Health checks in Docker in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple health check endpoint in Spring Boot.

Spring Boot

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HealthController {
    @GetMapping("/health")
    public String [1]() {
        return "OK";
    }
}

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

Ahealth

BcheckHealth

ChealthCheck

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated method names that confuse the purpose.

The method name can be anything, but health matches the endpoint and is clear.

2fill in blank

medium

Complete the Dockerfile line to add a health check that calls the Spring Boot health endpoint.

Spring Boot

HEALTHCHECK --interval=30s --timeout=5s CMD curl -f http://localhost:8080/[1] || exit 1

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

Astatus

Bhealth

Cping

Dcheck

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect endpoint paths that cause health check failures.

The health endpoint is usually /health in Spring Boot apps.

3fill in blank

hard

Fix the error in the Dockerfile health check command to correctly check the Spring Boot app health.

Spring Boot

HEALTHCHECK --interval=30s --timeout=5s CMD curl -f http://localhost:8080/[1] || exit 1

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

Alive

Bhealthz

Cstatus

Dhealth

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent endpoints causing health check failures.

The correct health endpoint is /health. Others will cause errors.

4fill in blank

hard

Fill both blanks to create a Spring Boot actuator health check configuration and Dockerfile health check command.

Spring Boot

management.endpoint.health.[1]=true
HEALTHCHECK --interval=30s CMD curl -f http://localhost:8080/actuator/[2] || exit 1

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

Aenabled

Bdisabled

Chealth

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Disabling the endpoint or using wrong endpoint paths.

Enable the health endpoint in Spring Boot actuator with enabled and check /actuator/health in Docker.

5fill in blank

hard

Fill all three blanks to define a custom health indicator bean and use it in the health endpoint.

Spring Boot

import org.springframework.boot.actuate.health.Health;
import org.springframework.boot.actuate.health.HealthIndicator;
import org.springframework.stereotype.Component;

@Component
public class CustomHealthIndicator implements HealthIndicator {
    @Override
    public Health [1]() {
        boolean serverUp = checkServer();
        if (serverUp) {
            return Health.[2]().withDetail("server", "Up").build();
        } else {
            return Health.[3]().withDetail("server", "Down").build();
        }
    }

    private boolean checkServer() {
        // Custom logic here
        return true;
    }
}

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

Ahealth

Bup

Cdown

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Wrong method names or using incorrect Health status methods.

The method overriding HealthIndicator is health(). Use Health.up() and Health.down() to indicate status.

Practice

1.

What is the main purpose of adding a HEALTHCHECK instruction in a Dockerfile for a Spring Boot application?

easy

A. To let Docker monitor if the app is running correctly and restart if needed

B. To speed up the app startup time inside the container

C. To reduce the container size by removing unused files

D. To automatically update the Spring Boot version inside the container

Solution

1. Step 1: Understand the role of HEALTHCHECK in Docker

   The HEALTHCHECK instruction tells Docker how to test if the container's app is healthy.

2. Step 2: Connect HEALTHCHECK to app monitoring

   It helps Docker detect problems and restart the app if it stops working properly.

3. Final Answer:

   To let Docker monitor if the app is running correctly and restart if needed -> Option A

4. Quick Check:

   HEALTHCHECK monitors app health [OK]

Hint: HEALTHCHECK = Docker watches app health and restarts if unhealthy [OK]

Common Mistakes:

• Thinking HEALTHCHECK speeds up app startup
• Confusing HEALTHCHECK with image size optimization
• Assuming HEALTHCHECK updates app versions automatically

2.

Which of the following is the correct syntax to add a health check in a Dockerfile for a Spring Boot app that exposes /actuator/health on port 8080?

?

easy

A. HEALTHCHECK RUN curl http://localhost:8080/actuator/health

B. HEALTHCHECK CMD curl --fail http://localhost:8080/actuator/health || exit 1

C. HEALTHCHECK CMD ping localhost:8080/actuator/health

D. HEALTHCHECK EXEC curl http://localhost:8080/actuator/health

Solution

1. Step 1: Identify correct HEALTHCHECK syntax

   The HEALTHCHECK instruction uses CMD followed by a command that returns 0 on success or non-zero on failure.

2. Step 2: Verify the command tests the health endpoint properly

   Using curl with --fail returns error if the endpoint is unhealthy, triggering Docker restart.

3. Final Answer:

   HEALTHCHECK CMD curl --fail http://localhost:8080/actuator/health || exit 1 -> Option B

4. Quick Check:

   HEALTHCHECK CMD + curl --fail = correct syntax [OK]

Hint: Use HEALTHCHECK CMD with curl --fail for health endpoint [OK]

Common Mistakes:

• Using RUN instead of CMD in HEALTHCHECK
• Using ping instead of curl for HTTP check
• Using EXEC keyword which is invalid

3.

Given this Dockerfile snippet, what will Docker report as the container health status if the Spring Boot app's /actuator/health endpoint returns HTTP 500?

HEALTHCHECK CMD curl --fail http://localhost:8080/actuator/health || exit 1

medium

A. healthy

B. unknown

C. starting

D. unhealthy

Solution

1. Step 1: Understand curl --fail behavior on HTTP 500

   The curl --fail option causes curl to exit with error on HTTP status codes 400 or above.

2. Step 2: Connect curl failure to Docker health status

   Since curl fails, the HEALTHCHECK command exits with 1, marking container as unhealthy.

3. Final Answer:

   unhealthy -> Option D

4. Quick Check:

   HTTP 500 triggers curl failure = unhealthy [OK]

Hint: curl --fail fails on HTTP 500, so container becomes unhealthy [OK]

Common Mistakes:

• Assuming HTTP 500 still means healthy
• Confusing 'starting' with unhealthy status
• Thinking Docker ignores health check failures

4.

Identify the error in this Dockerfile health check instruction for a Spring Boot app:

HEALTHCHECK CMD curl http://localhost:8080/actuator/health || exit 1

medium

A. Incorrect URL path for health endpoint

B. Using CMD instead of RUN for HEALTHCHECK

C. Missing --fail option in curl to detect HTTP errors

D. Missing port number in the URL

Solution

1. Step 1: Check curl command correctness for health check

   Without --fail, curl returns success even if HTTP status is error (like 500).

2. Step 2: Understand impact on Docker health status

   Docker will wrongly mark container healthy because curl does not fail on bad HTTP codes.

3. Final Answer:

   Missing --fail option in curl to detect HTTP errors -> Option C

4. Quick Check:

   curl needs --fail to detect HTTP errors [OK]

Hint: Always add --fail to curl in HEALTHCHECK to catch errors [OK]

Common Mistakes:

• Using RUN instead of CMD in HEALTHCHECK
• Wrong health endpoint URL
• Forgetting port number in URL

5.

You want Docker to check your Spring Boot app health every 30 seconds, retry 3 times before marking unhealthy, and start checking only after 10 seconds of container start. Which HEALTHCHECK instruction correctly implements this?

hard

A. HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 CMD curl --fail http://localhost:8080/actuator/health || exit 1

B. HEALTHCHECK --interval=10s --timeout=30s --start-period=3s --retries=5 CMD curl --fail http://localhost:8080/actuator/health || exit 1

C. HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 CMD curl http://localhost:8080/actuator/health || exit 1

D. HEALTHCHECK CMD curl --fail http://localhost:8080/actuator/health || exit 1

Solution

1. Step 1: Match timing parameters to requirements

   Interval 30s means check every 30 seconds, start-period 10s delays first check, retries 3 means try 3 times before unhealthy.

2. Step 2: Verify curl command correctness

   curl uses --fail to detect HTTP errors, with exit 1 to signal failure to Docker.

3. Final Answer:

   HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 CMD curl --fail http://localhost:8080/actuator/health || exit 1 -> Option A

4. Quick Check:

   Correct timing + curl --fail = HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 CMD curl --fail http://localhost:8080/actuator/health || exit 1 [OK]

Hint: Set interval, retries, start-period explicitly with curl --fail [OK]

Common Mistakes:

• Mixing up interval and timeout values
• Omitting --fail in curl command
• Not setting start-period to delay checks