Docker Compose

Why Docker Compose?

In the previous tutorial you started one Selenium Chrome container with a single docker run. That works fine. But what happens when your setup grows?

For example, a Selenium Grid has two parts: a hub (the coordinator) and one or more nodes (the actual browsers). You need both running at the same time, and the node needs to know the hub’s address.

You could run two separate docker run commands and wire them together manually. But that is tedious, error-prone, and hard to share with your team.

The Compose File Structure

Create a file named docker-compose.yml in your project folder. Its structure is simple:

services:
  service-name-1:
    image: some-image
    ports:
      - "host:container"

  service-name-2:
    image: another-image

Each item under services is one container. The name you give it (like service-name-1) is also how containers find each other on the network.

Real Example: Selenium Grid

Here is a complete Selenium Grid setup — hub plus one Chrome node:

services:

  hub:
    image: selenium/hub:4.18.1
    ports:
      - "4444:4444"

  chrome:
    image: selenium/node-chrome:4.18.1
    depends_on:
      - hub
    environment:
      - SE_EVENT_BUS_HOST=hub

That is 12 lines. Let us read each important part:

Part	What it means
hub:	The name of this service — also its hostname on the Docker network
ports: "4444:4444"	Expose the hub so your tests can reach it from outside
depends_on: - hub	Start the hub first, then start chrome
SE_EVENT_BUS_HOST=hub	Tell the Chrome node to register with the hub (using the service name “hub” as hostname)

Up and Down

With the docker-compose.yml file in your project folder, start everything:

docker compose up -d

Docker starts both containers. Verify they are running:

docker compose ps

NAME      SERVICE   STATUS    PORTS
hub       hub       running   0.0.0.0:4444->4444/tcp
chrome    chrome    running

Open http://localhost:4444 — you will see the Selenium Grid UI with one Chrome node registered.

When you are done, stop and remove everything with one command:

docker compose down

This stops all containers and removes them. Clean slate.

Connect Your Tests to the Grid

Your tests connect the same way as before — using RemoteWebDriver pointing at the hub:

ChromeOptions options = new ChromeOptions();

WebDriver driver = new RemoteWebDriver(
    new URL("http://localhost:4444"),
    options
);

The hub distributes your test to the Chrome node automatically. Add more nodes (Firefox, Edge) to the Compose file and the grid handles load balancing for you.

Scale Chrome Nodes for Parallel Tests

Want to run tests in parallel? Just tell Compose to start more Chrome nodes:

docker compose up -d --scale chrome=3

This starts 3 Chrome nodes, all registered to the same hub. Your parallel tests will now run on 3 browsers simultaneously. No extra configuration in your Compose file needed.

Useful Compose Commands

Command	What it does
docker compose up -d	Start all services in background
docker compose down	Stop and remove all containers
docker compose ps	List all running services
docker compose logs chrome	See logs from the chrome service
docker compose up -d --scale chrome=3	Start 3 Chrome nodes

What You Have Learned Across All 6 Tutorials

You started from zero and now you can:

• Explain what Docker is and why it matters for QA
• Pull images from Docker Hub and manage them
• Run containers with ports, names, and background mode
• Write a Dockerfile and build your own custom image
• Run Selenium Chrome in Docker — no browser install needed
• Spin up a full Selenium Grid with Docker Compose in seconds