Health checks in Docker in Spring Boot

Introduction

Health checks help Docker know if your Spring Boot app is running well or needs restarting. This keeps your app reliable and available.

When you want Docker to restart your app if it crashes or becomes unresponsive.

When running your Spring Boot app in a container and you want to monitor its status automatically.

When deploying microservices and you need to ensure each service is healthy before sending traffic.

When you want to avoid manual checks and let Docker manage app health.

When you want to improve uptime by detecting problems early.

Syntax

Spring Boot

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

This Dockerfile instruction tells Docker how to check your app's health.

It uses Spring Boot's built-in /actuator/health endpoint to verify the app is running.

Examples

Basic health check with default timing, checks the health endpoint.

Spring Boot

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

Health check runs every 10 seconds and waits up to 3 seconds for a response.

Spring Boot

HEALTHCHECK --interval=10s --timeout=3s CMD curl -f http://localhost:8080/actuator/health || exit 1

Gives the app 20 seconds to start before checking and retries 5 times before failing.

Spring Boot

HEALTHCHECK --start-period=20s --retries=5 CMD curl -f http://localhost:8080/actuator/health || exit 1

Sample Program

This Dockerfile builds a container for a Spring Boot app. It exposes port 8080 and adds a health check that calls the app's /actuator/health endpoint every 30 seconds. If the app is unhealthy, Docker can restart it automatically.

Spring Boot

FROM openjdk:17-jdk-slim
WORKDIR /app
COPY target/myapp.jar ./myapp.jar
EXPOSE 8080
HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
  CMD curl -f http://localhost:8080/actuator/health || exit 1
ENTRYPOINT ["java", "-jar", "myapp.jar"]

Important Notes

Make sure your Spring Boot app has the actuator dependency and the health endpoint enabled.

You can customize the health endpoint or create your own for more specific checks.

Use Docker commands like docker ps and docker inspect to see health status.

Summary

Health checks let Docker watch your Spring Boot app's health automatically.

Use the /actuator/health endpoint as the check target.

Configure timing options to fit your app's startup and response times.

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