Health checks in Compose in Docker - Mini Project: Build & Apply

Health checks in Compose

📖 Scenario: You are setting up a simple web service using Docker Compose. To make sure your service is running correctly, you want to add a health check. This health check will help Docker know if your service is healthy or not.

🎯 Goal: Build a Docker Compose file that defines a web service with a health check. The health check will run a command inside the container to verify the service is working.

📋 What You'll Learn

Create a Docker Compose file with a service named web

Use the official nginx:alpine image for the web service

Add a health check to the web service that runs wget --no-verbose --tries=1 --spider http://localhost/

Set the health check interval to 10 seconds

Print the health status of the web service using docker compose ps

💡 Why This Matters

🌍 Real World

Health checks help keep your applications running smoothly by letting Docker know if a service is working correctly. This is important in real projects to restart or replace failing containers automatically.

💼 Career

Understanding health checks in Docker Compose is useful for DevOps roles, site reliability engineers, and developers who deploy applications using containers.

1

Create the basic Docker Compose file

Create a file named docker-compose.yml with a service called web that uses the image nginx:alpine.

Docker

# Create a docker-compose.yml file with a service named web using nginx:alpine
# Your code here

Hint

Use the services key and define web under it with the image set to nginx:alpine.

2

Add a health check command

Add a healthcheck section under the web service with a test command that runs wget --no-verbose --tries=1 --spider http://localhost/.

Docker

version: '3.8'
services:
  web:
    image: nginx:alpine
    # Add healthcheck with test command here
    # Your code here

Hint

The test command should be an array starting with CMD followed by the wget command.

3

Set the health check interval

Add the interval key under healthcheck with the value 10s to run the health check every 10 seconds.

Docker

version: '3.8'
services:
  web:
    image: nginx:alpine
    healthcheck:
      test: ["CMD", "wget", "--no-verbose", "--tries=1", "--spider", "http://localhost/"]
      # Add interval: 10s here
      # Your code here

Hint

The interval key controls how often the health check runs. Use 10s for 10 seconds.

4

Display the health status of the web service

Run the command docker compose ps in your terminal to see the health status of the web service.

Docker

# Run the command below in your terminal
# Your code here

Hint

Use docker compose ps to check the status. The health column should show healthy after some time.

Practice

1. What is the main purpose of a health check in a Docker Compose service?

easy

A. To restart the container automatically

B. To verify if the container's application is running correctly

C. To build the container image

D. To expose ports to the host machine

Solution

1. Step 1: Understand health check purpose

   Health checks run commands inside the container to confirm the app is working well.

2. Step 2: Differentiate from other Docker tasks

   Restarting, building, and exposing ports are separate Docker tasks, not health checks.

3. Final Answer:

   To verify if the container's application is running correctly -> Option B

4. Quick Check:

   Health check = verify app status [OK]

Hint: Health checks confirm app health inside containers [OK]

Common Mistakes:

• Confusing health checks with container restart policies
• Thinking health checks build images
• Mixing health checks with port mapping

2. Which of the following is the correct syntax to add a health check in a Docker Compose service?

easy

A. healthcheck: { run: "curl http://localhost" }

B. health_check: { command: "curl http://localhost" }

C. healthcheck: { test: ["CMD", "curl", "-f", "http://localhost"] }

D. healthcheck: { check: "curl http://localhost" }

Solution

1. Step 1: Recall Docker Compose healthcheck syntax

   The correct key is 'healthcheck' with a 'test' array specifying the command.

2. Step 2: Validate command format

   The 'test' must be an array starting with 'CMD' followed by the command and arguments.

3. Final Answer:

   healthcheck: { test: ["CMD", "curl", "-f", "http://localhost"] } -> Option C

4. Quick Check:

   Use 'healthcheck' with 'test' array [OK]

Hint: Use 'healthcheck' and 'test' array with CMD for commands [OK]

Common Mistakes:

• Using wrong keys like 'health_check' or 'run'
• Passing command as a string instead of array
• Omitting 'CMD' in the test array

3. Given this Compose snippet, what will be the health status if the curl command fails?

healthcheck:
  test: ["CMD", "curl", "-f", "http://localhost"]
  interval: 10s
  retries: 3

medium

A. The container will be marked as unhealthy after 3 failed attempts

B. The container will restart automatically after the first failure

C. The container will ignore the failure and stay healthy

D. The container will stop immediately on failure

Solution

1. Step 1: Understand retries and failure effect

   The health check runs every 10 seconds and allows 3 retries before marking unhealthy.

2. Step 2: Identify container behavior on failure

   Failing the test 3 times marks the container unhealthy; it does not restart or stop automatically.

3. Final Answer:

   The container will be marked as unhealthy after 3 failed attempts -> Option A

4. Quick Check:

   Retries control unhealthy marking, not restart [OK]

Hint: Retries count failures before unhealthy status [OK]

Common Mistakes:

• Assuming container restarts on health check failure
• Thinking container stops immediately on failure
• Ignoring the retries setting

4. You added this health check to your Compose file but it never reports healthy:

healthcheck:
  test: curl -f http://localhost || exit 1
  interval: 5s
  retries: 2

What is the problem?

medium

A. The test command must be an array starting with CMD

B. The interval value must be in milliseconds

C. Retries cannot be less than 3

D. The healthcheck key should be under volumes

Solution

1. Step 1: Check healthcheck test syntax

   Docker Compose requires the test command as an array starting with 'CMD' or 'CMD-SHELL'.

2. Step 2: Identify why string command fails

   Using a plain string without array causes Compose to ignore or fail the health check.

3. Final Answer:

   The test command must be an array starting with CMD -> Option A

4. Quick Check:

   Use array with CMD for test command [OK]

Hint: Healthcheck test must be array with CMD or CMD-SHELL [OK]

Common Mistakes:

• Writing test as plain string instead of array
• Misplacing healthcheck under wrong keys
• Using invalid interval units

5. You want a service in Docker Compose to be considered healthy only if a custom script check.sh inside the container returns success within 5 seconds. Which Compose healthcheck configuration is best?

hard

A. healthcheck:
test: ["CMD-SHELL", "./check.sh"]
interval: 10s
retries: 3

B. healthcheck:
test: ["CMD-SHELL", "timeout 5s ./check.sh"]
interval: 10s
retries: 3

C. healthcheck:
test: ["CMD", "./check.sh"]
interval: 5s
retries: 1

D. healthcheck:
test: ["CMD", "./check.sh"]
timeout: 5s
interval: 10s
retries: 3

Solution

1. Step 1: Understand timeout usage in healthcheck

   The 'timeout' key sets max time for the test command to run; here 5 seconds is needed.

2. Step 2: Choose correct test format and timing

   Using 'CMD' with './check.sh' is valid; 'timeout' controls max duration; interval and retries set frequency and tolerance.

3. Final Answer:

   healthcheck: test: ["CMD", "./check.sh"] timeout: 5s interval: 10s retries: 3 -> Option D

4. Quick Check:

   Use 'timeout' key to limit test duration [OK]

Hint: Use 'timeout' key to limit healthcheck command time [OK]

Common Mistakes:

• Using shell timeout inside test instead of 'timeout' key
• Confusing interval and timeout values
• Omitting retries leading to quick unhealthy status