Container health checks in Docker - Mini Project: Build & Apply

Container health checks

📖 Scenario: You are managing a web application running inside a Docker container. To keep the app reliable, you want Docker to check if the app is healthy and restart it if it is not working properly.

🎯 Goal: Learn how to add a health check to a Docker container using a Dockerfile. You will create a simple web server, add a health check command, and see the health status.

📋 What You'll Learn

Create a Dockerfile with a basic web server

Add a health check command using HEALTHCHECK

Use a simple command to test the server health

Build and run the Docker container

Check the container health status

💡 Why This Matters

🌍 Real World

In real projects, health checks help keep applications running smoothly by automatically restarting containers that are not responding.

💼 Career

Knowing how to add and use container health checks is important for DevOps roles to maintain reliable and self-healing systems.

1

Create a basic Dockerfile with a web server

Create a file named Dockerfile with these exact lines to set up a simple web server using nginx image: FROM nginx:alpine

Docker

# Create a Dockerfile with FROM nginx:alpine
# Your code here

Hint

This sets the base image to nginx on Alpine Linux, which is a small web server.

2

Add a health check command to the Dockerfile

Add a HEALTHCHECK instruction to the Dockerfile that runs the command curl -f http://localhost/ || exit 1 every 30 seconds with a timeout of 5 seconds. Use HEALTHCHECK --interval=30s --timeout=5s CMD curl -f http://localhost/ || exit 1.

Docker

FROM nginx:alpine
# Add HEALTHCHECK instruction below
# Your code here

Hint

The health check runs curl to check if the web server responds successfully.

3

Build and run the Docker container with health check

Build the Docker image with the tag mynginx using docker build -t mynginx .. Then run the container in detached mode with the name webserver using docker run -d --name webserver mynginx.

Docker

FROM nginx:alpine
HEALTHCHECK --interval=30s --timeout=5s CMD curl -f http://localhost/ || exit 1
# Build and run commands below
# Your code here

Hint

Use the exact commands to build and run the container so Docker can track health.

4

Check the container health status

Use the command docker ps to see the container status. The STATUS column should show Up (healthy) if the health check passes.

Docker

FROM nginx:alpine
HEALTHCHECK --interval=30s --timeout=5s CMD curl -f http://localhost/ || exit 1

# Build the image
# docker build -t mynginx .

# Run the container
# docker run -d --name webserver mynginx

# Check container health status
# Your code here

Hint

Look for the word healthy in the status column to confirm the health check works.

Practice

1. What is the main purpose of a HEALTHCHECK in a Docker container?

easy

A. To restart the Docker daemon automatically

B. To update the container's image

C. To check if the container's application is running correctly

D. To increase the container's CPU usage

Solution

1. Step 1: Understand the role of HEALTHCHECK

   The HEALTHCHECK instruction runs a command inside the container to verify if the app is working properly.

2. Step 2: Identify what HEALTHCHECK does not do

   It does not restart Docker daemon, update images, or increase CPU usage.

3. Final Answer:

   To check if the container's application is running correctly -> Option C

4. Quick Check:

   HEALTHCHECK verifies app status = B [OK]

Hint: HEALTHCHECK tests app inside container, not Docker itself [OK]

Common Mistakes:

• Confusing HEALTHCHECK with Docker daemon restart
• Thinking HEALTHCHECK updates images
• Assuming HEALTHCHECK changes resource usage

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

easy

A. HEALTHCHECK CMD curl -f http://localhost/ || exit 1

B. HEALTHCHECK RUN curl -f http://localhost/ || exit 1

C. HEALTHCHECK EXEC curl -f http://localhost/ || exit 1

D. HEALTHCHECK CHECK curl -f http://localhost/ || exit 1

Solution

1. Step 1: Recall Dockerfile HEALTHCHECK syntax

   The correct syntax uses HEALTHCHECK CMD followed by the command to run.

2. Step 2: Identify incorrect keywords

   RUN, EXEC, CHECK are not valid keywords for HEALTHCHECK instruction.

3. Final Answer:

   HEALTHCHECK CMD curl -f http://localhost/ || exit 1 -> Option A

4. Quick Check:

   HEALTHCHECK uses CMD keyword = C [OK]

Hint: HEALTHCHECK always uses CMD keyword in Dockerfile [OK]

Common Mistakes:

• Using RUN instead of CMD in HEALTHCHECK
• Using EXEC or CHECK which are invalid
• Omitting CMD keyword entirely

3. Given this Dockerfile snippet:

HEALTHCHECK CMD curl -f http://localhost/health || exit 1
CMD ["./start-server.sh"]

What will be the health status if curl fails?

medium

A. unhealthy

B. unknown

C. starting

D. healthy

Solution

1. Step 1: Understand HEALTHCHECK command behavior

   If the command returns a non-zero exit code (like exit 1), Docker marks the container as unhealthy.

2. Step 2: Analyze the curl failure effect

   Since curl -f fails, it triggers exit 1, so health status becomes unhealthy.

3. Final Answer:

   unhealthy -> Option A

4. Quick Check:

   Failed HEALTHCHECK command = unhealthy [OK]

Hint: Failed health check command sets status to unhealthy [OK]

Common Mistakes:

• Assuming failure means healthy
• Confusing 'starting' with unhealthy
• Thinking status stays unknown on failure

4. You wrote this in your Dockerfile:

HEALTHCHECK CMD curl http://localhost/health || exit 1

But your container health status is always starting. What is the likely problem?

medium

A. The container has no network access

B. The HEALTHCHECK syntax is invalid

C. Docker daemon is not running

D. The health check command never succeeds within the start period

Solution

1. Step 1: Understand 'starting' health status meaning

   'starting' means the container is still within the initial grace period before health checks are considered final.

2. Step 2: Analyze why health check stays 'starting'

   If the command never succeeds during this period, Docker keeps status as 'starting'. Syntax is correct, Docker daemon running, and network likely accessible.

3. Final Answer:

   The health check command never succeeds within the start period -> Option D

4. Quick Check:

   Failing health check in start period = starting [OK]

Hint: Starting means health check not yet passed in grace period [OK]

Common Mistakes:

• Assuming syntax error causes 'starting' status
• Blaming Docker daemon when it runs fine
• Ignoring network issues but they are less likely here

5. You want to create a health check that retries 3 times before marking unhealthy and waits 10 seconds between checks. Which Dockerfile snippet correctly sets this up?

hard

A. HEALTHCHECK --timeout=10s --retries=3 CMD curl -f http://localhost/health || exit 1

B. HEALTHCHECK --interval=10s --retries=3 CMD curl -f http://localhost/health || exit 1

C. HEALTHCHECK --interval=3s --retries=10 CMD curl -f http://localhost/health || exit 1

D. HEALTHCHECK --start-period=10s --retries=3 CMD curl -f http://localhost/health || exit 1

Solution

1. Step 1: Identify correct flags for interval and retries

   --interval sets wait time between checks; --retries sets how many failures before unhealthy.

2. Step 2: Match values to requirements

   We want 10 seconds between checks and 3 retries, so --interval=10s --retries=3 is correct. Timeout and start-period are different settings.

3. Final Answer:

   HEALTHCHECK --interval=10s --retries=3 CMD curl -f http://localhost/health || exit 1 -> Option B

4. Quick Check:

   Interval=10s and retries=3 flags = A [OK]

Hint: Use --interval for wait, --retries for failure count [OK]

Common Mistakes:

• Confusing timeout with interval
• Swapping retries and interval values
• Using start-period instead of interval