Migrating to Docker Compose

Even with a simple image, we've already been dealing with plenty of command line options in both building, pushing and running the image.

Next we will switch to a tool called Docker Compose to manage these. Docker Compose used to be a separate tool but now it is integrated to Docker and can be used like the rest of the Docker commands.

Docker Compose is designed to simplify running multi-container applications to using a single command.

Assume that we are in the folder where we have our Dockerfile with the following content:

FROM ubuntu:18.04

WORKDIR /mydir

RUN apt-get update
RUN apt-get install -y curl python
RUN curl -L https://yt-dl.org/downloads/latest/youtube-dl -o /usr/local/bin/youtube-dl
RUN chmod a+x /usr/local/bin/youtube-dl

ENV LC_ALL=C.UTF-8

ENTRYPOINT ["/usr/local/bin/youtube-dl"]

Let us now create a file called docker-compose.yml:

version: '3.8'

services:
    youtube-dl-ubuntu:
      image: <username>/<repositoryname>
      build: .

The version setting is not very strict, it just needs to be above 2 because otherwise the syntax is significantly different. See https://docs.docker.com/compose/compose-file/ for more info. The key build: value can be set to a path (ubuntu), have an object with context and dockerfile keys or reference a url of a git repository.

Now we can build and push with just these commands:

$ docker compose build
$ docker compose push

Volumes in Docker Compose

To run the image as we did previously, we will need to add the volume bind mounts. Volumes in Docker Compose are defined with the following syntax location-in-host:location-in-container. Compose can work without an absolute path:

version: '3.8'

services:

    youtube-dl-ubuntu:
      image: <username>/<repositoryname>
      build: .
      volumes:
        - .:/mydir
      container_name: youtube-dl

We can also give the container a name it will use when running with container_name. And the service name can be used to run it:

$ docker compose run youtube-dl-ubuntu https://imgur.com/JY5tHqr

Exercise 2.1

Exercise 2.1

Let us now leverage the Docker Compose with the simple webservice that was we user eg in the Exercise 1.3

Without a command devopsdockeruh/simple-web-service will create logs into its /usr/src/app/text.log.

Create a docker-compose.yml file that starts devopsdockeruh/simple-web-service and saves the logs into your filesystem.

Submit the docker-compose.yml, make sure that it works simply by running docker compose up if the log file exists.

Web services in Docker Compose

Compose is really meant for running web services, so let's move from simple binary wrappers to running a HTTP service.

https://github.com/jwilder/whoami is a simple service that prints the current container id (hostname).

$ docker container run -d -p 8000:8000 jwilder/whoami
  736ab83847bb12dddd8b09969433f3a02d64d5b0be48f7a5c59a594e3a6a3541

Navigate with a browser or curl to localhost:8000, they both will answer with the id.

Take down the container so that it's not blocking port 8000.

$ docker container stop 736ab83847bb
$ docker container rm 736ab83847bb

Let's create a new folder and a Docker Compose file whoami/docker-compose.yml from the command line options.

version: '3.8'

services:
    whoami:
      image: jwilder/whoami
      ports:
        - 8000:8000

Test it:

$ docker compose up -d
$ curl localhost:8000

Environment variables can also be given to the containers in Docker Compose.

version: '3.8'

services:
  backend:
      image:
      environment:
        - VARIABLE=VALUE
        - VARIABLE2=VAULE2

Exercises 2.2 - 2.3

Exercise 2.2

Read about how to add command to docker-compose.yml from the documentation.

The familiar image devopsdockeruh/simple-web-service can be used to start a web service.

Create a docker-compose.yml and use it to start the service so that you can use it with your browser.

Submit the docker-compose.yml, make sure that it works simply by running docker compose up

Mandatory Exercise 2.3

As we saw previously, starting an application with two programs was not trivial and the commands got a bit long.

In the previous part we created Dockerfiles for both frontend and backend of the example application. Next, simplify the usage into one docker-compose.yml.

Configure the backend and frontend from part 1 to work in Docker Compose.

Submit the docker-compose.yml