How does Playwright run tests in parallel?

The Answer

Playwright spawns multiple worker processes that run tests simultaneously. Each worker operates independently with its own browser, browser context, and page — guaranteeing complete isolation.

Deep Dive Explanation

Worker count impacts resource usage. A good rule: set workers equal to the number of CPU cores available. In Docker-based CI, over-provisioning workers causes memory pressure and paradoxically slows down execution.

example.spec.ts

// playwright.config.ts
export default defineConfig({
  // Run ALL tests across files in parallel (no ordering)
  fullyParallel: true,

  // Control number of workers
  workers: process.env.CI ? 4 : '50%', // 4 in CI, half CPU cores locally

  // Override per describe block
});

// In test file - force parallel within a file
test.describe.configure({ mode: 'parallel' });

// Run with specific workers via CLI
// npx playwright test --workers=4

// Sharding for CI (split across multiple machines)
// Machine 1: npx playwright test --shard=1/3
// Machine 2: npx playwright test --shard=2/3
// Machine 3: npx playwright test --shard=3/3