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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsFor a local or small CI test suite, start Selenium Grid in Standalone mode and point your WebDriver client at http://localhost:4444. When you need more machines or browser and operating-system combinations, use a Hub with Nodes; reserve fully distributed mode for deployments that need Grid components operated separately. Grid routes WebDriver commands to remote browser instances so tests can run in parallel across environments. Selenium Grid documentation describes its purpose and components.
Choose a Grid deployment that fits your suite
The right mode depends on how many machines and browser/OS combinations you need, your target concurrent sessions and available resources, and how much operational separation you require.
| Mode | Best starting point | Trade-off |
|---|---|---|
| Standalone | Local debugging or a small CI job on one machine | All Grid components run in one process; capacity is limited to that machine. |
| Hub/Node | Multiple machines or browser/OS environments behind one client entry point | You operate the Hub and register Nodes, including their network connectivity. |
| Fully distributed | Deployments that need Grid roles separated across components or machines | More addresses, ports, configuration and operational coordination. |
Selenium’s rough sizing guide characterizes small grids as Standalone or Hub/Node with up to five Nodes, middle grids as Hub/Node with 6–60 Nodes, and large grids as Hub/Node with 60–100 Nodes or distributed above 100. These are planning estimates, not fixed limits or performance guarantees. See Selenium’s Grid getting-started guide.
Prepare Java, browsers and drivers
- Install Java 11 or later.
- Install the browser or browsers your tests will request.
- Obtain the Selenium Server JAR for the release pinned by your project. The current release number is not specified here; use the version selected by your project rather than copying an unpinned version into a command.
- Ensure browser drivers are available. When enabled, Selenium Manager can configure drivers automatically with
--selenium-manager true; otherwise install the required drivers and put them onPATH.
Start a local Standalone Grid
- From the directory containing the downloaded JAR, run:
java -jar selenium-server-<version>.jar standaloneReplace
<version>with the JAR filename’s version. - Set the test’s RemoteWebDriver URL to
http://localhost:4444, the default client endpoint. - Open
http://localhost:4444to view the Grid UI, or queryGET http://localhost:4444/statusto inspect status.
Standalone runs all Grid components in one process on one machine, which makes it the shortest path to local parallel testing. Selenium calls it the easiest mode to start. The client URL is the Standalone server in this mode. See Getting started with Selenium Grid and Grid endpoints.
Scale out with a Hub and Nodes
Start the Hub
java -jar selenium-server-<version>.jar hubThe Hub is the client entry point. Point RemoteWebDriver at the Hub address, normally http://<hub-host>:4444.
Register a Node
For a Node on the same host, run:
java -jar selenium-server-<version>.jar nodeFor a Node on another machine, give it the Hub address:
java -jar selenium-server-<version>.jar node --hub http://<hub-ip>:4444Nodes advertise browser slots and execute sessions. Adding or removing Nodes lets you change capacity and combine machines with different operating systems or browser versions. If you start multiple Nodes on one machine, assign distinct ports, such as 5555 and 6666, and configure each according to the installed release’s help output.
Allow the required internal traffic
For separate Hub and Node machines, allow the Hub event bus ports 4442 and 4443, along with 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 each Node. Verify hostnames and ports are reachable from the machines that need them; do not assume a command using one network layout applies unchanged to another.
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Restrict Grid endpoints to trusted test infrastructure and administrators. Selenium warns that an exposed Grid can be abused to access internal web applications and files or to run custom binaries. See the security guidance in Selenium’s getting-started documentation.
Use distributed mode only when separate component roles help
Fully distributed mode separates six roles: the Event Bus carries internal messages; the Session Queue holds new session requests; the Distributor matches requests to Nodes; the Session Map tracks session IDs and Nodes; the Router accepts client traffic and routes requests; and Nodes run browser sessions.
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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 example values, not a guarantee that the ports suit your network. Start components with reachable hostnames and ports, and consult the installed version’s --help and --config-help output. In this mode, configure clients to use the Router endpoint, normally on port 4444. The component model and examples are in Selenium’s Grid getting-started guide.
Set concurrency from observed capacity
Grid’s session capacity is constrained by browser workload and host resources; a slot count alone does not establish how many tests your machines can run reliably.
- Selenium’s guide gives a starting default of one concurrent session per CPU for Chromium-based browsers and Firefox, and one Safari session. These are defaults and may not suit every workload.
- The guide estimates around 1 GB of RAM per browser session as a reference, not a guaranteed requirement.
- Distributor session-creation concurrency depends on available processors.
- Measure the actual suite continuously and adjust for memory pressure, browser type and host configuration. Selenium favors smaller Nodes for process isolation and notes Docker as one way to achieve that approach.
These figures are operational starting points, not benchmark results. See Selenium’s sizing guidance.
Configure options with flags or TOML
Grid settings can be passed as command-line flags or placed in a TOML configuration file. Selenium recommends TOML for readability and source control, and flags can be combined with a TOML file. Examples cover Node session caps and driver implementations. Docker-backed sessions can be configured on Standalone or a Node, with suitable image-to-capability mapping and Docker daemon connectivity.
Configuration flags can change by release. Check the installed server’s help before relying on a copied setting: use component-specific --help and global --config-help. References: CLI options and TOML configuration options.
Validate registration and diagnose session failures
Use the Grid UI at http://localhost:4444 or request GET http://localhost:4444/status (substitute the Hub or Router host when remote). Status reports registered Nodes, their availability, sessions and slots. In Standalone mode the client targets the server; in Hub/Node mode it targets the Hub; in distributed mode it targets the Router.
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| Symptom | What to check | Fix |
|---|---|---|
| Node does not appear or is unavailable | Hub address, Node port, event bus ports and matching publish/subscribe addresses when non-default ports are used. | Correct the address and internal firewall rules, then check status again. |
| Session request waits or cannot be created | Whether a Node is registered, requested browser capabilities match an installed browser/driver, and slots are available. | Install or enable the needed browser and driver, request capabilities the Node supports, or add capacity after measuring host resources. |
| Browser fails to launch | Browser installation, driver availability and whether driver setup is enabled. | Install the browser/driver or enable Selenium Manager with --selenium-manager true. |
| Client cannot connect | Whether the client URL points to the correct server, Hub or Router and whether that endpoint is reachable. | Use the mode-specific endpoint and verify network access to its host and port. |
| Copied configuration flag is rejected | Whether the flag exists in the installed Selenium release. | Consult that server’s component --help and --config-help output. |
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Frequently Asked Questions
Can Selenium Grid run tests on different operating systems?
Yes. Hub/Node can combine machines with different operating systems and browser versions, provided the Nodes are reachable and advertise browsers that match the test capabilities.
Does a Grid session limit guarantee a particular test throughput?
No. Session capacity is only a starting configuration; actual throughput depends on the browser workload and available CPU, memory and host configuration.
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