Health checks in Compose in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a health check in Docker Compose?

A health check is a way to test if a container is working correctly by running a command inside it regularly. It helps Docker know if the app inside the container is healthy or not.

beginner

Which key is used to define a health check in a Docker Compose service?

The key healthcheck is used under a service in the docker-compose.yml file to define commands and settings for health checks.

intermediate

What does the test option inside a health check specify?

The test option defines the command Docker runs inside the container to check if it is healthy. For example, test: ["CMD", "curl", "-f", "http://localhost"].

intermediate

What happens if a health check fails multiple times in Docker Compose?

If the health check command fails repeatedly, Docker marks the container as unhealthy. This can be used to trigger restarts or alerts.

intermediate

Name two timing options you can set for a health check in Docker Compose.

You can set interval (how often to run the check) and timeout (how long to wait for the check to finish). Others include retries and start_period.

Which key defines a health check in a Docker Compose service?

A testhealth

B checkhealth

C health

D healthcheck

The correct key to define health checks in Docker Compose is healthcheck.

What does the test option inside a health check do?

A Runs a command inside the container to check health

B Sets the health check interval

C Defines the container restart policy

D Specifies the container image

The test option runs a command inside the container to verify if it is healthy.

If a health check fails repeatedly, Docker marks the container as:

A unhealthy

B stopped

C paused

D running

Repeated health check failures cause Docker to mark the container as unhealthy.

Which of these is NOT a valid health check timing option?

A retries

B timeout

C restart

D interval

restart is not a health check timing option; it is a separate container policy.

How often does Docker run the health check command by default if not specified?

A 10 seconds

B 30 seconds

C 1 minute

D 5 seconds

By default, Docker runs health checks every 30 seconds if interval is not set.

Explain how to add a health check to a service in a Docker Compose file.

Think about the keys under the service in docker-compose.yml

Describe what happens when a container's health check fails multiple times.

Focus on Docker's reaction to health check results

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