Mount volumes to persist data in local & initialize database in Docker

• When a docker container is deleted, relaunching it from the image will start a fresh new container without any of the changes made in the previously running container
  
• This happens because when we create a new container from an image, we add a new writable layer on top of the underlying stack of layers present in the base docker image.
  

• All changes made to the running container, such as creating new tables, inserting data etc are written to the writable container layer
  
• This writable layer does not persist after the container is deleted, therefore if you launch a postgreSQL container and create database and delete the container all the data will also get lost.
  
• In order to save or persist the data even after a container is deleted , you should use volumes.
  
• volumes are directories (or files) that are outside of the container and exist as normal directories and files on the host filesystem
  
• Idea is to launch the container but mount the volume on host file system so that even if the container is deleted , data can continue to persist in the host file system.
  
• I will explain it by mounting volume to a postgreSQL container and also initializing the database with some initialization scripts to create table and load data
  
• create a docker-compose.yml file
  

services: 
  db: 
    container_name: docker_pg
    environment: 
      POSTGRES_DB: northwind
      POSTGRES_PASSWORD: changeme
    image: "postgres:12"
    ports: 
      - "5432:5432"
    volumes: 
      - /Users/nola/docker/postgres/docker-pg-vol/data:/var/lib/postgresql/data
      - /Users/nola/docker/postgres/init/initData.sql:/docker-entrypoint-initdb.d/
version: "3"
volumes: 
  pg-data: 
    driver: local

• volumes: Users/nola/docker/postgres/docker-pg-vol/data:/var/lib/postgresql/data means mount the localhost data directory Users/nola/docker/postgres/docker-pg-vol/data to containers data directory /var/lib/postgresql/data. Write to data will happen now in this host location

• Initialize database: To initialize database i have created the initial scripts in my local at /Users/nola/docker/postgres/init/initData.sql and mounted that to /docker-entrypoint-initdb.d/ location of the container

• Rest of the entries in docker compose files are straight forward it has container name, database name, password, ports of container mapped to local host port and image name
  
• With this when you execute

docker compose up

• This will bring the new container up with data file mounted to host computer and initialization scripts

• docker compose file and data initialization is located at
• https://github.com/rajeshsgr/postgres-docker-compose

Thank you !