docker-compose.yaml

docker-compose allow us to start multiple container services at a time. The configuration is written in YAML format

Sample

Lets assume we have a EmployeeApp built using SpringBoot REST & uses MySQL for DB. Below is the sample for doing end to end containerization of the EmployeeApp application.

Dockerfile

FROM openjdk:8-jdk-alpine
COPY EmployeeApp.jar EmployeeApp.jar
CMD ["java", "-jar", "EmpApp.jar"]

docker-compose.yaml

version : '3'
services :
  webapp :
    build : .
    image : employee-app
    ports :
      - "8080:8080"
  dbapp :
    image : mysql:5.7
    restart : always
    environment :
      MYSQL_DATABASE : 'MY_DB'
      MYSQL_ROOT_PASSWORD : 'rootroot'
    expose :
      - "3306"
    ports :
      - "3306:3306"
    volumes :
      - ./sqlscript:/docker-entrypoint-initdb.d
      - ./data:/var/lib/mysql

docker-compose file explained

Docker-compose	Value	Description
version	3	Version of Docker compose we want to use. It should be the first line in docker-compose.yaml file
services		Configuration for each Docker container we want, is specified under services
webapp		User provided name for the Spring Boot web app service
build	. (Dot)	Specify Dockerfile location. Use (Dot) if the Dockerfile & docker-compose.yaml are in same directory
image	my-img	Custom Image name
ports	8080:8080	Ports we want to expose outside of Docker container. The format is <HOST_IP>:<CONTAINER_IP>
dbapp		User provided name for the Database container service
image	mysql:5.7	Image name & its version to download from docker hub
restart	always	To restart the container every time
environment		Environment variables we want to pass while starting the MySQL Container
MYSQL_DATABASE	MY_DB	Database to be created on Container startup
MYSQL_ROOT_PASSWORD	Rootroot	Password for MySQL Root user account
expose	8080	Port to be exposed for inter container communication
ports	8080:8080	Ports we want to expose outside of Docker container. The format is <HOST_IP>:<CONTAINER_IP>
volumes	./data:/var/lib/mysql	Volumes we want to mount from host to Docker container

Starting multiple containers with docker-compose.yaml

In order to start the application, use either of the following commands

> docker-compose up

or

> docker-compose up --build

---

Networking in Docker

In above docker-compose sample, we have started two containers, one for Employee Web Application & other is for MySQL Database. Typically when we want to connect Web application with MySQL, we use connection string like below

spring.datasource.url=jdbc:mysql://<b>localhost:3306</b>/MY_DB?useSSL=false&allowPublicKeyRetrieval=true

3306 is the default port of MySQL server.

After we containerized the entire application, MySQL will run in its own container (read as its own operating system) whereas the Spring Boot application will be running in its own container. So localhost of SpringBoot container is not the localhost of MySQL container. Hence connecting with localhost:3306 from SpringBoot container is not gonna work

Hence we need to rely on the networking capabilities of the docker-compose to make inter-container communication possible

When we start application using docker-compose up, docker-compose creates a default network bridge between all the containers it start. We can refer to any container started by docker-compose using this default network bridge using the name given for the container in docker-compose.yaml file

Fixing DB Connection between MySQL container and SpringBoot container

Replace the localhost in connection string with name given for docker container in docker-compose.yaml file can fix the inter-container communication

spring.datasource.url=jdbc:mysql://<del>localhost</del>:3306/ORDER_DB?useSSL=false&allowPublicKeyRetrieval=true

spring.datasource.url=jdbc:mysql://**dbapp**:3306/ORDER_DB?useSSL=false&allowPublicKeyRetrieval=true

dbapp is nothing but the name we given for the MySQL image in docker-compose.yaml file. Refer to line 31 above for dpapp