docker-compose for services in Spring Boot - Step-by-Step Execution

Concept Flow - docker-compose for services

Write docker-compose.yml

↓

Define services

↓

Set images, ports, env

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Run docker-compose up

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Docker creates containers

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Services start and communicate

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Access services via ports

This flow shows how you write a docker-compose file, define services, run it, and how containers start and communicate.

Execution Sample

Spring Boot

version: '3.8'
services:
  app:
    image: my-springboot-app
    ports:
      - "8080:8080"
  db:
    image: postgres
    environment:
      POSTGRES_PASSWORD: example

This docker-compose file defines two services: a Spring Boot app and a Postgres database, exposing app on port 8080.

Execution Table

Step	Action	Details	Result
1	Read docker-compose.yml	Parse version and services	Services 'app' and 'db' recognized
2	Pull images	Pull 'my-springboot-app' and 'postgres' images	Images downloaded or found locally
3	Create containers	Create container for 'app' and 'db'	Containers created but not started
4	Start containers	Start 'db' container first	Postgres database starts running
5	Start 'app' container	Spring Boot app container starts	App starts and tries to connect to db
6	Map ports	Host port 8080 maps to app container port 8080	App accessible at localhost:8080
7	Containers running	Services communicate internally	App can use database
8	Exit	docker-compose stops or user interrupts	Containers stop and exit

💡 Execution stops when containers are stopped or docker-compose is terminated

Variable Tracker

Variable	Start	After Step 3	After Step 5	Final
app container	none	created	running	running
db container	none	created	running	running
app port mapping	none	none	8080:8080	8080:8080
db environment	none	set POSTGRES_PASSWORD=example	set POSTGRES_PASSWORD=example	set POSTGRES_PASSWORD=example

Key Moments - 3 Insights

Why does the database container start before the app container?

In the execution_table at step 4 and 5, the db container starts first so the app can connect to it when it starts.

How does port mapping work in docker-compose?

Step 6 shows host port 8080 maps to container port 8080, making the app accessible on localhost:8080.

What happens if the images are not found locally?

Step 2 shows docker-compose pulls the images from the registry before creating containers.

Visual Quiz - 3 Questions

Look at the execution_table, at which step are the containers created but not started?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Check the 'Result' column in execution_table row for Step 3

According to variable_tracker, what is the state of the 'app container' after Step 5?

A none

B running

C created

D stopped

💡 Hint

Look at the 'app container' row under 'After Step 5' column in variable_tracker

If the POSTGRES_PASSWORD environment variable was missing, what step would be affected?

A Step 4 - Start db container

B Step 2 - Pull images

C Step 6 - Map ports

D Step 7 - Services communicate

💡 Hint

Check environment variable setup in variable_tracker and when db container starts in execution_table

Concept Snapshot

docker-compose.yml defines multiple services
Each service has image, ports, environment
Run 'docker-compose up' to start all
Containers start in order, can communicate
Ports map container to host for access
Environment variables configure services

Full Transcript

This visual trace shows how docker-compose manages multiple services for a Spring Boot app and a database. First, the docker-compose.yml file is read and parsed to find service definitions. Then, docker pulls the required images if not present locally. Containers for each service are created but not started immediately. The database container starts first to be ready for connections. Next, the app container starts and connects to the database. Ports are mapped from container to host so users can access the app on localhost. The services run and communicate internally. Finally, when docker-compose is stopped, containers exit. Variables like container states and environment variables change step-by-step as shown. This helps beginners see how docker-compose orchestrates multiple services simply and reliably.