Health checks in Docker in Spring Boot - Step-by-Step Execution

Concept Flow - Health checks in Docker

Start Docker Container

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Docker runs Health Check Command

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Health Check Command executes Spring Boot Actuator Endpoint

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Spring Boot returns status (UP/DOWN)

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Docker receives status

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Docker marks container as healthy or unhealthy

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Docker takes action based on health (restart, alert, etc.)

Docker runs a health check command that calls Spring Boot's actuator endpoint to get app status, then marks container health accordingly.

Execution Sample

Spring Boot

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

Docker runs this command every 10 seconds to check if the Spring Boot app is healthy.

Execution Table

Step	Action	Command Output	Docker Health Status	Container State
1	Docker starts container	N/A	starting	container running
2	Docker runs health check command	HTTP 200 {"status":"UP"}	healthy	container running
3	Wait 10 seconds	N/A	healthy	container running
4	Docker runs health check command	HTTP 200 {"status":"UP"}	healthy	container running
5	Wait 10 seconds	N/A	healthy	container running
6	Docker runs health check command	curl: (7) Failed to connect	unhealthy	container running
7	Docker marks container unhealthy	N/A	unhealthy	container running
8	Docker restarts container	N/A	starting	container restarting
9	Docker runs health check command	HTTP 200 {"status":"UP"}	healthy	container running

💡 Docker stops unhealthy state by restarting container after failed health check

Variable Tracker

Variable	Start	After 2	After 4	After 6	After 8	After 9
Docker Health Status	starting	healthy	healthy	unhealthy	starting	healthy
Container State	container running	container running	container running	container running	container restarting	container running

Key Moments - 3 Insights

Why does Docker mark the container unhealthy even though it is still running?

Docker bases health on the health check command result, not container running state. See step 6 in execution_table where command fails but container is still running.

What happens after Docker marks the container unhealthy?

Docker restarts the container to try to fix the problem, as shown in step 8 and 9 in execution_table.

Why use Spring Boot actuator endpoint for health check?

Because it returns a simple UP/DOWN status that Docker can easily interpret, shown in steps 2 and 4 where HTTP 200 with status UP means healthy.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Docker Health Status at step 4?

A starting

B unhealthy

C healthy

D restarting

💡 Hint

Check the 'Docker Health Status' column at step 4 in execution_table

At which step does the health check command fail causing Docker to mark the container unhealthy?

A Step 2

B Step 6

C Step 4

D Step 8

💡 Hint

Look for 'curl: (7) Failed to connect' in 'Command Output' column in execution_table

If the health check command always returns HTTP 200 with status UP, what will Docker do?

A Keep container healthy and running

B Restart container repeatedly

C Mark container unhealthy

D Stop the container

💡 Hint

Refer to steps 2 and 4 in execution_table where HTTP 200 means healthy

Concept Snapshot

Docker health checks run a command periodically.
Spring Boot actuator /health endpoint returns app status.
Docker marks container healthy if command succeeds.
If command fails, Docker marks unhealthy and can restart.
Use HEALTHCHECK in Dockerfile with curl to actuator.
This helps keep apps running smoothly.

Full Transcript

This visual execution shows how Docker performs health checks on a Spring Boot application. Docker starts the container and runs a health check command every 10 seconds. The command calls the Spring Boot actuator health endpoint. If the endpoint returns HTTP 200 with status UP, Docker marks the container healthy. If the command fails, Docker marks the container unhealthy but the container keeps running. After marking unhealthy, Docker restarts the container to recover. Variables like Docker Health Status and Container State change step by step. Key moments include understanding that health is based on the command result, not container running state, and that Docker restarts unhealthy containers. The visual quiz tests understanding of health status at different steps and Docker's reaction to health check results.