Docker-based Grid in Selenium Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Selenium Grid Node Registration Log

You start a Selenium Grid node using Docker with the command:

docker run -d --net=host -e SE_EVENT_BUS_HOST=localhost -e SE_EVENT_BUS_PUBLISH_PORT=4442 -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 selenium/node-chrome:latest

What will be the expected output in the node's Docker logs indicating successful registration?

A ERROR - Could not connect to the hub at http://localhost:4444

B INFO - Registered a node with the hub: http://localhost:4444

C WARN - Node is running but not registered to any hub

D FATAL - Missing environment variables for event bus

💡 Hint

Look for logs that confirm successful connection to the hub.

When a Selenium Grid node starts correctly with proper event bus environment variables, it logs an INFO message confirming registration to the hub URL.

❓ assertion

intermediate

Correct Assertion for Node Availability in Selenium Grid

You want to write a test to assert that a Selenium Grid node is available and ready to accept sessions. Which assertion correctly checks the node's readiness using Selenium Python bindings?

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.CHROME)

# Which assertion below correctly verifies the node is ready?

A assert driver.session_id is not None

B assert driver is None

C assert driver.title == ''

D assert driver.current_url == 'about:blank'

💡 Hint

A valid session_id means the node accepted the session.

If the driver.session_id is not None, it means the remote node accepted the session and is ready.

🔧 Debug

advanced

Debugging Selenium Grid Node Connection Failure

You run a Selenium Grid hub and a node in Docker containers on the same machine. The node fails to register with the hub. The node's Docker logs show:

ERROR - Could not connect to the hub at http://localhost:4444

What is the most likely cause?

A The node image is outdated and incompatible with the hub.

B The hub is not running on port 4444.

C Docker network is misconfigured to block all traffic.

D The node container cannot reach the hub at localhost because localhost refers to itself inside the container.

💡 Hint

Remember how localhost behaves inside Docker containers.

Inside a Docker container, localhost refers to the container itself, not the host machine. The node tries to connect to its own container instead of the hub.

❓ framework

advanced

Best Practice for Scaling Selenium Grid with Docker

You want to scale your Selenium Grid to run multiple Chrome and Firefox nodes using Docker Compose. Which approach correctly defines the services for scaling nodes?

Selenium Python

version: '3'
services:
  selenium-hub:
    image: selenium/hub:latest
    ports:
      - '4444:4444'
  chrome:
    image: selenium/node-chrome:latest
    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:latest
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443

A Manually start multiple containers with different ports mapped.

B Run multiple docker-compose files separately for each node.

C Add 'deploy: replicas: 3' under chrome and firefox services to scale nodes.

D Use 'scale' command in docker-compose CLI to add nodes dynamically.

💡 Hint

Docker Compose version 3 supports replicas for scaling services.

Using 'deploy: replicas: 3' under each node service in Docker Compose allows scaling multiple node containers automatically.

🧠 Conceptual

expert

Understanding Event Bus Ports in Selenium Grid Docker Setup

In a Docker-based Selenium Grid setup, the environment variables SE_EVENT_BUS_HOST, SE_EVENT_BUS_PUBLISH_PORT, and SE_EVENT_BUS_SUBSCRIBE_PORT are set for nodes. What is the primary purpose of these event bus ports?

A They enable communication between the hub and nodes for session management and status updates.

B They expose the browser debugging ports for remote inspection.

C They are used to forward HTTP traffic from the hub to the nodes.

D They configure the ports for Selenium WebDriver commands to reach the browsers.

💡 Hint

Think about how the hub and nodes coordinate in a distributed grid.

The event bus ports allow the hub and nodes to communicate asynchronously for managing sessions and sharing node status.