selenium-grid-4-runner

Author and operate Selenium Grid 4 - self-hosted distributed WebDriver. Covers the six-component architecture (Router / Distributor / Session Map / Event Bus / New Session Queue / Node), standalone vs hub-and-node deployment modes, Docker-image stack (selenium/standalone-chrome, selenium/hub, selenium/node-chrome), node registration, session-queue tuning, and observability. Use for self-hosted cross-browser testing - alternative to cloud grids (BrowserStack / Sauce Labs / LambdaTest) when data residency or cost-control require an on-prem solution.

selenium-grid-4-runner

Overview

Selenium Grid 4 is the self-hosted distributed WebDriver infrastructure - the open-source alternative to cloud-grid SaaS providers. Per selenium.dev/documentation/grid.

Composes with browser-matrix-strategy-reference for matrix planning. Routed by selenium-grid-orchestrator for cross-grid workflows.

When to use

Data-residency requirements forbid cloud grids.
Cost: high-volume testing where cloud per-session pricing exceeds self-hosting break-even.
Internal-network apps where tunnel solutions (BrowserStackLocal / Sauce Connect) are insufficient.
Custom browser builds / non-standard configurations cloud grids don't support.

For cloud-hosted alternatives see browserstack-automate, saucelabs-automate, lambdatest-automate.

Authoring

Six-component architecture

Per Selenium docs:

Component	Role
Router	Entry point; routes WebDriver requests to the right session
Distributor	Allocates new sessions to available Nodes based on capabilities
Session Map	Tracks active sessions (session ID → Node URL)
Event Bus	Internal messaging between components
New Session Queue	Holds pending session requests when no Node available
Node	Runs actual browser instances; registers with the Distributor

Standalone mode (development / small teams)

All components in one JVM:

# Download from selenium.dev/downloads
java -jar selenium-server-<version>.jar standalone

Default port 4444. WebDriver clients connect to http://localhost:4444/wd/hub.

Standalone mode is suitable for a single developer's machine or a small CI runner with co-located browsers.

Hub-and-node mode (production)

Hub on one machine, Nodes on others:

# Hub
java -jar selenium-server-<version>.jar hub

# Node (on another machine)
java -jar selenium-server-<version>.jar node \
  --hub http://hub-host:4444 \
  --port 5555

Or distributed (all components separated):

# Event Bus
java -jar selenium-server.jar event-bus --port 5557

# New Session Queue
java -jar selenium-server.jar sessionqueue --port 5559

# Session Map
java -jar selenium-server.jar sessions --port 5556

# Distributor
java -jar selenium-server.jar distributor --port 5553 \
  --sessions http://sessions-host:5556 \
  --sessionqueue http://queue-host:5559 \
  --bind-bus-events false \
  --publish-events tcp://event-bus-host:4442 \
  --subscribe-events tcp://event-bus-host:4443

# Router
java -jar selenium-server.jar router --port 4444 \
  --sessions http://sessions-host:5556 \
  --distributor http://distributor-host:5553 \
  --sessionqueue http://queue-host:5559

# Node(s)
java -jar selenium-server.jar node \
  --publish-events tcp://event-bus-host:4442 \
  --subscribe-events tcp://event-bus-host:4443

Per Selenium docs, the fully-distributed mode is for very large deployments; most teams run hub-and-node.

Docker stack

Per Selenium docs Docker images are at selenium/* on Docker Hub:

# docker-compose.yml
services:
  selenium-hub:
    image: selenium/hub:4.21.0
    ports: ["4442:4442", "4443:4443", "4444:4444"]

  chrome:
    image: selenium/node-chrome:4.21.0
    shm_size: 2gb
    depends_on: [selenium-hub]
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
      - SE_NODE_MAX_SESSIONS=2

  firefox:
    image: selenium/node-firefox:4.21.0
    shm_size: 2gb
    depends_on: [selenium-hub]
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443

  edge:
    image: selenium/node-edge:4.21.0
    shm_size: 2gb
    depends_on: [selenium-hub]
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443

shm_size: 2gb is required for Chrome (shared-memory bloat with many tabs).

Standalone Docker (simpler)

docker run -d -p 4444:4444 -p 7900:7900 --shm-size="2g" \
  selenium/standalone-chrome:4.21.0

Port 7900 exposes noVNC (browser at http://localhost:7900) for live-session viewing.

Running

Connect a WebDriver client

from selenium import webdriver

driver = webdriver.Remote(
    command_executor="http://grid-router:4444/wd/hub",
    options=webdriver.ChromeOptions(),
)
driver.get("https://example.com")
# ...
driver.quit()

For Kubernetes-deployed Grid use the cluster-internal DNS: http://selenium-router.test-ns.svc.cluster.local:4444/wd/hub.

Capabilities

Standard W3C - no grid-specific options needed:

{
  "browserName": "chrome",
  "browserVersion": "stable",
  "platformName": "linux"
}

Session-queue tuning

Per Selenium docs, key knobs:

Setting	Effect
`--session-request-timeout`	How long a queued session waits before failing (default 300s)
`--session-retry-interval`	Polling interval for matching capabilities (default 5s)
`SE_NODE_MAX_SESSIONS`	Max concurrent sessions per Node (default 1)
`SE_NODE_SESSION_TIMEOUT`	Inactive session cleanup (default 300s)

Tune SE_NODE_MAX_SESSIONS per Node's CPU + memory budget; typical: 2 Chrome / 1 Firefox per 2-core / 4 GB Node.

Observability

Grid 4 exposes:

GraphQL endpoint at /graphql for session inspection
Status endpoint at /status for ready-check
Prometheus metrics at /wd/hub/status (Grid health, session counts)

For production add a Grafana dashboard polling Prometheus.

Parsing results

Grid 4 doesn't add session videos / HAR by default - that's the test client's responsibility (or via a sidecar like selenoid + selenoid-ui for Grid 3-style recording).

Logs at /var/log/seluser/ inside Docker containers, exportable via volume mount.

CI integration

on: pull_request
jobs:
  e2e:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - name: Start Selenium Grid
        run: docker-compose -f selenium-grid.yml up -d
      - name: Wait for Grid ready
        run: |
          for i in {1..30}; do
            curl -s http://localhost:4444/wd/hub/status | grep -q '"ready":true' && break
            sleep 2
          done
      - name: Run E2E tests
        run: pytest tests/e2e/ --grid-url=http://localhost:4444/wd/hub
      - name: Tear down Grid
        if: always()
        run: docker-compose -f selenium-grid.yml down

Anti-patterns

Anti-pattern	Why it fails	Fix
`shm_size` default (64 MB) on Chrome node	Chrome crashes mid-session	Always `shm_size: "2g"`
`SE_NODE_MAX_SESSIONS` too high	OOM / CPU thrashing	Conservative: 2 per 2-core Node
Hub-and-node without health check	Failed nodes silently drop sessions	Wait for `/status` ready before tests start
Standalone in production	No HA; single point of failure	Hub-and-node minimum for prod
Manually allocating ports for Nodes	Conflicts	Let Docker assign + use service DNS
No session-queue timeout	Sessions wait forever; CI hangs	Set `--session-request-timeout` bounded
Mixing Selenium versions across Hub + Nodes	Capability negotiation breaks	Pin same Grid version across all components

Limitations

No real-device support out of box. Mobile testing needs Appium server + emulators / real devices - significant additional infrastructure.
Browser-version matrix is self-managed. Cloud grids ship 3000+ combinations; self-hosted ≤ what your team builds.
HA + scaling is your problem. Cloud grids handle it; on self-hosted you do.
macOS / iOS Safari requires actual macOS hosts. Linux can run Chrome / Firefox / Edge but not Safari.

References

Selenium Grid 4 documentation - selenium.dev/documentation/grid.
Selenium Docker images - github.com/SeleniumHQ/docker-selenium.
W3C WebDriver specification - w3.org/TR/webdriver2/.
Composes: browser-matrix-strategy-reference.
Cloud-grid alternatives: browserstack-automate, saucelabs-automate, lambdatest-automate.
Routed by: selenium-grid-orchestrator.