Docker notes #3

The original Docker logo

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We continue to learn Docker. See the first part, second part on the learning notes.

docker compose

A tool for running multi-container application. An application may need postgresql, redis, or any other 3rd party servers.

Building all of them inside a single container doesn't make any sense in practical.

Example: Running two containers; first one is the web app, second one is the mongodb instance. (filename convention: docker-compose.yml.

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services:
     web:
         image: "emrebeyler/my-web-app"
     database:
         image: "mongodb"

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use docker compose up to run the containers.

Port mapping

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services:
     web:
         image: "emre/my-web-app"
         ports:
              - "80:5000"

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Volume mapping

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services:
        database:
             image: "mongodb"
             volumes:
                 - /opt/data:/var/lib/mongodb

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Stop or Remove containers

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• docker-compose stop to stop containers.

• docker-compose down to stop everything and remove the containers entirely.

Passing environment files to containers

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services:
        database:
             image: "mongodb"
             environment:
                - MYSQL_ROOT_PASSWORD=password

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It's also possible to set a environment file:

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web:
  env_file:
    - web-variables.env

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It's a good practice to share the configuration information via environment variables.

Placing dependencies on containers

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It's not a good idea to fire up the web app before the database.

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version: '3'
services:
  web:
    depends_on:
      - db
  db:
    image: postgres

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docker compose will start the postgres first, and web later since it. It follows the dependency order.

Networking between services

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By default, every service can reach each other by their image names. No need to know which IP they operate on.

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version: '3'
services:
  web:
    depends_on:
      - db
  db:
    image: postgres

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web service can reach the postgres server by using postgres://db:5432 as a connection string.