How do I run Playwright tests in parallel, and what are workers?

Playwright already runs test files in parallel, each in a worker: a separate operating-system process with its own browser. You set how many workers run at once with --workers or the workers config option, and you let tests inside one file run in parallel with fullyParallel: true or test.describe.configure({ mode: 'parallel' }).

This answer is one piece of Chapter 16: The Test Runner.

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A worker is an independent process that the test runner starts and hands tests to. Each worker launches its own browser, and workers cannot talk to each other, so parallel tests must not share state or global variables. That isolation is what makes parallel runs safe.

By default, different files run at the same time in different workers, but the tests inside one file run in order in the same worker. Two switches change that. fullyParallel: true in the config spreads every test across workers. test.describe.configure({ mode: 'parallel' }) does the same for one file.

The number of workers is a limit you choose. --workers=2 on the command line, or workers: process.env.CI ? 2 : undefined in the config, caps it. --workers=1 turns parallelism off, which is useful when you suspect two tests are interfering.

The example puts four page checks in one file, opts that file into parallel mode and runs it with two workers. The log shows the tests shared out between worker 0 and worker 1, read from testInfo.workerIndex.

parallel.spec.js(https://practice.learnplayforge.com)

import { test, expect } from '@playwright/test';

test.describe.configure({ mode: 'parallel' });

for (const path of ['/catalog.html', '/orders.html', '/api-lab.html', '/login.html']) {
  test(`opens ${path}`, async ({ page }, testInfo) => {
    await page.goto(`${path}?mock=static&seed=42&autologin=1`);
    await expect(page.getByTestId('site-header')).toBeVisible();
    console.log(`${path} ran on worker ${testInfo.workerIndex}`);
  });
}

Output

$ npx playwright test parallel.spec.js --workers=2
Running 4 tests using 2 workers

/catalog.html ran on worker 0
/orders.html ran on worker 1
  ok 1 [chromium] › parallel.spec.js:6:3 › opens /catalog.html (650ms)
  ok 2 [chromium] › parallel.spec.js:6:3 › opens /orders.html (644ms)
/api-lab.html ran on worker 0
/login.html ran on worker 1
  ok 3 [chromium] › parallel.spec.js:6:3 › opens /api-lab.html (439ms)
  ok 4 [chromium] › parallel.spec.js:6:3 › opens /login.html (443ms)

  4 passed (2.8s)

Test yourself

A file has five tests and no parallel settings anywhere. You run it with --workers=4. How do those five tests run?

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How do I run Playwright tests in parallel, and what are workers?

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