Selenium grid: all you need to know

12 min read
September 6, 2023

Selenium is an open-source automation testing framework for web apps. It helps testers and developers to automate tests across browsers and platforms. 

But as web apps and testing scenarios become more complex, a single Selenium server might struggle to handle the load. 

That’s where Selenium Grid comes to the rescue. 

It’s a powerful extension that enables parallel test execution. It does so by distributing test cases across multiple nodes with unique browser configurations. 

This speeds up testing and reduces execution time. Here we’ll explore Selenium Grid’s crucial role in cross-browser and cross-platform testing. 

We’ll also talk about its architecture, setup, CI/CD integration, and a comparison between Selenium Grid 4 and its predecessors.

Let’s get into it!

What is Selenium grid? 

Selenium Grid is an advanced testing framework for developers and testers. 

It helps them perform tests across various web browsers and platforms simultaneously. 

By distributing testing across multiple machines, it significantly reduces the time needed for comprehensive testing. 

Selenium Grid is made up of a hub and nodes architecture. 

The hub acts as a central control point that manages and delegates test execution to multiple nodes. Each node represents a different browser and platform configuration.

 The Latest Selenium Grid 4 represents a significant upgrade to its predecessors.

It introduced components and capabilities that enhance its testing efficiency.

Selenium Grid components

Now that we got the definition out of the way, let’s talk about the key components of Selenium Grid.


The core of Selenium Grid’s architecture is the Hub.

It’s a central unit of the testing process. 

It efficiently manages incoming test requests and directs them to individual Nodes for execution. 

Unlike Selenium Grid 3, Selenium Grid 4 improves the Hub with features like distributed tracing and improved communication with Nodes. 

It does so through the introduction of components such as:

  • Router
  • Distributor
  • Session Map
  • New Session Queue
  • Event Bus

These components show an evolution in Selenium Grid’s capabilities.


Nodes are distinct execution environments where tests are carried out. 

Each Node is configured with specific browser and platform settings. It enables parallel test execution across various combinations. 

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Source: Selenium Grid

In Selenium Grid 3, Nodes played a similar role, executing tests based on the Hub’s instructions. 

And here’s how Selenium Grid 4 is designed, for comparison:


Source: Selenium Grid

Selenium Grid 4 Nodes continue this role, but with enhancements that include:

  • Improved command execution flow
  • Refined resource management
  • Increased scalability

These upgrades enable Nodes to handle complex testing scenarios more efficiently.


A notable addition in Selenium Grid 4, the Router, improves the overall architecture. 

It serves as the gateway to the Grid, receiving all external requests and directing them to the appropriate component. 

This dynamic allocation optimizes the distribution of test execution. 

And that contributes to load balancing and efficient resource utilization. 

By ensuring test task allocation to the most suitable Nodes, the Router significantly enhances the speed and efficiency of testing.


The Distributor in Selenium Grid 4 handles two main tasks.

First, it manages the registration of Nodes. Nodes communicate their presence by sending registration events through the Event Bus. 

The Distributor validates them via HTTP requests and keeps a record of their capabilities in the GridModel.

Second, the Distributor oversees session management. 

It keeps an eye on the New Session Queue, where the Router directs the pending session requests.

 When a request appears, the Distributor takes charge. 

It identifies a suitable Node for session creation and maintains the relationship between the session and the executing Node in the Session Map.

Session Map

The Session Map is a data repository responsible for maintaining the correlation between session IDs and the specific Node where the session is active. 

It plays a crucial role in assisting the Router when it needs to route a request to the appropriate Node. 

When needed, the Router will inquire with the Session Map to determine the Node linked to a particular session ID.

New Session Queue

The New Session Queue stores incoming session requests in a FIFO order. That allows customization of request timeout and retry intervals.

The Router adds new session requests to the queue and monitors for timeouts, removing expired requests.

The Distributor checks for available slots and attempts to create sessions for matching requests. 

If slots are full, it returns requests to the queue. It rejects requests timing out during retries or while at the front of the queue.

On successful session creation, the Distributor sends session information back through the queue to the Router and then to the client.

Event Bus

The Event Bus is a communication mechanism that improves interaction between different parts of the Selenium Grid architecture. 

This enhanced communication leads to smoother data exchange between components. And that contributes to improved efficiency and coordination. 

When initializing the Grid in the distributed mode, the Event Bus should be the very first component put into operation.

Why you should use Selenium grid

Selenium Grid is a game-changer in the world of automated testing. 

There are several reasons why you should consider add it to your testing workflow. 

Scalability and Parallel Testing

Selenium Grid helps you scale your test infrastructure effortlessly.

By distributing tests across multiple nodes, it enables parallel testing across different browsers and platforms.

This leads to faster test execution and reduced test cycle times. 


Selenium Grid uses resources effectively. 

By sharing browser instances across multiple tests, it minimizes hardware and software costs. This makes it a budget-friendly choice for teams with diverse testing needs.

Cross-Browser Testing

Selenium Grid ensures consistent performance across different browsers.

It enables simultaneous testing on different browser setups. This makes cross-browser testing seamless and reliable.

Improved Test Coverage

Selenium Grid improves test coverage by running tests at the same time on multiple nodes.

This allows you to find issues across different environments, increasing the accuracy and scope of your test suites. 

Real-Time Visualization

The inclusion of VNC support in Selenium Grid allows you to monitor test execution in real-time. 

This feature simplifies debugging and fosters better collaboration among team members.

How to set up Selenium grid 

Setting up Selenium Grid will revolutionize your test automation efforts. 

It’ll do so by enabling parallel test execution across browsers and platforms. 

Latest Selenium Grid 4 offers 3 modes:


  • All components on one machine.
  • Simple setup.
  • Ideal for local testing or CI/CD.

Hub and Node

  • Central Hub manages Nodes.
  • Nodes on different machines.
  • Scalable and versatile for various environments.


  • Components on separate machines.
  • Event Bus, Queue, Map, Distributor, and Router.
  • Complex setup for large-scale testing.
  • Offers precise control and scalability.

In this step-by-step guide, we’ll focus on the standalone Selenium Grid mode. We’ll walk you through the process of setting it up using Python. 

By following these steps, you’ll be able to streamline your testing efforts. This will lead to improved efficiency and more robust test automation.

Visit Getting started with Selenium Grid for deeper understanding of each mode and their setup processes.

Let’s get started!

  1. Install Dependencies

Make sure you have Python installed, along with the Selenium library. You can install Selenium using pip:

pip install selenium
  1. Download Selenium Server (Grid 4)

Download the latest Selenium Server (Grid 4) JAR file from the official Selenium website:

Save the JAR file in a directory of your choice.

  1. Start the Selenium standalone and configure drivers

Open a terminal and navigate to the directory where you downloaded the Selenium Server JAR file. Run the following command to start the Selenium Grid Hub:

java -jar selenium-server-<version>.jar standalone --selenium-manager true

Note: Make sure to replace <version> in the command with the actual version number of the    Selenium Server JAR file you downloaded. Adding –selenium-manager true will configure the drivers configuration automatically. You don’t have to download them manually.

Your terminal should look similar to the screenshot below:

  1. Create a test script

Now let’s write a simple test script. 

The test script will:

  • launch the official Selenium website
  • navigate to the ‘Downloads’ page
  • check for any mention of a stable Python version
  • capture a screenshot if it finds a Python version

all of this happens concurrently for multiple web browsers using the ‘multiprocessing’ module, enabling parallel testing.

import logging
import multiprocessing

from selenium import webdriver
from selenium.common.exceptions import WebDriverException
from import Options as ChromeOptions
from import By
from selenium.webdriver.edge.options import Options as EdgeOptions
from selenium.webdriver.firefox.options import Options as FirefoxOptions
from import expected_conditions as EC
from import WebDriverWait

# Configure logging
# Define browser options and URLs
browser_options = {
    "chrome": ChromeOptions(),
    "firefox": FirefoxOptions(),
    "edge": EdgeOptions(),
hub_url = 'http://localhost:4444/wd/hub'

# Use 'http://localhost:4444/wd/hub' for local testing.
# If running tests on a remote hub, replace 'localhost' with the hub's IP address.
def setup_driver(browser):
        options = browser_options.get(browser)
        driver = webdriver.Remote(command_executor=hub_url, options=options)
        return driver
    except WebDriverException as e:
        logging.error(f"Error setting up {browser} WebDriver: {e}")
        return None

def test_check_stable_python_version(browser):
    driver = setup_driver(browser)
    if driver is None:
        downloads_link = WebDriverWait(driver, 10).until(
            EC.element_to_be_clickable((By.LINK_TEXT, "Downloads"))
        WebDriverWait(driver, 10).until(
            EC.presence_of_element_located((By.XPATH, "//p[contains(text(), 'Python')]"))
        # Take a screenshot when the condition is met
        driver.save_screenshot(f"{browser}_screenshot.png")"Screenshot taken for {browser}")
    except Exception as e:
        logging.error(f"Error in {browser} test: {e}")

if __name__ == "__main__":
    browsers = ["chrome", "firefox", "edge"]  # Define the browsers to test
    processes = []
    for browser in browsers:
        process = multiprocessing.Process(target=test_check_stable_python_version, args=(browser,))
    for process in processes:
  1. Run the script

Execute the script using a Python interpreter (Python 3.x) from the command line or your preferred development environment:


If you’d like to run the script in headless mode, modify the browser options accordingly:

# Enable headless mode for Chrome


# Enable headless mode for Firefox


# Add headless mode for Edge if supported

  1.  Monitor test sessions with Selenium Grid Console

As you execute tests on the Selenium Grid, you can monitor and manage them using the Selenium Grid Console. 

Access the console by opening a web browser and navigating to the following URL:


Replace localhost with the hostname or IP address of the machine where you started the Selenium Grid Hub. 

The console provides graphical insights into connected nodes, active test sessions, browser configurations, and the overall status of your Selenium Grid setup. 

Here you can see the parallel runs of your tests: 

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Now let’s take a look how you can use Selenium Grid with your CI/CD tools.

How to integrate Selenium grid with CI/CD tools 

If you want to improve your testing and achieve faster, more reliable software delivery, integrating Selenium Grid with CI/CD tools is a must. Here’s how to do it:

Choose Your CI/CD Tool

Selecting the right CI/CD tool that suits your team’s workflow is the first step. Whether you opt for Jenkins, GitLab CI, or another tool, it’s essential to find the perfect fit.

Set Up Selenium Grid:

Ensure you have a robust Selenium Grid in place with the hub and nodes ready to roll. With a diverse set of browser configurations, your Grid is all set for parallel test execution.

Configure the CI/CD Pipeline:

Craft a seamless CI/CD pipeline, including the test stage. Trigger the execution of your Selenium test scripts during this phase to activate automated testing.

Run Tests:

Use test runners like TestNG , Cucumber or Behave to execute your Selenium tests. Use the Selenium Grid URL to distribute tests across the registered nodes for simultaneous execution.

Analyze and React:

Capture and analyze test results to gain valuable insights into the stability and performance of your web applications. Swiftly identify and address issues to maintain a robust and resilient product.

Consider Docker Selenium

In addition to these steps, consider using Docker Selenium to further enhance your integration.

Docker provides an efficient way to provision and scale Selenium Grid infrastructure using containers. 

Selenium project maintains Docker images for both Firefox and Chrome nodes. This makes it easy to set up and manage your Selenium Grid within Docker containers. 

You can find detailed instructions on provisioning a grid in the Docker Selenium repository.

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Source: LinkedIn

By integrating Selenium Grid with your chosen CI/CD tool you can significantly improve your testing process. 

This powerful automation solution will:

  • boost productivity.
  • enhance test coverage.
  • enable the delivery of high-quality software.

All at an accelerated pace.


In conclusion, Selenium Grid is a game-changer in test automation, especially for cross-browser testing. 

It speeds up tests and makes them more efficient by running them simultaneously. 

The integration of Selenium Grid with CI/CD tools presents a powerful approach to optimizing software delivery processes. 

Embracing Selenium Grid can make your testing process smoother, ensuring websites work well for users everywhere. 

So, whether you’re testing big or small, Selenium Grid is a tool worth considering for top-notch results.
If you’d like to read more about all things software, check out our blog!

Written by

Marko Brajer

QA Engineer

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