• Docker Compose Basics
• How docker-compose up/down Handles Images and Containers

Docker Compose Basics

You could organize your docker container configurations and create multiple containers at once with docker-compose.

version: '3'

services:
  nats-mon:
    container_name: nats-mon
    image: nats-mon:latest
    ports:
      - 8282:8282
    environment:
      - STAN_URL=nats://db01.d2sphere.com:4222
      - STAN_MONITOR_URL=http://db01.d2sphere.com:8222
      - STAN_CLUSTER=test-cluster

  nats-mon2:
    container_name: nats-mon2
    image: nats-mon:latest
    volumes:
      - /var/lib/docker/volumes/nats_vol2/_data:/datastore
    ports:
      - 8283:8283
    environment:
      - STAN_URL=nats://db01.d2sphere.com:4223
      - STAN_MONITOR_URL=http://db01.d2sphere.com:8223
      - STAN_CLUSTER=test-cluster

networks:
  default:
    external:
      name: nats-cluster-docker-compose_default

• A service definition contains configuration that is applied to each container started for that service
• The volumes tag creates a directory within the filesystem which is accessible by the container and maps it to a name. In this case, the application accesses this directory with /datastore
• The network configuration here tells docker that these containers will join an existing docker network named nats-cluster-docker-compose_default

How docker-compose up/down Handles Images and Containers

If docker-compose up is run for the first time, docker-compose up will do the following:

• Build image
• Create container
• Start container

When we run docker-componse down, docker-compose will:

• Stop container
• Remove container

At this point, the docker image previously built is still there. If we run docker-compose up again, the existing image is used to create and start new container(s).

If we want to force docker-compose to rebuild a new image (instead of using the existing one), we can do this by docker-compose up --build.