Docker-based Grid in Selenium Python

Introduction

Docker-based Grid helps run many browser tests at the same time easily. It uses containers to keep tests separate and fast.

You want to run tests on different browsers without installing them all on your computer.

You need to run many tests at once to save time.

You want to share the same test setup with your team easily.

You want to avoid conflicts between different browser versions.

You want to run tests on a clean environment every time.

Syntax

Selenium Python

docker run -d -p 4444:4444 --name selenium-grid selenium/standalone-chrome

# In Python test script:
from selenium import webdriver
from selenium.webdriver.common.desired_capabilities import DesiredCapabilities

driver = webdriver.Remote(
    command_executor='http://localhost:4444/wd/hub',
    desired_capabilities=DesiredCapabilities.CHROME
)
driver.get('https://example.com')
print(driver.title)
driver.quit()

The first command starts a Docker container with a Chrome browser ready for testing.

The Python code connects to this container to run browser commands remotely.

Examples

This starts a Docker container with Firefox browser for testing.

Selenium Python

docker run -d -p 4444:4444 --name selenium-grid selenium/standalone-firefox

This Python code connects to the Firefox Docker container to run tests.

Selenium Python

from selenium import webdriver
from selenium.webdriver.common.desired_capabilities import DesiredCapabilities

driver = webdriver.Remote(
    command_executor='http://localhost:4444/wd/hub',
    desired_capabilities=DesiredCapabilities.FIREFOX
)
driver.get('https://example.com')
print(driver.title)
driver.quit()

Sample Program

This script connects to the Docker Selenium Grid with Chrome, opens example.com, prints the page title, and closes the browser.

Selenium Python

from selenium import webdriver
from selenium.webdriver.common.desired_capabilities import DesiredCapabilities

# Connect to Docker-based Selenium Grid running Chrome

driver = webdriver.Remote(
    command_executor='http://localhost:4444/wd/hub',
    desired_capabilities=DesiredCapabilities.CHROME
)

# Open a website
driver.get('https://example.com')

# Print the page title
print(driver.title)

# Close the browser
driver.quit()

Important Notes

Make sure Docker is installed and running before starting the Selenium Grid container.

Use the correct port (default 4444) and URL when connecting from your test script.

Always quit the driver to free resources after the test finishes.

Summary

Docker-based Grid lets you run browser tests in isolated containers.

You connect your test scripts to the grid using Remote WebDriver.

This setup helps run many tests faster and keeps your computer clean.

Practice

1. What is the main purpose of using a Docker-based Selenium Grid?

easy

A. To run tests only on the local machine without network

B. To write test scripts without any browser

C. To replace Selenium WebDriver with a new tool

D. To run browser tests in isolated containers for better scalability

Solution

1. Step 1: Understand Docker-based Grid concept

   Docker-based Grid runs browsers inside containers, isolating tests and allowing many tests to run in parallel.

2. Step 2: Compare options with this concept

   Only To run browser tests in isolated containers for better scalability correctly describes running tests in isolated containers for scalability.

3. Final Answer:

   To run browser tests in isolated containers for better scalability -> Option D

4. Quick Check:

   Docker Grid = isolated, scalable browser tests [OK]

Hint: Docker Grid isolates browsers to run many tests fast [OK]

Common Mistakes:

• Thinking Docker replaces Selenium WebDriver
• Believing tests run only locally without network
• Confusing Docker Grid with test script writing

2. Which Python code snippet correctly creates a Remote WebDriver session to connect to a Docker-based Selenium Grid at URL http://localhost:4444/wd/hub for Chrome browser?

easy

A. driver = webdriver.Remote('http://localhost:4444/wd/hub', browser='chrome')

B. driver = webdriver.Chrome('http://localhost:4444/wd/hub')

C. driver = webdriver.Remote(command_executor='http://localhost:4444/wd/hub', options=webdriver.ChromeOptions())

D. driver = webdriver.ChromeOptions().Remote('http://localhost:4444/wd/hub')

Solution

1. Step 1: Recall correct Remote WebDriver syntax

   Remote WebDriver requires command_executor URL and browser options object, e.g., ChromeOptions.

2. Step 2: Match options with correct syntax

   driver = webdriver.Remote(command_executor='http://localhost:4444/wd/hub', options=webdriver.ChromeOptions()) uses webdriver.Remote with command_executor and ChromeOptions correctly; others misuse classes or parameters.

3. Final Answer:

   driver = webdriver.Remote(command_executor='http://localhost:4444/wd/hub', options=webdriver.ChromeOptions()) -> Option C

4. Quick Check:

   Remote WebDriver needs command_executor and options [OK]

Hint: Use webdriver.Remote with command_executor and options [OK]

Common Mistakes:

• Using webdriver.Chrome instead of webdriver.Remote for grid
• Passing URL directly to webdriver.Chrome constructor
• Incorrect chaining of ChromeOptions and Remote

3. Given this Python code connecting to a Docker Selenium Grid, what will print(driver.title) output if the page at https://example.com has title "Example Domain"?

from selenium import webdriver

driver = webdriver.Remote(
    command_executor='http://localhost:4444/wd/hub',
    options=webdriver.FirefoxOptions()
)
driver.get('https://example.com')
print(driver.title)
driver.quit()

medium

A. Example Domain

B. https://example.com

C. None

D. Error: NoSuchWindowException

Solution

1. Step 1: Understand code flow with Remote WebDriver

   The code opens Firefox browser via Docker Grid, navigates to example.com, then prints the page title.

2. Step 2: Recall page title for https://example.com

   The official example.com page title is "Example Domain", so print(driver.title) outputs this string.

3. Final Answer:

   Example Domain -> Option A

4. Quick Check:

   driver.title = page title = "Example Domain" [OK]

Hint: Page title prints exactly what the website sets [OK]

Common Mistakes:

• Confusing URL with page title
• Expecting None if using Remote WebDriver
• Assuming an error without browser session

4. You run this Python test script with Docker Selenium Grid but get a connection error. What is the likely cause?

from selenium import webdriver

driver = webdriver.Remote(
    command_executor='http://localhost:5555/wd/hub',
    options=webdriver.ChromeOptions()
)
driver.get('https://example.com')

medium

A. Docker Selenium Grid is not running on port 5555

B. ChromeOptions cannot be used with Remote WebDriver

C. The URL 'https://example.com' is invalid

D. webdriver.Remote requires a local Chrome browser

Solution

1. Step 1: Analyze connection error cause

   Connection errors usually mean the Selenium Grid server is unreachable at the given URL and port.

2. Step 2: Check port and server status

   Port 5555 is uncommon; default Docker Grid uses 4444. If Grid is not running on 5555, connection fails.

3. Final Answer:

   Docker Selenium Grid is not running on port 5555 -> Option A

4. Quick Check:

   Connection error = wrong port or server down [OK]

Hint: Check Docker Grid port matches command_executor URL [OK]

Common Mistakes:

• Thinking ChromeOptions can't be used remotely
• Blaming website URL for connection errors
• Assuming Remote needs local browser installed

5. You want to run tests on Chrome and Firefox browsers simultaneously using Docker Selenium Grid. Which Docker Compose service setup is correct to start both browser nodes connected to the hub?

hard

A. version: '3'
services:
selenium-hub:
image: selenium/hub:4.10.0-20230628
ports:
- '4444:4444'
chrome:
image: selenium/node-chrome:4.10.0-20230628
environment:
- SE_EVENT_BUS_HOST=selenium-hub
firefox:
image: selenium/node-firefox:4.10.0-20230628
environment:
- SE_EVENT_BUS_HOST=selenium-hub

B. version: '3'
services:
selenium-hub:
image: selenium/hub:4.10.0-20230628
ports:
- '4444:4444'
chrome:
image: selenium/node-chrome:4.10.0-20230628
depends_on:
- selenium-hub
environment:
- SE_EVENT_BUS_HOST=selenium-hub
- SE_EVENT_BUS_PUBLISH_PORT=4442
- SE_EVENT_BUS_SUBSCRIBE_PORT=4443
firefox:
image: selenium/node-firefox:4.10.0-20230628
depends_on:
- selenium-hub
environment:
- SE_EVENT_BUS_HOST=selenium-hub
- SE_EVENT_BUS_PUBLISH_PORT=4442
- SE_EVENT_BUS_SUBSCRIBE_PORT=4443

C. version: '3'
services:
selenium-hub:
image: selenium/hub:4.10.0-20230628
ports:
- '4444:4444'
chrome:
image: selenium/node-chrome:4.10.0-20230628
depends_on:
- selenium-hub
firefox:
image: selenium/node-firefox:4.10.0-20230628
depends_on:
- selenium-hub

D. version: '3'
services:
selenium-hub:
image: selenium/hub:4.10.0-20230628
ports:
- '4444:4444'
chrome:
image: selenium/node-chrome:4.10.0-20230628
depends_on:
- selenium-hub
environment:
- SE_EVENT_BUS_HOST=selenium-hub
- SE_EVENT_BUS_PUBLISH_PORT=4443
- SE_EVENT_BUS_SUBSCRIBE_PORT=4442
firefox:
image: selenium/node-firefox:4.10.0-20230628
depends_on:
- selenium-hub
environment:
- SE_EVENT_BUS_HOST=selenium-hub
- SE_EVENT_BUS_PUBLISH_PORT=4442
- SE_EVENT_BUS_SUBSCRIBE_PORT=4443

Solution

1. Step 1: Understand Docker Compose setup for Selenium Grid

   The hub service exposes port 4444. Each node (chrome, firefox) must depend on hub and set environment variables for event bus host and ports.

2. Step 2: Check environment variables and dependencies

   version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 correctly sets SE_EVENT_BUS_HOST, SE_EVENT_BUS_PUBLISH_PORT=4442, and SE_EVENT_BUS_SUBSCRIBE_PORT=4443 for both nodes, matching Selenium Grid docs.

3. Final Answer:

   version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 -> Option B

4. Quick Check:

   Correct event bus ports and depends_on = version: '3' services: selenium-hub: image: selenium/hub:4.10.0-20230628 ports: - '4444:4444' chrome: image: selenium/node-chrome:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 firefox: image: selenium/node-firefox:4.10.0-20230628 depends_on: - selenium-hub environment: - SE_EVENT_BUS_HOST=selenium-hub - SE_EVENT_BUS_PUBLISH_PORT=4442 - SE_EVENT_BUS_SUBSCRIBE_PORT=4443 [OK]

Hint: Set SE_EVENT_BUS_HOST and ports exactly as docs show [OK]

Common Mistakes:

• Omitting depends_on causing nodes to start before hub
• Wrong or missing event bus port environment variables
• Swapping publish and subscribe port numbers