Docker images for the Selenium Grid Server

The project is made possible by volunteer contributors who have put in thousands of hours of their own time, and made the source code freely available under the Apache License 2.0.

Community

Do you need help using these Docker images? Here are all the contact points for the different Selenium projects: https://www.selenium.dev/support/

Quick start

1. Start a Docker container with Firefox

$ docker run -d -p 4444:4444 --shm-size 2g selenium/standalone-firefox:4.0.0-alpha-7-prerelease-20200826
# OR
$ docker run -d -p 4444:4444 -v /dev/shm:/dev/shm selenium/standalone-firefox:4.0.0-alpha-7-prerelease-20200826

2. Point your WebDriver tests to http://localhost:4444/wd/hub

3. That's it!

To inspect visually the browser activity, see the Debugging section for details.

☝️ When executing docker run for an image that contains a browser please either mount -v /dev/shm:/dev/shm or use the flag --shm-size=2g to use the host's shared memory.

Why is -v /dev/shm:/dev/shm or --shm-size 2g necessary? This is a known workaround to avoid the browser crashing inside a docker container, here are the documented issues for Chrome and Firefox. The shm size of 2gb is arbitrary but known to work well, your specific use case might need a different value, it is recommended to tune this value according to your needs. Along the examples -v /dev/shm:/dev/shm will be used, but both are known to work.

☝️ Always use a tag with an element suffix to pin a specific browser version. See Tagging Conventions for details.

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Standalone

Firefox

$ docker run -d -p 4444:4444 -v /dev/shm:/dev/shm selenium/standalone-firefox:4.0.0-alpha-7-prerelease-20200826

Chrome

$ docker run -d -p 4444:4444 -v /dev/shm:/dev/shm selenium/standalone-chrome:4.0.0-alpha-7-prerelease-20200826

Opera

$ docker run -d -p 4444:4444 -v /dev/shm:/dev/shm selenium/standalone-opera:4.0.0-alpha-7-prerelease-20200826

Note: Only one Standalone container can run on port 4444 at the same time.

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Selenium Grid Hub and Nodes

There are different ways to run the images and create a Grid with a Hub and Nodes, check the following options.

Docker networking

The Hub and Nodes will be created in the same network and they will recognize each other by their container name. A Docker network needs to be created as a first step.

$ docker network create grid
$ docker run -d -p 4442-4444:4442-4444 --net grid --name selenium-hub selenium/hub:4.0.0-alpha-7-prerelease-20200826
$ docker run -d --net grid -e SE_EVENT_BUS_HOST=selenium-hub -e SE_EVENT_BUS_PUBLISH_PORT=4442 -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 -v /dev/shm:/dev/shm selenium/node-chrome:4.0.0-alpha-7-prerelease-20200826
$ docker run -d --net grid -e SE_EVENT_BUS_HOST=selenium-hub -e SE_EVENT_BUS_PUBLISH_PORT=4442 -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 -v /dev/shm:/dev/shm selenium/node-firefox:4.0.0-alpha-7-prerelease-20200826
$ docker run -d --net grid -e SE_EVENT_BUS_HOST=selenium-hub -e SE_EVENT_BUS_PUBLISH_PORT=4442 -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 -v /dev/shm:/dev/shm selenium/node-opera:4.0.0-alpha-7-prerelease-20200826

When you are done using the Grid and the containers have exited, the network can be removed with the following command:

# Removes the grid network
$ docker network rm grid

Docker Compose

Docker Compose is the most simple way to start a Grid. Use the linked resources below, save them locally, and check the execution instructions on top of each file.

Version 2

docker-compose-v2.yml

Version 3

docker-compose-v3.yml

To stop the Grid and cleanup the created containers, run docker-compose down.

Version 3 with Swarm support

docker-compose-v3-swarm.yml

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Selenium Grid - Router, Distributor, EventBus, SessionMap and Nodes

It is possible to start a Selenium Grid with its five components apart. For simplicity, only an example with docker-compose will be provided. Save the file locally, and check the execution instructions on top of it.

docker-compose-v3-full-grid.yml

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Deploying to Kubernetes (:warning: not tested yet with Selenium 4 images)

Check out the Kubernetes examples on how to deploy selenium hub and nodes on a Kubernetes cluster.

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Configuring the containers

SE_OPTS Selenium Configuration Options

You can pass SE_OPTS variable with additional commandline parameters for starting a hub or a node.

$ docker run -d -p 4444:4444 -e SE_OPTS="-debug" --name selenium-hub selenium/hub:4.0.0-alpha-7-prerelease-20200826

JAVA_OPTS Java Environment Options

You can pass JAVA_OPTS environment variable to java process.

$ docker run -d -p 4444:4444 -e JAVA_OPTS=-Xmx512m --name selenium-hub selenium/hub:4.0.0-alpha-7-prerelease-20200826

Node configuration options

The Nodes register themselves through the Event Bus. When the Grid is started in its typical Hub/Node setup, the Hub will be the one acting as the Event Bus, and when the Grid is started with all its five elements apart, the Event Bus will be running on its own.

In both cases, it is necessary to tell the Node where the Event Bus is, so it can register itself. That is the purpose of the SE_EVENT_BUS_HOST, SE_EVENT_BUS_PUBLISH_PORT and SE_EVENT_BUS_SUBSCRIBE_PORT environment variables.

Here is an example with the default values of these environment variables:

$ docker run -d --e SE_EVENT_BUS_HOST=<event_bus_ip|event_bus_name> -e SE_EVENT_BUS_PUBLISH_PORT=4442 -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 -v /dev/shm:/dev/shm selenium/node-chrome:4.0.0-alpha-7-prerelease-20200826

Setting Screen Resolution

By default, nodes start with a screen resolution of 1360 x 1020 with a color depth of 24 bits and a dpi of 96. These settings can be adjusted by specifying SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_DEPTH, and/or SCREEN_DPI environmental variables when starting the container.

docker run -d -e SCREEN_WIDTH=1366 -e SCREEN_HEIGHT=768 -e SCREEN_DEPTH=24 -e SCREEN_DPI=74 selenium/standalone-firefox

Running in Headless mode

Firefox, Chrome and Opera support running tests in headless mode. When using headless mode, there's no need for the Xvfb server to be started.

To avoid starting the server you can set the START_XVFB environment variable to false (or any other value than true), for example:

$ docker run -d --net grid -e SE_EVENT_BUS_HOST=selenium-hub -e SE_EVENT_BUS_PUBLISH_PORT=4442 -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 -e START_XVFB=false -v /dev/shm:/dev/shm selenium/node-chrome

For more information, see this GitHub issue.

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Building the images

Clone the repo and from the project directory root you can build everything by running:

$ VERSION=local make build

If you need to configure environment variable in order to build the image (http proxy for instance), simply set an environment variable BUILD_ARGS that contains the additional variables to pass to the docker context (this will only work with docker >= 1.9)

$ BUILD_ARGS="--build-arg http_proxy=http://acme:3128 --build-arg https_proxy=http://acme:3128" make build

Note: Omitting VERSION=local will build the images with the released version but replacing the date for the current one.

Using the images

Example: Spawn a container for testing in Firefox

$ docker network create grid
$ docker run -d -p 4442-4444:4442-4444 --net grid --name selenium-hub selenium/hub:4.0.0-alpha-7-prerelease-20200826
$ docker run -d --net grid -e SE_EVENT_BUS_HOST=selenium-hub \
    -e SE_EVENT_BUS_PUBLISH_PORT=4442 -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 \
    -v /dev/shm:/dev/shm \
    -v /e2e/uploads:/e2e/uploads selenium/node-firefox:4.0.0-alpha-7-prerelease-20200826

Note: -v /e2e/uploads:/e2e/uploads is optional in case you are testing browser uploads on your web app you will probably need to share a directory for this.

This command line for Opera or Chrome is the virtually the same, only remember to replace the image name for node-opera or node-crhome. Remember that the Selenium running container is able to launch either Chrome, Opera or Firefox, the idea around having 3 separate containers, one for each browser is for convenience plus avoiding certain :focus issues your web app may encounter during end-to-end test automation.

Note: Since a Docker container is not meant to preserve state and spawning a new one takes less than 3 seconds you will likely want to remove containers after each end-to-end test with --rm command. You need to think of your Docker containers as single processes, not as running virtual machines.

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Waiting for the Grid to be ready

It is a good practice to check first if the Grid is up and ready to receive requests, this can be done by checking the /wd/hub/status endpoint.

A Grid that is ready, composed by a hub and two nodes, could look like this:

{
  "value": {
    "ready": true,
    "message": "Selenium Grid ready.",
    "nodes": [
      {
        "id": "6c0a2c59-7e99-469d-bbfc-313dc638797c",
        "uri": "http:\u002f\u002f172.19.0.3:5555",
        "maxSessions": 4,
        "stereotypes": [
          {
            "capabilities": {
              "browserName": "firefox"
            },
            "count": 4
          }
        ],
        "sessions": [
        ]
      },
      {
        "id": "26af3363-a0d8-4bd6-a854-2c7497ed64a4",
        "uri": "http:\u002f\u002f172.19.0.4:5555",
        "maxSessions": 4,
        "stereotypes": [
          {
            "capabilities": {
              "browserName": "chrome"
            },
            "count": 4
          }
        ],
        "sessions": [
        ]
      }
    ]
  }
}

The "ready": true value indicates that the Grid is ready to receive requests. This status can be polled through a script before running any test, or it can be added as a HEALTHCHECK when the docker container is started.

Adding a HEALTHCHECK to the Grid

The script check-grid.sh, which is included in the images, can be used to poll the Grid status.

This example checks the status of the Grid every 15 seconds, it has a timeout of 30 seconds when the check is done, and it retries up to 5 times until the container is marked as unhealthy. Please use adjusted values to fit your needs, (if needed) replace the --host and --port parameters for the ones used in your environment.

$ docker network create grid
$ docker run -d -p 4444:4444 --net grid --name selenium-hub \
    --health-cmd='/opt/bin/check-grid.sh --host 0.0.0.0 --port 4444' \
    --health-interval=15s --health-timeout=30s --health-retries=5 \
    selenium/hub:4.0.0-alpha-7-prerelease-20200826
$ docker run -d --net grid -e HUB_HOST=selenium-hub -v /dev/shm:/dev/shm selenium/node-chrome:4.0.0-alpha-7-prerelease-20200826
$ docker run -d --net grid -e HUB_HOST=selenium-hub -v /dev/shm:/dev/shm selenium/node-firefox:4.0.0-alpha-7-prerelease-20200826
$ docker run -d --net grid -e HUB_HOST=selenium-hub -v /dev/shm:/dev/shm selenium/node-opera:4.0.0-alpha-7-prerelease-20200826

Note: The \ line delimiter won't work on Windows based terminals, try either ^ or a backtick.

The container health status can be checked by doing docker ps and verifying the (healthy)|(unhealthy) status or by inspecting it in the following way:

$ docker inspect --format='{{json .State.Health.Status}}' selenium-hub
"healthy"

Using a bash script to wait for the Grid

A common problem known in docker is that a running container does not always mean that the application inside it is ready. A simple way to tackle this is by using a "wait-for-it" script, more information can be seen here.

The following script is an example of how this can be done using bash, but the same principle applies if you want to do this with the programming language used to write the tests.

#!/bin/bash
# wait-for-grid.sh

set -e

cmd="$@"

while ! curl -sSL "http://localhost:4444/wd/hub/status" 2>&1 \
        | jq -r '.value.ready' 2>&1 | grep "true" >/dev/null; do
    echo 'Waiting for the Grid'
    sleep 1
done

>&2 echo "Selenium Grid is up - executing tests"
exec $cmd

Will require jq installed via apt-get, else the script will keep printing Waiting without completing the execution.

Note: If needed, replace localhost and 4444 for the correct values in your environment. Also, this script is polling indefinitely, you might want to tweak it and establish a timeout.

Let's say that the normal command to execute your tests is mvn clean test. Here is a way to use the above script and execute your tests:

$ ./wait-for-grid.sh mvn clean test

Like this, the script will poll until the Grid is ready, and then your tests will start.

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Debugging

In the event you wish to see what the browser is doing, you can check what is going inside by connecting to the VNC server running on port 5900 inside the browser container.

You are free to map that port to any free external port that you wish. Keep in mind that you will only be able to run one node per port. If you wish to include a second node (or more), you will have to use different ports.

The internal 5900 port will need to remain the same because that is the configured port for the VNC server running inside the container.

Here is an example with the standalone images, the same concept applies to the node images.

$ docker run -d -p 4444:4444 -p 5900:5900 -v /dev/shm:/dev/shm selenium/standalone-chrome:4.0.0-alpha-7-prerelease-20200826
$ docker run -d -p 4445:4444 -p 5901:5900 -v /dev/shm:/dev/shm selenium/standalone-firefox:4.0.0-alpha-7-prerelease-20200826
$ docker run -d -p 4446:4444 -p 5902:5900 -v /dev/shm:/dev/shm selenium/standalone-opera:4.0.0-alpha-7-prerelease-20200826

Then, you would use in your VNC client:

• Port 5900 to connect to the Chrome container
• Port 5901 to connect to the Firefox container
• Port 5902 to connect to the Opera container

In case you have RealVNC binary vnc in your path, you can always take a look, select view only to avoid messing around your tests with an unintended mouse click or keyboard interrupt:

$ ./bin/vncview 127.0.0.1:5900

When you are prompted for the password it is secret. If you wish to change this then you should either change it in the /NodeBase/Dockerfile and build the images yourself, or you can define a Docker image that derives from the posted ones which reconfigures it:

#FROM selenium/node-chrome:4.0.0-alpha-7-prerelease-20200826
#FROM selenium/node-firefox:4.0.0-alpha-7-prerelease-20200826
#FROM selenium/node-opera:4.0.0-alpha-7-prerelease-20200826
#Choose the FROM statement that works for you.

RUN x11vnc -storepasswd <your-password-here> /home/seluser/.vnc/passwd

If you want to run VNC without password authentication you can set the environment variable VNC_NO_PASSWORD=1.

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Troubleshooting

All output is sent to stdout so it can be inspected by running:

$ docker logs -f <container-id|container-name>

You can turn on debugging by passing environment variable to the hub and the nodes containers:

GRID_DEBUG=true

Headless

If you see the following selenium exceptions:

Message: invalid argument: can't kill an exited process

or

Message: unknown error: Chrome failed to start: exited abnormally

The reason might be that you've set the START_XVFB environment variable to "false", but forgot to actually run Firefox, Chrome or Opera in headless mode.