Volumes and Docker Compose

Introduction

By default, anything a container writes disappears when the container is removed. That is fine for a web server, but not for a database. Volumes keep data safe, and Docker Compose lets you run several connected containers with one command.

Volumes

A volume is storage managed by Docker that lives outside any single container.

$ docker volume create mydata
$ docker run -d --name db -v mydata:/var/lib/postgresql/data -e POSTGRES_PASSWORD=change-me postgres:16

-v mydata:/var/lib/postgresql/data connects the volume to a folder inside the container. If you remove and recreate the container, the data in mydata is still there. The -e option sets an environment variable. Use a strong password of your own, not the example one.

A bind mount links a folder from your computer instead. It is handy while developing, because you can edit code on your computer and the container sees the change:

$ docker run -p 8080:80 -v "$(pwd)":/usr/share/nginx/html nginx

Docker Compose

Real apps often have several parts, such as a web app and a database. Compose describes them all in one file, usually named compose.yaml:

services:
  web:
    build: .
    ports:
      - "5000:5000"
    depends_on:
      - db
  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: change-me
    volumes:
      - dbdata:/var/lib/postgresql/data

volumes:
  dbdata:

• services lists each container.
• web is built from your Dockerfile, and db uses a ready-made image.
• Compose puts them on the same network, so web can reach the database using the name db.
• The volumes section keeps the database data.

Compose commands

$ docker compose up -d      # build if needed and start everything
$ docker compose ps         # see what is running
$ docker compose logs web   # logs from one service
$ docker compose down       # stop and remove the containers

docker compose down keeps your named volumes. Add -v only if you really want the data deleted.

Good habits

• Keep secrets out of the Compose file you share. Use an .env file that is not committed, or a secrets tool.
• Pin versions, such as postgres:16, rather than relying on latest.
• Do not run containers with more access than they need.

Try it yourself

Write a compose.yaml that runs nginx on port 8080 with docker compose up -d, then take it down again with docker compose down.

Summary

• Containers lose their data when removed, so use volumes for anything that must last.
• Bind mounts share a folder from your computer, which is useful while developing.
• Docker Compose describes and runs several containers together from one file.