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Choose where Chrome runs
There are two supported arrangements:
| Arrangement | Browser location | What you configure | Main trade-off |
|---|---|---|---|
| Local headless Chrome | The Rails test machine, runner, or container | Selenium, Chrome, and a compatible driver | Simpler networking; browser and tests share one runtime |
| Remote browser | A Selenium server or Chrome container | Remote URL, browser options, and network-reachable Capybara host | Separates workloads but adds service, DNS, port, and security work |
Use local execution when your Rails process and browser can live in the same image or CI runner. Use a remote service when browser dependencies are maintained separately or several jobs share a browser environment.
Prerequisites and version boundaries
- A Rails application with system tests enabled.
- Selenium and Capybara as provided by, or configured for, your Rails test stack.
- Google Chrome (or the Chromium build your environment supports) installed in the environment that launches the session.
- A browser driver that Selenium can discover, with a version compatible with the installed browser.
- For a remote setup, a Selenium/Chrome service reachable from the Rails process and able to reach the Rails test server.
Driver installation is platform- and runtime-specific. Do not copy a package command intended for one operating system into another image. Check the documentation for your Rails version, Selenium integration, browser build, and driver distribution, then verify the executables inside the actual CI or container runtime.
Local headless Chrome in a Rails system test
1. Configure the system-test base class
Rails places this configuration in application_system_test_case.rb:
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require "test_helper"
class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
driven_by :selenium, using: :headless_chrome
endThis tells Rails to create Selenium sessions with Chrome’s headless mode. The browser and driver must be available to the process running the test.
2. Add a real system test
require "application_system_test_case"
class HomePageTest < ApplicationSystemTestCase
test "shows the welcome page" do
visit "/"
assert_text "Welcome"
end
endRun the system-test command used by your Rails version and test framework. If you use RSpec, Cucumber, a custom Capybara driver, or a production automation job, configure that integration point instead of copying the Rails test-case class unchanged.
3. Add driver options only when needed
Rails allows additional Capybara and driver settings in the system-test case. Keep options close to the integration that consumes them, and confirm their names against the versions installed in your application. Options accepted by one Selenium, Capybara, or Rails release are not automatically portable to another.
Running Chrome in a separate container or service
Remote Selenium configuration
Rails’ remote-browser pattern reads a URL from SELENIUM_REMOTE_URL. The following uses http://localhost:4444/wd/hub only as an example endpoint; the correct path depends on the Selenium version and service you deploy.
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require "test_helper"
class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
url = ENV.fetch("SELENIUM_REMOTE_URL", nil)
options = if url
{ browser: :remote, url: url }
else
{ browser: :chrome }
end
driven_by :selenium, using: :headless_chrome, options: options
endWhen the variable is absent, the test uses a local Chrome session. When it is present, Selenium connects to the remote endpoint.
Make the Rails app reachable from the browser
A browser in another container cannot normally reach a Rails server bound only to loopback. Bind Capybara to an address available on the container network and set an app host that the browser service can resolve. The hostname and port must match your orchestration network:
if ENV["SELENIUM_REMOTE_URL"]
Capybara.server_host = "0.0.0.0"
Capybara.app_host = "http://rails-test:3000"
endHere, rails-test is an example container or service name, not a universal value. Use the actual DNS name and exposed port from Docker Compose, Kubernetes, or your CI network. A loopback address inside the browser container refers to that container itself, not the Rails container.
Network checklist
- Confirm the Rails test server listens on the configured interface and port.
- Resolve the Rails service name from the browser container.
- Test TCP reachability from the browser container to the Rails port.
- Confirm the remote Selenium URL is reachable from the Rails process.
- Keep the Selenium control port on a private, trusted network.
Chrome and driver reliability
Driver discovery
A missing driver executable is a common first-run failure. Inspect the environment inside the process that launches Selenium, not only your interactive shell. Check the executable search path, file permissions, and the driver’s ability to start under the test user.
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Browser-driver compatibility
Keep Chrome and its driver current and verify that their versions work together in the selected runtime. There is no single version pairing that applies to every operating system, Chrome channel, Selenium release, or container image.
Linux startup crashes
If Chrome exits immediately, collect Chrome and Selenium logs and check the runtime user. Launching Chrome as root can cause startup failures and is an unsafe deployment choice. Run the browser as a dedicated, non-privileged account instead.
Security for a server deployment
ChromeDriver exposes powerful browser automation controls. Chrome for Developers recommends using a test account without access to sensitive local or network data, never running ChromeDriver with a privileged account, and isolating it in a protected container or virtual machine.
- Run Chrome and ChromeDriver as a non-root, least-privilege user.
- Keep Chrome, ChromeDriver, Selenium Server, and base images updated.
- Restrict the browser-control port with network policy or a firewall.
- If remote access is required, allow only the intended client addresses.
- Do not publish Selenium or ChromeDriver ports to the public internet to fix a test failure.
- Give the browser test account only the files, credentials, and network routes it needs.
Troubleshooting common failures
“Unable to find driver” or a missing executable
Cause: Selenium cannot locate the driver in the runtime environment. Fix: install a compatible driver in the image or runner, put it on the process path (or configure its explicit location), and verify permissions as the test user.
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Chrome starts locally but not in CI
Cause: the CI image lacks Chrome, has a different browser channel, or runs under a user with different permissions. Fix: inspect installed versions and browser logs inside CI, then align the image, driver, and user rather than relying on your workstation setup.
Chrome exits during startup on Linux
Cause: an unsuitable runtime user or container environment; root execution is a known risk. Fix: run as a non-privileged account, inspect stderr and Selenium logs, and confirm the browser has the libraries and writable directories it needs.
Remote session is created but the page will not load
Cause: the browser cannot resolve or connect to the Rails app host. Fix: set Capybara.server_host to a reachable interface, use the container DNS name in Capybara.app_host, expose the correct port, and test connectivity from the browser container.
Options are rejected after an upgrade
Cause: Rails, Capybara, Selenium, or the browser driver changed the accepted configuration shape. Fix: read the guide and API documentation for the installed Rails version and confirm the driver integration’s current option names.
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Performance and operational trade-offs
Local execution removes a network hop and keeps failure diagnosis in one process, but every runner must carry a browser installation. A remote service centralizes browser maintenance and can isolate heavy browser processes, while introducing endpoint availability, DNS, port, and app-host dependencies. The supplied guidance does not establish a universal speed advantage for either mode, so measure your own suite if runtime is a deciding factor.
For repeatable CI runs, pin the browser image and driver policy, record versions in job logs, retain Selenium and Chrome logs on failure, and make the remote URL and app host explicit environment variables. Treat browser availability as infrastructure: health-check the service before tests and fail with a clear connectivity message.
Or skip the browser setup
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FAQ
Can I use headless Chrome in production code?
Yes, but the Rails system-test example is specifically for tests. Production automation should configure its own job lifecycle, credentials, timeouts, logging, and isolation.
Does headless mode remove the need for Chrome?
No. Headless means no visible window; the Chrome browser binary and a compatible Selenium driver are still required.
Is the sample remote URL always /wd/hub?
No. Treat that path as an example and use the endpoint documented by your Selenium service and version.
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