Answer

Why Visual Comparisons Fail in Parallel Mode

When you run tests in parallel mode, visual comparisons can fail. This is because parallel tests are executed in separate worker processes and can't share any state or global variables. This can lead to unexpected results due to interference between tests.

Visual comparisons depend on capturing screenshots or videos of the application under test. In parallel mode, multiple instances of the application may be running at the same time, each capturing its own screenshots or videos. This can introduce timing issues and race conditions.

Additionally, running tests in parallel mode can lead to resource contention issues. If multiple tests require heavy CPU usage or network bandwidth at the same time, it can lead to slower execution times and potential failures in visual comparisons.

How to Address These Issues

To improve the reliability and accuracy of visual comparisons when running tests in parallel mode, you can:

1. Ensure proper synchronization mechanisms: If your testing framework supports it, use synchronization mechanisms like locks or semaphores to control access to shared resources during visual comparison steps.

// Example using Playwright Test
import { test } from '@playwright/test';

test('visual comparison', async ({ page }) => {
  // Lock access to shared resources
  await page.evaluate(() => {
    // Your synchronization code here
  });

  // Your visual comparison code here
});

2. Use unique identifiers: Assign unique identifiers to captured screenshots or videos to avoid conflicts.

// Example using Playwright Test
import { test } from '@playwright/test';

test('visual comparison', async ({ page }) => {
  // Capture screenshot with unique identifier
  await page.screenshot({ path: `screenshot-${Date.now()}.png` });
});

3. Optimize resource usage: Analyze the resource requirements of your tests and optimize their execution to minimize contention.

For more tips on how to optimize your Playwright tests, check out this blog post.