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Development Workflow

This exercise is meant to take someone through the process of converting an already-working application into a containerized application.

An application typically may have one or more external services it utilizes. Simulating these services locally is a great use of the docker-compose utility. Docker Compose is a utility from Docker that makes it easy to spin up and connect multiple containers together, locally. It is configured via a YAML file, typically named docker-compose.yaml

Docker Compose File Reference

Docker Compose Reference Commands

Command Description
docker-compose build Builds any containers in docker-compose.yaml that do not reference an image
docker-compose up Starts all containers, and outputs logs to the terminal
docker-compose up -d Starts all containers in daemon mode
docker-compose logs Outputs all container logs to the terminal
docker-compose logs -f Outputs and follows container logs to the terminal
docker-compose down Stops all containers
docker-compose rm Deletes all stopped containers part of this application

Docker Compose CLI Reference

Exercise

Run a multi-container application locally using docker-compose.

  • Create a .env file with the following values:

    REDIS_HOST=redis
REDIS_PORT=6379
FLASK_PORT=8080

  • Edit app.py to look for and use these environment variables

    os.environ.get('ENV_VAR', 'DEFAULT')

  • Update docker-compose.yaml to have the web container inherit environment variables from the new .env file

    env_file: 
    - ./.env

  • Verify that everything still works by running docker-compose up and, when running, opening http://localhost:8080 in a browser.

  • Change the value of FLASK_PORT in .env to be 9000

  • Change the port mapping for web in docker-compose.yaml to be 9000:9000

  • Verify that the config changes have worked by running docker-compose up and, when running, opening http://localhost:9000 in a browser.

[Solution]