Docker Selenium Grid in Selenium Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Selenium Grid Node Registration Log

You have started a Selenium Grid node using Docker. The node logs show the following line:

"Registered a node with the hub: http://hub:4444"

What does this log line indicate?

A The node is running but has not yet attempted to connect to the hub.

B The node failed to connect to the hub due to network issues.

C The hub is down and the node is running standalone.

D The node has successfully connected and registered with the Selenium Grid hub.

💡 Hint

Look for keywords about registration and connection in the log line.

The log line explicitly states the node has registered with the hub URL, meaning the connection was successful.

❓ assertion

intermediate

Correct Assertion for Checking Node Availability

In a Selenium Grid test setup using Java, you want to assert that a remote WebDriver session is created successfully on a node. Which assertion is correct to verify the session is active?

Selenium Java

RemoteWebDriver driver = new RemoteWebDriver(new URL("http://hub:4444/wd/hub"), new ChromeOptions());
// Which assertion below correctly checks the session?

A assertEquals(driver.getSessionId(), null);

B assertTrue(driver.getSessionId().toString().isEmpty());

C assertNotNull(driver.getSessionId());

D assertFalse(driver.getSessionId() != null);

💡 Hint

A valid session ID means the session is active and not null.

The session ID is assigned when the session is created. Checking it is not null confirms the session exists.

🔧 Debug

advanced

Debugging Node Connection Failure in Docker Selenium Grid

You have a Docker Selenium Grid setup with one hub and two nodes. One node fails to register with the hub and logs show:

"Connection refused: connect"

What is the most likely cause?

A The node has incorrect browser version installed.

B The node container cannot reach the hub container due to network misconfiguration.

C The hub is running but has no available slots.

D The node's Docker image is outdated but still connects.

💡 Hint

Connection refused usually means no service is listening at the target address.

If the node cannot connect to the hub, it is often due to Docker network issues preventing communication between containers.

❓ framework

advanced

Choosing the Correct Docker Compose Service for Selenium Grid Hub

In a Docker Compose file for Selenium Grid, which service configuration correctly defines the hub to expose port 4444 and allow nodes to connect?

Selenium Java

services:
  selenium-hub:
    image: selenium/hub:4.10.0-20230628
    ports:
      - "4444:4444"
    environment:
      - GRID_MAX_SESSION=5
      - GRID_BROWSER_TIMEOUT=60
    # What else is needed here?

A Add 'network_mode: bridge' to allow communication with nodes on the same network.

B Add 'expose: 5555' to open the node port on the hub service.

C Add 'depends_on: [selenium-node-chrome]' to start nodes before the hub.

D Add 'restart: always' to ensure the hub restarts on failure.

💡 Hint

Nodes and hub must be on the same Docker network to communicate.

Setting 'network_mode: bridge' or defining a shared network allows containers to communicate internally.

🧠 Conceptual

expert

Understanding Session Distribution in Docker Selenium Grid

In a Selenium Grid setup with multiple Docker nodes, how does the hub decide which node to assign a new test session?

A The hub assigns sessions based on node availability and browser capabilities matching the test request.

B The hub assigns sessions randomly to any node regardless of browser type or load.

C The hub always assigns sessions to the newest node started in Docker.

D The hub assigns sessions only to nodes running on the same physical machine.

💡 Hint

Think about how the hub matches test requirements to node capabilities.

The hub matches requested browser and platform capabilities with nodes that have free slots and the required browsers.

Practice

1. What is the main purpose of using Docker Selenium Grid in automated testing?

easy

A. To debug Java code in Selenium tests

B. To write test scripts faster

C. To replace Selenium WebDriver completely

D. To run tests on multiple browsers and machines simultaneously

Solution

1. Step 1: Understand Docker Selenium Grid's role

   Docker Selenium Grid allows running tests on different browsers and machines at the same time, improving test coverage and speed.

2. Step 2: Compare options with this purpose

   Only To run tests on multiple browsers and machines simultaneously matches this main purpose. Options B, C, and D describe unrelated or incorrect uses.

3. Final Answer:

   To run tests on multiple browsers and machines simultaneously -> Option D

4. Quick Check:

   Purpose = run tests on many browsers [OK]

Hint: Docker Selenium Grid = run tests on many browsers [OK]

Common Mistakes:

• Thinking it speeds up writing tests
• Confusing it with Selenium WebDriver itself
• Assuming it is for debugging code

2. Which Docker command correctly starts a Selenium Grid Hub container?

easy

A. docker start selenium/node-chrome

B. docker build -t selenium/hub .

C. docker run -d -p 4444:4444 selenium/hub

D. docker stop selenium/hub

Solution

1. Step 1: Identify the command to run a container

   The command to start a container in detached mode and map port 4444 is 'docker run -d -p 4444:4444 selenium/hub'.

2. Step 2: Check other options

   docker start selenium/node-chrome starts an existing container, C builds an image, and D stops a container. Only B starts the hub container correctly.

3. Final Answer:

   docker run -d -p 4444:4444 selenium/hub -> Option C

4. Quick Check:

   Start hub = docker run -d -p 4444:4444 selenium/hub [OK]

Hint: Use 'docker run -d -p 4444:4444 selenium/hub' to start hub [OK]

Common Mistakes:

• Using 'docker start' instead of 'docker run' to create container
• Confusing build and run commands
• Stopping instead of starting the container

3. Given this Java code snippet connecting to a Docker Selenium Grid hub:

RemoteWebDriver driver = new RemoteWebDriver(
  new URL("http://localhost:4444/wd/hub"),
  new ChromeOptions()
);
driver.get("https://example.com");
System.out.println(driver.getTitle());

What will be printed if the page title is "Example Domain"?

medium

A. Example Domain

B. http://localhost:4444/wd/hub

C. null

D. NoSuchSessionException

Solution

1. Step 1: Understand RemoteWebDriver usage

   The code creates a RemoteWebDriver connected to the Selenium Grid hub URL and opens the page https://example.com.

2. Step 2: Check what driver.getTitle() returns

   driver.getTitle() returns the page title, which is "Example Domain" for that URL.

3. Final Answer:

   Example Domain -> Option A

4. Quick Check:

   driver.getTitle() = page title [OK]

Hint: driver.getTitle() prints page title loaded [OK]

Common Mistakes:

• Confusing URL with page title
• Expecting null if no errors
• Assuming exceptions without errors

4. You run this command to start a Chrome node:

docker run -d --link selenium-hub:hub selenium/node-chrome

But your tests fail to connect to the node. What is the likely problem?

medium

A. The Docker daemon is stopped

B. The node container is not linked properly to the hub

C. The Chrome browser is not installed inside the node container

D. The hub container is not running

Solution

1. Step 1: Analyze the linking option

   The '--link' option is deprecated and may not correctly link the node to the hub, causing connection failure.

2. Step 2: Check other possible causes

   Docker daemon (A) or hub not running (B) issues would cause broader failures. Chrome is preinstalled in official node images (C) so unlikely cause.

3. Final Answer:

   The node container is not linked properly to the hub -> Option B

4. Quick Check:

   Deprecated --link causes node-hub connection fail [OK]

Hint: Avoid deprecated --link; use network for node-hub [OK]

Common Mistakes:

• Assuming Chrome is missing inside node container
• Ignoring hub container status
• Not checking Docker daemon status

5. You want to run tests on Chrome and Firefox browsers simultaneously using Docker Selenium Grid. Which setup is correct?

hard

A. Start hub container, then start both selenium/node-chrome and selenium/node-firefox containers connected to the hub network

B. Start only selenium/node-chrome container; Firefox tests will run on Chrome node

C. Start hub container and run tests locally without nodes

D. Start selenium/node-firefox container first, then hub container

Solution

1. Step 1: Understand multi-browser setup

   To run tests on multiple browsers, you must start the hub and connect nodes for each browser to the hub's network.

2. Step 2: Evaluate options

   Start hub container, then start both selenium/node-chrome and selenium/node-firefox containers connected to the hub network correctly starts hub and both browser nodes connected properly. Start only selenium/node-chrome container; Firefox tests will run on Chrome node misses Firefox node, A misses nodes, and D starts nodes before hub which is incorrect.

3. Final Answer:

   Start hub container, then start both selenium/node-chrome and selenium/node-firefox containers connected to the hub network -> Option A

4. Quick Check:

   Hub + all browser nodes connected = multi-browser tests [OK]

Hint: Start hub first, then all browser nodes on same network [OK]

Common Mistakes:

• Starting nodes before hub
• Running tests without nodes
• Assuming one node can run all browsers