TESTING DOCKER CONTAINER STRUCTURE

A Brief History of Docker Containers

In the early days of software development, developers ran their applications on their local machines during testing and deployed them to a production virtual machine.

The problem with this approach is that some dependencies which might work in the development environment might not work in production. Hence the famous line by engineers, "it is working on my local machine but not working in production!" Or worse, it could be working on the local machine of some engineers but not working for others. Docker addressed this problem.

Docker containers provide applications with all it needs to run in a containerized environment. Therefore, eliminating the need for applications to depend on environment-specific system requirements. As long as it runs in a development environment, it would certainly work in a production environment.

Containers in Docker are spun up from images. Images provide a convenient way to package applications with the preconfigured server environment. So, Docker images are the package and containers are the package in current use.

Testing Docker Container Structure

What if there is a way to make sure that our Docker images have all the required files and directory to run correctly, execute commands correctly and generally reduce the rate of deploying a problematic image to production?

There is a way! Google's container structure test is the answer. container-structure-test.

Let's say you have folders called src and test at the root of your application and a Dockerfile for your Node js application with these commands:

FROM node:14-alpine

# Create app directory

WORKDIR /usr/node-app

ENV PORT=3000

COPY . .

RUN npm install


EXPOSE 3000

ENTRYPOINT ["npm", "run", "start" ]

Build the Docker image with this command:

docker build -t my-container-name .

Follow these commands to install Container Structure Tests on MacOs or Linux:

OS X

Install via brew:

$ brew install container-structure-test

curl -LO https://storage.googleapis.com/container-structure-test/latest/container-structure-test-darwin-amd64 && chmod +x container-structure-test-darwin-amd64 && sudo mv container-structure-test-darwin-amd64 /usr/local/bin/container-structure-test

Linux

curl -LO https://storage.googleapis.com/container-structure-test/latest/container-structure-test-linux-amd64 && chmod +x container-structure-test-linux-amd64 && sudo mv container-structure-test-linux-amd64 /usr/local/bin/container-structure-test

If you want to avoid using sudo:

curl -LO https://storage.googleapis.com/container-structure-test/latest/container-structure-test-linux-amd64 && chmod +x container-structure-test-linux-amd64 && mkdir -p $HOME/bin && export PATH=$PATH:$HOME/bin && mv container-structure-test-linux-amd64 $HOME/bin/container-structure-test

Create a file called config.yaml at the root of your project. Write the following in that file:

schemaVersion: 2.0.0
metadataTest:
  env:
    - key: PORT
      value: 3000
  exposedPorts: ["3000"]
  # volumes: ["/test"]
  entrypoint: ["npm", "run", "start" ]
  # cmd: ["/bin/bash"]
  workdir: "/usr/node-app"
  # user: "luke"

fileExistenceTests:
  - name: 'src directory exists'
    path: '/usr/node-app/src'
    shouldExist: true
  - name: 'data directory exists'
    path: '/usr/node-app/test'
    shouldExist: true

Save the file and run this command:

container-structure-test test --image my-container-name \
--config config.yaml

This should give a successful test output.

Congratulations! You just tested your first docker container structure! You can research further ways to add it to a deployment pipeline or automate the process of testing. Thanks for your time.