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To run Selenium Grid with Docker, first choose whether Docker will run the Grid service, the browsers, or both. For a simple local or CI setup, start with a Standalone Grid container; for browser sessions launched in separate Docker containers, configure Grid’s Docker support with browser-image mappings and a reachable Docker daemon. A multi-machine deployment needs additional network paths beyond port 4444.
Two ways to combine Selenium Grid and Docker
These patterns solve different problems and can be combined:
- Run Grid in Docker: The Grid server itself runs as a container. This packages the service but does not, by itself, mean that each browser session runs in its own container.
- Have Grid launch browser containers: Grid maps requested browser capabilities to browser images and asks a Docker daemon to start containers for sessions. This requires Docker configuration and network access from Grid to that daemon.
Selenium’s Docker project links to its image resources and detailed setup instructions at the official Docker Selenium project. Follow the current README there for a Compose file and compatible image tags. The project landing page alone does not establish a current canonical Compose recipe or tag set, so do not treat an old example or an unverified latest tag as a production pin.
Choose a Grid topology before exposing ports
| Topology | When to use it | Operations and scaling |
|---|---|---|
| Standalone | Local development, quick pre-push runs, or straightforward CI on one machine. | All Grid components run in one process on one machine. It is the simplest starting point, but it does not span machines. |
| Hub and Node | When one endpoint should coordinate browser capacity across multiple machines. | Nodes register with a Hub; add or remove Nodes to change capacity. You must manage Hub-to-Node HTTP, Event Bus, and Node-port connectivity. |
| Distributed | Larger or more controlled deployments where Grid components should be deployed separately. | Each component runs separately, ideally on different machines according to Selenium’s guide. This gives deployment control but requires more services, port planning, and operational coordination. |
Selenium documents the default RemoteWebDriver endpoint for a local Standalone Grid as http://localhost:4444. The right topology depends on where browsers need to run and how much infrastructure you are prepared to operate—not merely on whether the Grid server is containerized. See Selenium Grid’s getting-started guide for topology details and documented defaults.
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Start with a Dockerized Standalone Grid
For a quick setup, use the current Selenium Docker project instructions to select a compatible image and run a Standalone Grid container. The exact command or Compose configuration depends on the image tag and release you choose; use the project’s current README rather than copying an unverified recipe. Once started, make port 4444 reachable from your test runner and open http://localhost:4444 when the runner is on the same machine.
This basic approach is distinct from configuring Grid to create a new Docker container for every browser session. If you need that second pattern, configure the Node and Docker mappings as described below.
Configure Grid to launch browser containers
For Docker-backed sessions, the Node needs capability stereotypes paired with browser images, and Grid needs a Docker daemon endpoint it can reach. Selenium’s TOML configuration documents this structure; the abbreviated example below shows the relevant shape. Replace the illustrative image and browser version with tags verified for your chosen Selenium release. Do not copy old browser-version values as current recommendations.
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[node]
detect-drivers = false
max-sessions = 4
[docker]
# Set this to the Docker daemon address reachable from the Grid process.
url = "http://docker-host:2375"
configs = [
"selenium/standalone-chrome:REPLACE_WITH_VERIFIED_TAG", "{"browserName":"chrome"}",
"selenium/standalone-firefox:REPLACE_WITH_VERIFIED_TAG", "{"browserName":"firefox"}"
]This is a configuration pattern, not a ready-to-deploy file: verify the supported TOML keys and syntax against the documentation for the exact Grid release, and use actual compatible image tags. The documented example pairs Docker image names with capability JSON, disables automatic driver detection, sets a maximum session count, and configures the Docker URL. Some setups also configure a video image.
The Docker daemon must be exposed through HTTP/TCP for this Grid configuration. Treat that endpoint as privileged infrastructure: make it reachable only from the Grid host or containers that require it, and do not publish it publicly. Selenium describes Docker-related configuration options including Docker host, Docker URL, Docker API version, browser-server startup timeout, and graceful stop period. For the exact release’s options, consult the Grid CLI options documentation and the server’s own help output.
Plan network reachability for your topology
Port 4444 is the documented default Grid UI/client port, but it is not sufficient for every multi-node or distributed deployment. Allow only the paths required by your topology, and make sure peer services can reach the hostnames Grid advertises or reports.
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- Standalone: Test clients need access to the Grid endpoint, normally port 4444.
- Hub and Node: Nodes communicate with the Hub over HTTP and the Event Bus. The documented default Event Bus ports are 4442 and 4443, and the Node’s port must also be reachable from the Hub or other required peers.
- Distributed: Separate components require the ports for the components actually deployed. Selenium documents default Event Bus ports 4442, 4443, and 5557, along with separate ports for the Session Map, New Session Queue, Distributor, Router, and Node. Check the getting-started guide and your release configuration before opening firewall rules.
Container networking changes what “reachable” means: a service may resolve on one Docker network but not from another host. Verify connectivity in the direction each component initiates it; do not assume publishing the client-facing port also connects the Event Bus or Nodes.
Verify Grid health and create a session
- Open the Grid UI at
http://<grid-host>:4444, or requesthttp://<grid-host>:4444/status. Uselocalhostin place of<grid-host>when testing locally. - Inspect status for registered Nodes, available slots, and sessions. A live endpoint alone does not prove a requested browser can start.
- Run a RemoteWebDriver test using the desired browser capability against the Grid endpoint. Confirm that the session is created and that the intended Node or browser container receives it.
Selenium’s Grid endpoints documentation covers the status endpoint and related endpoints.
Troubleshoot common startup and session failures
| Symptom | Likely cause | What to check |
|---|---|---|
| No Node appears in the UI or status. | The Node cannot reach the Hub/Event Bus, or registration details advertise an unreachable host or port. | Check Hub HTTP and Event Bus paths, the Node port, Docker network membership, and the hostname peers use to reach the Node. |
| A browser session cannot be matched to a Node. | The requested capabilities do not match a configured stereotype, or the image mapping is missing or malformed. | Compare the test capabilities with the JSON capability mapped to the browser image; verify the configuration format for the running Grid version. |
| Grid cannot start a browser container. | The Docker daemon URL is unreachable, the image is unavailable, or daemon access is blocked. | Test daemon reachability from the Grid process, check the configured Docker URL and API version, and confirm the image can be pulled from that environment. |
| The endpoint responds but sessions time out. | Browser startup may exceed the configured timeout, the host may be resource constrained, or required network paths may be blocked. | Check Grid logs and CLI settings for browser-server startup timeout; inspect CPU, memory, slots, and connectivity before increasing timeouts or concurrency. |
| A copied CLI option is rejected. | Options can differ by Grid release or role. | Run java -jar selenium-server.jar --config-help or the relevant role-specific --help for the deployed server and use its output as the version-specific reference. |
Secure the Grid and size concurrency from measurements
Do not expose an unauthenticated Grid endpoint or Docker daemon to the public internet. Selenium warns that failure to protect a Grid can let third parties access internal web applications and files or run custom binaries. Restrict access with firewall and network rules; where appropriate, also configure the basic authentication options documented for the Grid release.
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Selenium offers 1 CPU and 1 GB of RAM per browser as a rule-of-thumb starting reference, not a universal requirement or benchmark. Actual capacity depends on the browser workload, page complexity, and environment. Measure your own tests over time: watch queued sessions, slot use, CPU and memory pressure, and session failures before raising the maximum session count or adding Nodes.
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Frequently Asked Questions
Can Selenium Grid run browser tests in parallel?
Yes. Grid can allocate sessions across available slots and Nodes; the practical parallel limit depends on configured capacity and measured workload.
Can I run browser sessions in containers without splitting Grid across machines?
Yes. Grid’s Docker-backed session configuration can launch browser containers through a reachable Docker daemon; that is separate from choosing a multi-machine Grid topology.
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