Setting Up Grid — Standalone Mode and Docker Compose

Understanding Grid architecture is useful, but you need a running Grid to test against. This lesson walks through two practical deployment methods: standalone mode for quick local setup (one command, everything in one process) and Docker Compose for multi-node environments that mirror real CI/CD infrastructure. By the end, you will have a Grid running with Chrome and Firefox nodes ready to accept tests.

Deploying Grid — Standalone and Docker Compose #

Standalone mode is ideal for local development and learning. Docker Compose is ideal for CI/CD pipelines and team environments where reproducibility and scaling matter.

# ── Method 1: Standalone Mode ──
# Requirements: Java 11+ installed, selenium-server JAR downloaded
# Download from: https://github.com/SeleniumHQ/selenium/releases

STANDALONE_SETUP = {
    "steps": [
        "1. Download selenium-server-4.x.jar from GitHub releases",
        "2. Run: java -jar selenium-server-4.18.1.jar standalone",
        "3. Grid starts on http://localhost:4444",
        "4. Grid UI available at http://localhost:4444/ui",
        "5. All components run in one process (Router + Distributor + Node)",
    ],
    "browsers": "Automatically detects installed browsers (Chrome, Firefox, Edge)",
    "max_sessions": "Default: number of available CPU cores",
    "pros": ["One command setup", "No Docker needed", "Good for local dev"],
    "cons": ["Single machine only", "Manual browser/driver management", "Not reproducible"],
}

# ── Method 2: Docker Compose (Recommended for CI/CD) ──
DOCKER_COMPOSE = """
# docker-compose.yml — Selenium Grid with Chrome and Firefox

version: '3.8'

services:
  selenium-hub:
    image: selenium/hub:4.18.1
    container_name: selenium-hub
    ports:
      - "4442:4442"
      - "4443:4443"
      - "4444:4444"

  chrome-node:
    image: selenium/node-chrome:4.18.1
    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=4
    deploy:
      replicas: 2     # 2 Chrome nodes x 4 sessions = 8 parallel Chrome tests

  firefox-node:
    image: selenium/node-firefox:4.18.1
    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=4
    deploy:
      replicas: 1     # 1 Firefox node x 4 sessions = 4 parallel Firefox tests
"""

# ── Docker commands ──
DOCKER_COMMANDS = [
    ("docker compose up -d",                "Start Grid in background"),
    ("docker compose ps",                   "Check running containers"),
    ("docker compose logs selenium-hub",    "View Hub logs"),
    ("docker compose up -d --scale chrome-node=4", "Scale Chrome to 4 nodes"),
    ("docker compose down",                 "Stop and remove all containers"),
]

# ── Verify Grid is running ──
VERIFY_STEPS = [
    "1. Open http://localhost:4444/ui in your browser",
    "2. You should see the Grid console with registered nodes",
    "3. Check that Chrome and Firefox nodes show 'Available' slots",
    "4. Run a quick test to verify connectivity (next lesson)",
]

print("Method 1: Standalone Mode")
print("=" * 55)
for step in STANDALONE_SETUP["steps"]:
    print(f"  {step}")

print(f"\n\nMethod 2: Docker Compose (Recommended)")
print("=" * 55)
print(DOCKER_COMPOSE)

print(f"\nDocker Commands:")
for cmd, desc in DOCKER_COMMANDS:
    print(f"  $ {cmd}")
    print(f"    → {desc}")

print(f"\nVerification:")
for step in VERIFY_STEPS:
    print(f"  {step}")

Common Mistakes #

Mistake 1 — Not setting shm_size for Chrome Docker nodes #

❌ Wrong: Running Chrome nodes without shm_size: 2gb — Chrome crashes randomly with “page crash” errors on complex pages.

✅ Correct: Always including shm_size: 2gb (or higher) in the Docker Compose configuration for Chrome nodes.

Mistake 2 — Using :latest tag for Selenium Docker images in CI/CD #

❌ Wrong: image: selenium/node-chrome:latest — pipeline breaks when a new version is published with a breaking change.

✅ Correct: image: selenium/node-chrome:4.18.1 — pinned version ensures the same behaviour every time the pipeline runs.