Container health checks in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of the health check status command?

You have a Docker container running with a health check configured. You run the command docker inspect --format='{{.State.Health.Status}}' mycontainer. What output do you expect if the container is healthy?

A starting

B unhealthy

C healthy

D exited

💡 Hint

Think about the possible health states Docker reports.

The .State.Health.Status field shows the current health status of the container. If the container passes all health checks, it reports healthy.

🧠 Conceptual

intermediate

Which Dockerfile instruction defines a health check?

In a Dockerfile, which instruction is used to define a container health check?

A STATUSCHECK

B HEALTHCHECK

C RUNHEALTH

D CHECKHEALTH

💡 Hint

It is a single word instruction starting with HEALTH.

The HEALTHCHECK instruction in a Dockerfile defines how Docker tests the container's health.

❓ Configuration

advanced

Identify the correct health check configuration in Dockerfile

Which of the following Dockerfile snippets correctly configures a health check that runs curl -f http://localhost/ || exit 1 every 30 seconds with a timeout of 10 seconds?

A HEALTHCHECK CMD curl -f http://localhost/ || exit 1 --interval=30s --timeout=10s

B HEALTHCHECK --timeout=10s --interval=30s RUN curl -f http://localhost/ || exit 1

C HEALTHCHECK CMD --interval=30s --timeout=10s curl -f http://localhost/ || exit 1

D HEALTHCHECK --interval=30s --timeout=10s CMD curl -f http://localhost/ || exit 1

💡 Hint

Options flags come immediately after HEALTHCHECK, before CMD.

The correct syntax places options like --interval and --timeout right after HEALTHCHECK and before CMD. The command follows CMD.

❓ Troubleshoot

advanced

Why does the health check always show 'starting' status?

You configured a health check in your Docker container, but docker inspect always shows the status as starting. What is the most likely cause?

A The health check command never exits or takes longer than the timeout

B The container is stopped and not running

C The Docker daemon is not running

D The health check command is missing from the Dockerfile

💡 Hint

Think about what 'starting' means in health check lifecycle.

If the health check command never finishes or exceeds the timeout, Docker keeps the status at starting because it never gets a success or failure result.

✅ Best Practice

expert

What is the best practice for health check command exit codes?

For Docker container health checks, which exit code convention should the health check command follow to indicate a healthy or unhealthy state?

A Exit code 0 means healthy; any non-zero exit code means unhealthy

B Exit code 1 means healthy; exit code 0 means unhealthy

C Exit code 2 means healthy; exit code 1 means unhealthy

D Exit code 0 means unhealthy; exit code 1 means healthy

💡 Hint

Think about standard Unix command exit codes.

By Unix convention, exit code 0 means success (healthy), and any non-zero exit code means failure (unhealthy). Docker health checks follow this.

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