Container health checks in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a container health check in Docker?

A container health check is a command or script that Docker runs inside a container to check if the container is working properly. It helps to know if the app inside the container is healthy or needs attention.

beginner

Which Dockerfile instruction is used to define a health check?

The HEALTHCHECK instruction is used in a Dockerfile to specify how Docker should test the container's health by running a command inside it.

beginner

What does the HEALTHCHECK CMD command do?

It runs a command inside the container at regular intervals. If the command succeeds, the container is healthy. If it fails, Docker marks the container as unhealthy.

beginner

What are the possible health statuses Docker can show for a container?

Docker can show three health statuses: starting (health check is running but not ready), healthy (container passed the health check), and unhealthy (container failed the health check).

intermediate

How can health checks improve container management?

Health checks help automatically detect problems inside containers. This allows tools to restart unhealthy containers or alert users, making apps more reliable and easier to maintain.

Which Dockerfile instruction defines a container health check?

A CMD

B HEALTHCHECK

C RUN

D EXPOSE

The HEALTHCHECK instruction tells Docker how to test the container's health.

What does Docker do if a health check command fails?

A Marks the container as unhealthy

B Stops the container immediately

C Ignores the failure

D Restarts the Docker daemon

Docker marks the container as unhealthy if the health check command fails.

How often does Docker run the health check command by default?

A Every 30 seconds

B Every 5 minutes

C Every 1 second

D Every hour

By default, Docker runs the health check every 30 seconds.

What is the purpose of the HEALTHCHECK CMD in a Dockerfile?

A To start the container

B To copy files into the container

C To expose ports

D To specify the command Docker runs to check container health

HEALTHCHECK CMD defines the command Docker uses to test if the container is healthy.

Which health status means the container is still starting and health checks are not yet conclusive?

A unhealthy

B healthy

C starting

D stopped

The 'starting' status means the container is initializing and health checks are in progress.

Explain how to add a health check to a Docker container using a Dockerfile.

Think about the Dockerfile instruction and what it does.

Why are container health checks important in managing Docker containers?

Consider how health checks help keep apps running smoothly.

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