router while the Distributor has no matching slot, or a Node may fail to register because its Event Bus path is blocked. Selenium’s component documentation describes the responsibilities and Node behavior.
When to use Distributed mode
Distributed mode separates the Grid services so they can be started and managed independently. It is appropriate when that separation fits your deployment and operations model, not simply because you have multiple browsers. Start the Event Bus first, then Session Queue, Session Map, Distributor, Router, and Nodes; configure each component with addresses and ports reachable from the components that communicate with it.
Selenium’s getting-started guide lists these documented defaults: Event Bus 4442, 4443, and 5557; Session Map 5556; Distributor 5553; New Session Queue 5559; Router 4444; and Node 5555. They are defaults, not mandatory universal values. The guide’s examples assume local components; for a multi-host deployment, set addresses and ports to match the actual network design. Check the current Grid guide and CLI options rather than copying old command examples unmodified.
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Size the Grid and verify it under load
Selenium’s current guidance offers starting points, not guarantees: expect around 1 GB of RAM per browser session as a planning reference; default Node concurrency is limited by available CPUs; and documented defaults generally provide one slot per CPU for Chromium browsers and Firefox, with one Safari slot by default. Actual memory and throughput depend on browser behavior, pages, test workload, and host resources. Selenium recommends small Nodes for isolation and notes that the defaults may not suit every context (getting-started guide; components guide).
Before increasing slots or adding machines, run the real suite and observe CPU and memory use, session startup time, queue time, and failure rates. A slot count that looks adequate on paper can still overload a host or create long queues when tests open heavy pages. Use the Grid UI or /status for basic status; Selenium also provides GraphQL for querying Grid state and metadata.
Secure the Grid endpoint
Do not expose an unprotected Grid to the public internet. Selenium warns that an exposed Grid can give third parties access to its infrastructure and internal applications or files, and allow custom binaries to run. Restrict client access to trusted systems, and allow only the component-to-component network paths required by your chosen topology. Use firewall rules and an appropriately protected network boundary; the official quick start warns of the risk but does not prescribe one universal production security architecture (Selenium Grid getting started).
Troubleshoot common setup failures
- The JAR does not start or Java is rejected: confirm Java 11 or higher is installed and available to the shell, and that the command names the downloaded JAR exactly.
- Driver-location or browser startup error: verify the browser is installed and the driver is compatible and discoverable on PATH. If using Selenium Manager, check that the environment can resolve and download the driver; otherwise install the driver explicitly. Selenium’s driver troubleshooting guide covers driver discovery.
- Connection refused at port 4444: ensure the Standalone or Hub process is still running, the client uses the correct hostname and port, and local or network firewall rules allow access.
- Session request waits or cannot find a slot: compare requested
browserName,browserVersion, andplatformNamewith the slots shown in the Grid UI. Remove unnecessary version/platform constraints or register a Node with a compatible browser. - A Node does not appear in the Hub: check its Hub URL, Event Bus reachability on the required ports, the Hub’s ability to reach the Node HTTP port, and any non-default Event Bus configuration.
- Sessions start slowly or fail under parallel load: reduce concurrent sessions, inspect CPU and memory pressure, and measure queue and startup times using the actual suite before raising capacity.
- Tests pass locally but fail remotely: check differences in browser version, operating system, viewport, locale, timezone, permissions, and network access. Request the intended environment explicitly and confirm that a matching slot exists.
Or skip the browser setup:
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