Health checks in Compose in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a health check command in Docker Compose.

Docker

healthcheck:
  test: ["CMD", [1]]

Drag options to blanks, or click blank then click option'

A"echo healthy"

B"curl -f http://localhost"

C"ping -c 1 localhost"

D"curl -f http://localhost/ || exit 1"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a command that always succeeds like 'echo healthy'.

Omitting the 'exit 1' on failure.

The health check command must return 0 on success. Using curl -f http://localhost/ || exit 1 ensures failure if the service is down.

2fill in blank

medium

Complete the code to set the health check interval to 30 seconds.

Docker

healthcheck:
  interval: [1]

Drag options to blanks, or click blank then click option'

A10s

B30s

C1m

D5s

Attempts:

3 left

💡 Hint

Common Mistakes

Using '1m' which is 1 minute, not 30 seconds.

Using a number without 's' suffix.

The interval defines how often the health check runs. Setting it to 30s means every 30 seconds.

3fill in blank

hard

Fix the error in the health check restart policy syntax.

Docker

restart: [1]

Drag options to blanks, or click blank then click option'

Aunless-stopped

Bon-failure

Crestart-always

Dalways

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid value like 'restart-always'.

Confusing 'on-failure' with 'always'.

The correct restart policy to restart containers unless stopped manually is unless-stopped.

4fill in blank

hard

Fill both blanks to set retries to 5 and timeout to 10 seconds in health check.

Docker

healthcheck:
  retries: [1]
  timeout: [2]

Drag options to blanks, or click blank then click option'

A5

B3

C10s

D30s

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping retries and timeout values.

Using timeout without 's' suffix.

Retries define how many times to retry before marking unhealthy. Timeout is how long to wait for each check. Here, 5 retries and 10 seconds timeout are set.

5fill in blank

hard

Fill all three blanks to create a health check with command 'curl -f http://localhost/', interval 20 seconds, and start period 5 seconds.

Docker

healthcheck:
  test: ["CMD", [1]]
  interval: [2]
  start_period: [3]

Drag options to blanks, or click blank then click option'

A"curl -f http://localhost/ || exit 1"

B20s

C5s

D"curl http://localhost"

Attempts:

3 left

💡 Hint

Common Mistakes

Using incomplete curl command.

Omitting start_period or using wrong format.

The test command must fail if the service is unhealthy. Interval is how often to run the check. Start period is the initial delay before checks start.

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