Riccardo Coppola

Dockerize your tests to run everywhere

September 05, 2016

With integration tests being an integral part of a webapp workflow, we should always try to find ways to make them run smoother and make our lives easier.

Note: This post was updated on May 2019

My dilemma

I’ve been working with Selenium Webdriver/WebdriverIO for years now, and the biggest complaint I have is that I need to have a selenium server running before starting my tests.

This may seem like a minor problem, but it means having another tab open in my terminal, starting/stopping that process, and it all get way more difficult when you try to automate it in a CI environment. In addition to that, you need Java installed to run the Selenium server (or you can use the selenium-standalone npm package, which removes the dependency from Java but still needs to be started/stopped).

While solutions like Browserstack and Saucelabs are quite popular, they are not the quickest way of getting a feedback and sometimes you just want a quick run on Chrome to check that everything is ok.

So how do you solve the problem of having to constantly start and stop your Selenium server? In other words, how do you create a container that runs in isolation and takes care of starting/stopping a selenium server for you? (plus running your tests, of course).

Enter the Docker (and Docker Compose) World

I am addicted to Docker: I’d containerize everything now.
It is such a clean way of dealing with (or not dealing with, even better) OS compatibility that it’s a no brainer.

Let’s apply it to a simple test case: running the webdriverIO example.

const assert = require('assert');

describe('webdriver.io page', () => {
  it('should have the right title', () => {
    const title = browser.getTitle();
    assert.equal(title, 'WebdriverIO · Next-gen WebDriver test framework for Node.js');

Of course, our application will also have its own package.json with all the dependencies listed there and a wdio.conf with all the WebdriverIO configuration, but hey, we all know that!

├── Dockerfile
├── README.md
├── docker-compose.yml
├── package.json
├── test
│   └── index.spec.js
└── wdio.conf.js

Our package.json defines a test script like this: "test: wdio wdio.conf".

To run the tests we need to install the npm dependencies and run the npm command:

$ npm i
$ npm test

Building the app

No need to install node.js, no need to install npm. Just Docker and a Dockerfile, like this:

FROM node:8
ADD . /app
RUN npm i

This is just a definition of what we want, a recipe. To build an image from it, we need some Docker magic (docker build): docker build -t webdriverapp .

In plain English: “Build the Dockerfile in this folder and name the result ‘webdriverapp’“. The result of a build is an image.

To run the image, and create a container, we need some more Docker magic (docker run): docker run --rm -ti webdriverapp npm test

In plain English: “Run the command npm test from the image named “webdriverapp” in interactive mode (-ti) and remove the container afterwards (--rm).

We’re good! Run it, and you’ll get an error saying that we need a Selenium server running!


We definitely need more than just a containerized version of our app: we need a whole stack (which, in this case, is a Selenium server, but it could be a Redis server, a mongodb server, etc) of applications that run in isolation and can communicate with each other.

Docker Hub is like npm for Docker, you can find all sorts of pre-built images there. Turns out, the Selenium guys published a dockerized version of a standalone Selenium, selenium/standalone-chrome. Running it with docker will create a container with a selenium server listening on the default port 4444 (always remember that when you run a container that listens on a port, you have to map that port on your host).

docker run -p 4444:4444 selenium/standalone-chrome

What we want now is to compose an architecture where the Selenium server is listening and our app is started and connects to it.

docker-compose is the answer.
It lets us create a file (docker-compose.yml) where we define our stack and how different applications interact with each other.
Easier done than said:

  build: .
  command: npm test -- --host selenium
    - selenium

  image: selenium/standalone-chrome
    - "4444"
  log_driver: "none"

The file defines two applications: app and selenium:

  • app needs to be built, and the Dockerfile is in the same directory as docker-compose.yml (build: .).
    When started, we want to run a specific command: command: npm test -- --host selenium
    We also want it to have a link to the selenium app.
  • selenium, instead, doesn’t need to be built, it uses a pre-built image from Docker Hub: image: selenium/standalone-chrome. It also exposes the port 4444.

Note that docker-compose creates a host for every application that exposes a port, and the hostname is the application name: selenium in this case. This is why we pass the option --host selenium to WebdriverIO: to tell it where the server is listening.

Time to build the stack: docker-compose build .

And run it: docker-compose up .


GitHub repo

A fully working code example can be found on GitHub.


Docker and docker-compose can help us in many different ways: we can compose entire stacks in minutes. Need a Redis database? Add it to docker-compose. Need a RabbitMq queue? Add it!
It is a simple and effective way to package an app and its dependencies.
Of course in production you’ll have your services hosted somewhere: no problem! You can have different docker-compose files for different environments, all defining different stacks!

There is so much to learn and experiment with!
My suggestion is to start with the Docker blog and the docker weekly newsletter, then the sky is the limit!

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