Selenium Docker: Parallel execution made easy

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Selenium Docker: Parallel execution made easy

Selenium Grid supports distributed test execution and allows running tests on different machines against different browsers in parallel. This reduces execution time from days to hours. However Selenium Grid setup and maintenance requires effort and has its own limitations.

  1. User needs to install browsers manually as required
  2. User needs to ensure proper selenium libraries are in place in hub and node machines
  3. User needs to ensure proper browser drivers are in place in node machines
  4. In case of same browser with different versions are used, User need to setup all versions manually and ensure selenium node setup command is proper.
  5. Scaling of browsers available is not easy; Node has to be brought down to increase browser instances and session which is not possible when tests are already running
  6. Requires constant maintenance as the machines had to be kept up and running at all times
  7. Setup cannot be changed from one machine to other machine easily

Selenium Docker

Selenium Docker helps in setting up test labs with grid in very easy and simple steps by removing all complexities. This has vastly improved the efficiency of automation and provides an ideal environment for distributed testing by addressing above pain points. It allows us to spin up the grid/nodes and tear them down all in simple commands. By end of this article, we will get solution for all the limitations mentioned above.

Please visit for more information on Docker. This article requires docker installation as prerequisite –

You can verify docker installation by running any of below commands

docker ps -a
docker version
docker info

Deploying Selenium Grid on a Standalone Docker Container

Important point to note and understand – the test scripts are running in local machine or invoked from local machine. docker container provides only the browser

Selenium provides standalone grid server Docker images with Chrome and Firefox browsers already installed on it.

docker run -d -P -p "4444:4444" --name standalone_grid_chrome selenium/standalone-chrome

With this command, the Selenium standalone Chrome image will be downloaded and the server will be started on your local machine on port 4444.

Similarly Firefox standalone server also can be started.

docker run -d -P -p "4444:4444" --name standalone_grid_firefox selenium/standalone-firefox

Verify whether the running container is running or not using the following command or visiting http://[your machine IP]:4444/grid/console url.

docker ps -a

Please note that the execution happens on these images is headless using Xvfb.

If you want to see your execution or debug your script step by step, you can use the selenium/standalone-chrome-debug or Firefox images. These images have a VNC server installed so you can use VNC viewer or any other VNC client.

docker run -d -P -p "4444:4444" --name standalone_grid_firefox selenium/standalone-firefox-debug

docker run -d -P -p "4444:4444" --name standalone_grid_chrome selenium/standalone-chrome-debug

By default, when there is no image tag specified, latest image with the latest browser version is used. Specific version of the browser can also be used to create standalone grid.

You should use docker image available with that browser version. For example, if you want to use Firefox 51.0, you can use the tag 3.5 tag next to the image name.

docker run -d -P -p "5554:4444" --name firefox_3.0 --link hub:hub selenium/node-firefox:3.0

All images are maintained with the tag on the Selenium Docker Hub –

Deploying Selenium Grid on Multiple Containers Using Docker Compose

Docker Compose is the tool that lets you deploy Selenium Grid in multiple containers. Docker Compose uses YAML files to configure application services like a hub. Chrome and Firefox will be the services in this case.

Create a docker-compose.yml file on your machine.

version: "2"
   image: selenium/hub
    - 4444:4444
   image: selenium/node-firefox
    - hub
    HUB_HOST: hub
   image: selenium/node-chrome
    - hub
    HUB_HOST: hub
  • image – Selenium docker image name
  • environment – used to configure environment variables for the hub and nodes
  • depends_on – used to maintain start up order
  • HUB_HOST – used to connect nodes to the hub with its service name
  • NODE_MAX_INSTANCES – number of instances of same version of browser that can run in node
  • NODE_MAX_SESSION – number of browsers (Any browser and version) that can run in parallel at a time in node

Note: NODE_MAX_SESSION overrides max instances settings by restricting the number of browser instances that can run in parallel.

Save the docker-compose.yml file and start the grid using the command:

docker-compose up -d

This will pull and run the images as hub, and nodes. The nodes will register and start communicating with the hub. You can check whether everything is running using below command.

docker ps -a

Also you can verify it in browser – http://[your machine IP]:4444/grid/console

If you want to increase the number of browser containers, you can use ‘scale‘ option. This will create specified number of chrome containers and number of containers includes already running containers. Same ‘scale’ option can be used to reduce browser containers, just need to provide lesser numbers than existing number of containers(For example, to remove 3 from 5 containers, use scale=2).

docker-compose scale chrome=2

To stop all containers created using compose file:

docker-compose stop

To start all containers created using compose file:

docker-compose start

To start/stop specific containers created using compose file:

docker-compose start <container id|container name>
docker-compose stop <container id|container name>

You can get container ID using ‘docker ps -a’ command.

To stop and remove containers:

docker-compose down

Deploying Selenium Grid on Multiple Containers with different browsers and versions:

docker run -d -P -p "4444:4444" --name hub selenium/hub

docker run -d -P -p "5551:4444" --name chrome_latest --link hub:hub selenium/node-chrome

docker run -d -P -p "5552:4444" --name chrome_3.4.0 --link hub:hub selenium/node-chrome:3.4.0

docker run -d -P -p "5553:4444" --name chrome_3.0 --link hub:hub selenium/node-chrome:3.0

docker run -d -P -p "5556:4444" --name firefox_latest --link hub:hub selenium/node-firefox

docker run -d -P -p "5555:4444" --name firefox_3.4.0 --link hub:hub selenium/node-firefox:3.4.0

docker run -d -P -p "5554:4444" --name firefox_3.0 --link hub:hub selenium/node-firefox:3.0

After running above commands, a grid will be setup with 3 different versions of chrome and firefox each.

Difference between images and containers


  • The file system and configuration of application which are used to create containers.


  • It is running instance of Docker images

You can see all tagged versions of selenium images on docker hub. For example, standalone-chrome tags can be found in

Please note the difference between different image names

  • standalone-firefox – Image to create standalone grid
  • standalone-firefox-debug – Image to create standalone grid with debugging capability
  • node-firefox – Image to create selenium node that can be registered to hub

Hope we have got solutions for all limitations mentioned. Happy reading!!!

5 Responses

  1. Lolo says:

    By following the docker-compose.yml, it does not works on docker version 18.06
    ERROR: In file ‘./docker-compose.yml’, service ‘image’ must be a mapping not a string.
    How to fix this ?

    • admin says:

      docker-compose.yml is a YAML file which follows certain format and indentation. The issue you faced is due to incorrect indentation. I updated post. Please try and let me know.

      • Ricky says:


        We want to setup a Selenium Docker Hub Node Environment.
        I have successfully set it up using following:
        docker run -d -p 4444:4444 –net inpronet –name selenium-hub selenium/hub:3.11.0-dysprosium

        docker run -d –net inpronet -e HUB_HOST=selenium-hub -v /dev/shm:/dev/shm selenium/node-chrome:3.11

        docker run -d –net inpronet -e HUB_HOST=selenium-hub -v /dev/shm:/dev/shm selenium/node-firefox:3.11.0-dysprosium

        docker run -d -P -p 5900:5900 –net inpronet -e HUB_HOST=selenium-hub -v /dev/shm:/dev/shm selenium/node-chrome-debug:3.11.0-dysprosium

        docker run -d -P -p 5901:5900 –net inpronet -e HUB_HOST=selenium-hub -v /dev/shm:/dev/shm selenium/node-firefox-debug:3.11.0-dysprosium

        I have configured my C Sharp based Selenium Tests as following:
        IWebDriver driver;
        DesiredCapabilities capability = DesiredCapabilities.Chrome();
        capability.SetCapability(“platformName”, “ANY”);

        capability.SetCapability(“version”, “ANY”);
        driver = new RemoteWebDriver(
        new Uri(“”),
        capability, TimeSpan.FromSeconds(1600));
        {// NOTE: connection timeout of 600 seconds or more required for time to launch grid nodes if non are available.

        Now the challenge is that their are around 1500 tests that need to be executed on say 3-4 different URLs and also on Chrome and Firefox nodes simultaneously;

        Example :

        Chrome Node1 : Tests running on URL1
        Chrome Node2 : Tests running on URL2
        Firefox Node1 : Tests running on URL 3
        ForeFox Node 2 : Test running on URL4

        or :

        Chrome Node1 : Tests running on URL1
        Chrome Node2 : Tests running on URL2

        Once the tests complete on Node 1 on Node 2 on chrome
        Firefox Node1 : Tests running on URL 1
        ForeFox Node 2 : Test running on URL2;

        We need this setup as we have the automation fraework so created that we cannot run the tests parallely because it results in data creation deadlocks and the business decision as of now is not to modify the framweork rather look at ways to run the tests sequentially on different URLs and different Nodes.
        Is it possible please?
        How are the URLs mapped to Nodes i.e how is it decided that which URL will run on which chrome node(assuming there are 2-3 chrome nodes);
        How can we setup in Selenium Automation Framework(NUnit- C Sharp Based) to run the tests in selquence on 2-3 different URLs.
        We are aiming to reduce the no. of physical servers we have to execute our selenium tests on different URLs by implementing the DOcker Selenium Hub-Node concept.

        I am stuck on this since a week and any advice would be very very helpful.

  2. Mislav says:

    nice post.

    I tried to run multiplier browsers in parallel for web scraping, but never couldn’t get more than 3 stable processes. When I ran say 5 browsers in parallel, there were always some errors appeared.

    can you also write a post about how to implement this docker hub for parallel testing, with real code in python on some other language?

  3. […] I hope that you found this useful in solving the limitations mentioned. You can also find this post on my blog. […]

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