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Docker Compose Integration to GitLab CI

This is a guide for the integration into GitLab CI of the code for the Dockerizing a Flask-MySQL app with docker-compose article.

Introduction

GitLab, just like GitHub, is a web-based versioning control system powered by Git. In addition, GitLab aims to support the whole development lifecycle. One of its features is Continuous Integration (CI).

The CI feature triggers a pipeline for building and testing upon every code submission. This ensures automatically that the latest code works and allows that multiple contributors can continuously integrate (merge) their code into the main repository with confidence.

Nevertheless, every CI system requires some extra effort from the developers: They have to provide instructions on how the code builds and how the application is tested.

The rest of this guide describes the .gitlab-ci.yml file that contains the commands for the Docker Compose, Flask, and MySQL combination, based on the aforementioned article and a Stack Overflow related answer.

Base Docker Image

Every time a commit is submitted, one or more Docker containers are spawned in GitLab CI. These Docker containers serve as an isolated volatile environment to check the whole codebase, with the latest changes included of course.

GitLab CI is enabled if a .gitlab-ci.yml file exists in the root directory of a GitLab project. All the code snippets that follow are taken from this project's .gitlab-ci.yml. This file starts with the declaration of a Docker image. In our case, we will use the official docker/compose image. Besides, we need to execute several docker-compose commands in it.

image:
  name: docker/compose:1.24.1

We stick to a specific docker/compose image version, e.g. 1.24.1. This is done because, as we can see in the related versions page, no default latest tag exists.

  entrypoint: ["sh", "-c"]

The above statement is needed in order to execute shell commands. If we omit it, only plain docker-compose commands can be executed.

Docker in Docker

We are going to execute docker commands inside a docker/compose container. This isn't as straightforward as it seems, because docker is designed to run directly in the host machine and not inside a Docker container. This limitation can be eliminated using the Docker in Docker dind service.

services:
  - docker:dind

variables:
  DOCKER_HOST: tcp://docker:2375
  DOCKER_DRIVER: overlay2

The above two variables are set according to GitLab instructions, to facilitate the communication between the Docker container and the host machine.

Execution

In GitLab CI, a job is created by simply writing its name followed by a colon. Below, we define the Build and Test job.

Build and Test:
  before_script:
    - apk add --update curl
    - docker version
    - docker-compose version

The commands of a job are divided into three sections: before_script, script, and after_script. In the first section above, we install curl, and we print the versions of the Docker commands.

  script:
    - docker-compose build
    - docker-compose up --detach

The main commands are under the script section. The containers are being run in detached mode. This allows the execution of more commands after docker-compose up. With the following commands we ensure that the application and database are up and running.

    - until [ $(docker-compose logs app | grep -c "Running on ") -eq 1 ];
      do
        sleep 2;
      done
    - until [ $(docker-compose logs db | grep -c "ready for connections") -eq 2 ];
      do
        sleep 2;
      done

docker-compose logs command gives the output of the corresponding service. The database should output twice the ready for connections message: one at the beginning and another after the database is fed with the database schema.

Testing

Last but not least, after all services are initialized, we should check that everything works as expected. We store the expected output into the expected file. The - > symbols below are used to overcome the need of escaping a bunch of quotes that follow.

    - >
      echo -n '{"favorite_colors": [{"Lancelot": "blue"}, {"Galahad":
      "yellow"}]}' > expected

Then, we get the real server response via curl. Instead of hitting the localhost domain, we use the docker keyword, due to the Docker in Docker limitations.

    - curl -f http://docker:5000 > output

Finally, we compare the expected to the real output. If the comparison or any of the commands fail, the success file will never be created.

    - diff expected output
    - touch success

If the success file doesn't exist, all the logs are printed for debugging purposes.

  after_script:
    - if [ ! -e success ]; then
        docker-compose logs;
      fi

See the output of the CI job here.