Implementation:SeleniumHQ Selenium Standalone And Hub Commands

Knowledge Sources	Selenium Selenium Grid
Domains	Distributed_Testing, Architecture, Selenium_Grid
Last Updated	2026-02-11 00:00 GMT

Overview

Concrete tool for selecting and running Grid deployment topologies provided by the Selenium Grid CLI commands.

Description

The Standalone and Hub classes both extend TemplateGridServerCommand and implement the CliCommand interface, registered via @AutoService(CliCommand.class) for ServiceLoader discovery. Standalone creates all Grid components in a single process: it instantiates LocalSessionMap, LocalNewSessionQueue, LocalDistributor, Router, GraphqlHandler, and GridUiRoute, plus a local Node via NodeOptions.getNode(). Hub creates the same central components (Router, Distributor, Session Map, Session Queue, Event Bus) but omits the local Node, expecting separate Node processes to connect. Both commands support an optional readinessCheck at the /readyz endpoint for Kubernetes liveness probes.

Usage

Invoke via the Grid server JAR: java -jar selenium-server.jar standalone or java -jar selenium-server.jar hub. Configuration via TOML file or CLI flags.

Code Reference

Source Location

• Repository: Selenium
• File: java/src/org/openqa/selenium/grid/commands/Standalone.java (L80-288)
• File: java/src/org/openqa/selenium/grid/commands/Hub.java (L73-247)

Signature

@AutoService(CliCommand.class)
public class Standalone extends TemplateGridServerCommand {
    // getName() returns "standalone"
    // getDescription() returns "The selenium server, running everything in-process."
    public Set<Role> getConfigurableRoles();   // DISTRIBUTOR, EVENT_BUS, HTTPD, NODE, ROUTER, SESSION_QUEUE
    protected Handlers createHandlers(Config config);
    protected void execute(Config config);
}

@AutoService(CliCommand.class)
public class Hub extends TemplateGridServerCommand {
    // getName() returns "hub"
    // getDescription() returns "A grid hub, composed of sessions, distributor, and router."
    public Set<Role> getConfigurableRoles();   // DISTRIBUTOR, EVENT_BUS, HTTPD, SESSION_QUEUE, ROUTER
    protected Handlers createHandlers(Config config);
    protected void execute(Config config);
}

Import

// These are CLI entry points, not directly imported in user code
// Invoked via: java -jar selenium-server.jar standalone|hub|node

I/O Contract

Inputs

Name	Type	Required	Description
CLI command	String	Yes	"standalone", "hub", "node", "distributor", "router", "sessions", "sessionqueue"
--config	File path	No	TOML configuration file
--port	int	No	HTTP server port (default: 4444)
--host	String	No	Hostname to bind to
--max-sessions	int	No	Maximum concurrent sessions per node

Outputs

Name	Type	Description
Grid Server	HTTP Server	Running Grid on configured port, accepting WebDriver sessions
/status	JSON	Grid status endpoint reporting readiness and node info
/readyz	Text	Kubernetes-compatible liveness probe endpoint
/ui	HTML	Grid web UI (can be disabled via RouterOptions.disableUi())

Usage Examples

Standalone Mode

# Start all-in-one Grid
java -jar selenium-server.jar standalone --port 4444

# With TOML config
java -jar selenium-server.jar standalone --config grid-config.toml

Hub-and-Node Mode

# Start Hub
java -jar selenium-server.jar hub --port 4444

# Start Node (on same or different machine)
java -jar selenium-server.jar node --hub http://hub-host:4444

Related Pages

Implements Principle

• Principle:SeleniumHQ_Selenium_Grid_Deployment_Topology

Requires Environment

• Environment:SeleniumHQ_Selenium_Grid_Deployment_Environment