Health checks in Docker in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding Docker HEALTHCHECK command behavior

Consider a Dockerfile with the following HEALTHCHECK instruction:

HEALTHCHECK --interval=10s --timeout=5s CMD curl -f http://localhost:8080/actuator/health || exit 1

What will Docker report as the container health status if the Spring Boot app's /actuator/health endpoint returns HTTP 503 for 3 consecutive checks?

A Docker will restart the container immediately after the first failed health check.

B The container health status will remain 'healthy' because the app is running.

C The container health status will be 'starting' indefinitely.

D The container health status will be 'unhealthy' after the third failed check.

💡 Hint

Docker marks a container unhealthy only after consecutive failed health checks.

Docker runs the HEALTHCHECK command periodically. If the command fails consecutively (default 3 times), Docker marks the container as 'unhealthy'. HTTP 503 causes curl -f to fail, so after 3 failures, status is 'unhealthy'.

📝 Syntax

intermediate

Correct syntax for Docker HEALTHCHECK with Spring Boot

Which of the following Dockerfile HEALTHCHECK instructions correctly checks the Spring Boot actuator health endpoint at http://localhost:8080/actuator/health?

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

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

C HEALTHCHECK CMD curl -s http://localhost:8080/actuator/health

D HEALTHCHECK CMD curl -f http://localhost:8080/actuator/health && exit 1

💡 Hint

The -f flag makes curl fail on HTTP errors.

Option A uses 'curl -f' which fails on HTTP errors, triggering the health check failure correctly. Option A lacks -f so it won't fail on HTTP errors. Option A is silent but does not fail on errors. Option A wrongly uses && exit 1 which always exits with failure.

❓ state_output

advanced

Spring Boot actuator health status effect on Docker health

If a Spring Boot application exposes /actuator/health with the following JSON response:

{"status":"DOWN","components":{"db":{"status":"DOWN"}}}

and the Docker HEALTHCHECK runs:

curl -f http://localhost:8080/actuator/health || exit 1

What will be the Docker container health status?

A healthy, because the HTTP status code is 200 even if status is DOWN

B unhealthy, because the JSON contains 'DOWN' status

C starting, because the health endpoint is not reachable

D unhealthy, because the HTTP status code is 503 or 500

💡 Hint

Docker health checks rely on command exit codes, not JSON content.

The curl command fails only if HTTP status is an error (4xx or 5xx). Spring Boot actuator returns HTTP 200 even if status is DOWN. So curl -f succeeds, Docker marks container healthy.

🔧 Debug

advanced

Diagnosing failing Docker health check with Spring Boot

A Docker container running a Spring Boot app has this Dockerfile line:

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

But Docker always shows the container as unhealthy. Which of the following is the most likely cause?

A Docker does not support health checks for Spring Boot apps.

B The curl command is missing the -f flag to fail on HTTP errors.

C The Spring Boot actuator endpoint is disabled or not exposed on port 8080.

D The Dockerfile is missing EXPOSE 8080 instruction.

💡 Hint

Check if the health endpoint is reachable inside the container.

If the actuator endpoint is disabled or the app listens on a different port, curl fails, causing health check failure. Missing -f flag would not cause failure. Docker supports health checks. EXPOSE is optional for health checks.

🧠 Conceptual

expert

Best practice for integrating Spring Boot health with Docker health checks

Which approach best ensures Docker health status accurately reflects Spring Boot application health?

A Parse the JSON response from /actuator/health in a custom script inside Docker HEALTHCHECK.

B Configure Spring Boot actuator to return HTTP 503 when unhealthy and use curl -f in Docker HEALTHCHECK.

C Use Docker HEALTHCHECK to ping any open port without checking application health.

D Disable Docker HEALTHCHECK and rely on container restart policies only.

💡 Hint

Think about how Docker interprets health check command success or failure.

Configuring Spring Boot to return HTTP 503 when unhealthy allows curl -f to fail, signaling Docker correctly. Parsing JSON in Docker HEALTHCHECK is complex and error-prone. Pinging ports does not reflect app health. Disabling health checks loses health visibility.