Uncategorized

Playwright vs k6: When to Test the Interface vs the Infrastructure

13 min read

Playwright and k6 answer different questions about your application. Playwright asks: "Does this feature work correctly for one user?" k6 asks: "Does this feature still work when thousands of users hit it simultaneously?"

These tools do not compete. They protect different parts of your system. Playwright detects broken buttons, failed form submissions, and visual bugs. k6 detects slow APIs, database bottlenecks, and servers that crash under pressure.

Many teams make the mistake of choosing one or the other. The reality is that most production applications need both.

This guide explains what each tool does, when to use each one, and how they work together in a mature testing strategy.

Quick Comparison Table

These tools operate in different domains. This table highlights their distinct purposes:

Aspect

Playwright

k6

Primary Purpose

Functional UI testing

Load and performance testing

What It Tests

User interface behavior

Backend capacity and speed

Test Environment

Real browser

HTTP/protocol level

Simulates

One user interacting with the UI

Thousands of concurrent users

Catches

Broken features, UI bugs, regressions

Slow responses, crashes under load, bottlenecks

Resource Usage

High (runs actual browsers)

Low (no browser needed)

Execution Speed

Minutes for full UI suite

Can simulate hours of traffic in minutes

Language

JavaScript, TypeScript, Python, Java, C#

JavaScript (specialized runtime)

Created By

Microsoft

Grafana Labs

Typical Run Frequency

Every commit, every PR

Nightly, weekly, or before releases

Output

Pass/fail, screenshots, traces

Response times, throughput, error rates

What Playwright Actually Does

Playwright launches real browsers and mimics human actions. It clicks buttons, fills forms, navigates pages, and checks that elements appear correctly. Every test runs exactly how a real user would experience your application.

A Simple Playwright Test

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

test('user can submit contact form', async ({ page }) => {
  await page.goto('/contact');
 
  await page.getByLabel('Name').fill('John Smith');
  await page.getByLabel('Email').fill('[email protected]');
  await page.getByLabel('Message').fill('Hello, I have a question.');
 
  await page.getByRole('button', { name: 'Send Message' }).click();
 
  await expect(page.getByText('Thank you for your message')).toBeVisible();
});

This test confirms that the contact form works. It fills fields, clicks submit, and verifies the success message appears. If any step fails, the button doesn't exist, the form doesn't submit, or the confirmation never shows, the test catches it.

What Playwright Validates

  • Forms are submit correctly

  • Navigation flows work end-to-end

  • Dynamic content loads properly

  • Error messages display when expected

  • Visual elements render correctly

  • JavaScript interactions function properly

What Playwright Cannot Tell You

  • How fast the form submission API responds

  • Whether the server handles 100 simultaneous submissions

  • If the database slows down under heavy write load

  • Whether the email service queues messages efficiently

  • How response times degrade as traffic increases

What k6 Actually Does

k6 sends HTTP requests directly to your backend without any browser. It simulates hundreds or thousands of users making API calls simultaneously. This reveals how your infrastructure performs under stress.

A Simple k6 Test

javascript
import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  vus: 100,           // 100 virtual users
  duration: '2m',     // Run for 2 minutes
};

export default function () {
  const payload = JSON.stringify({
    name: 'John Smith',
    email: '[email protected]',
    message: 'Hello, I have a question.',
  });

  const response = http.post('https://api.example.com/contact', payload, {
    headers: { 'Content-Type': 'application/json' },
  });

  check(response, {
    'status is 200': (r) => r.status === 200,
    'response time < 500ms': (r) => r.timings.duration < 500,
  });

  sleep(1); // Wait 1 second between requests
}

This test hammers the contact form API with 100 virtual users for 2 minutes. Each user submits the form once per second. That equals roughly 12,000 submissions, far more than manual testing could ever achieve.

What k6 Validates

  • API response times under various load levels

  • Maximum throughput before errors occur

  • How response times degrade as load increases

  • Which endpoints become bottlenecks first

  • Whether the system recovers after traffic spikes

  • Database and third-party service performance

What k6 Cannot Tell You

  • Whether the submit button is visible

  • If the form validation works in the browser

  • Whether CSS renders correctly

  • If JavaScript errors break the UI

  • How the success message animates

  • Whether the page works on mobile devices

The Testing Pyramid: Where Each Tool Fits

Software testing works best in layers. Each layer catches different problems at different costs.

Unit Tests (Bottom Layer)

Test individual functions in isolation. Fast and cheap. Run thousands in seconds.

API Tests (Middle Layer)

Test backend endpoints without browsers. k6 operates here for performance. Tools like Postman or REST Assured handle functional API testing.

End-to-End Tests (Top Layer)

Test complete user flows through real browsers. Playwright operates here. Expensive to run and maintain, but catches integration problems that nothing else can find.

Where Load Testing Fits

k6 can stress-test any layer that involves network calls. Most commonly, teams use it against:

  • REST APIs

  • GraphQL endpoints

  • WebSocket connections

  • Microservice communication

  • Database queries (indirectly)

Same Feature, Different Tests

The best way to understand these tools is to see how they test the same feature from different angles.

Feature: User Registration

What could go wrong:

  • The registration form might not submit (UI bug)

  • The API might reject valid data (backend bug)

  • The database might fail under many signups (capacity issue)

  • The email service might queue too slowly (integration issue)

Playwright Test: Does Registration Work?

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

test('new user can register successfully', async ({ page }) => {
  await page.goto('/register');
 
  // Fill registration form
  await page.getByLabel('Full Name').fill('Jane Doe');
  await page.getByLabel('Email').filljane${Date.now()}@example.com);
  await page.getByLabel('Password').fill('SecurePass123!');
  await page.getByLabel('Confirm Password').fill('SecurePass123!');
 
  // Accept terms and submit
  await page.getByLabel('I agree to the terms').check();
  await page.getByRole('button', { name: 'Create Account' }).click();
 
  // Verify success
  await expect(page.getByText('Welcome, Jane')).toBeVisible();
  await expect(page).toHaveURL('/dashboard');
});

A user can fill the form, submit it, and reach their dashboard. One user, one registration, works correctly.

k6 Test: Can Registration Handle Launch Day Traffic?

javascript
import http from 'k6/http';
import { check, sleep } from 'k6';
import { randomString } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';

export const options = {
  stages: [
    { duration: '1m', target: 50 },   // Ramp up to 50 users
    { duration: '3m', target: 200 },  // Ramp up to 200 users
    { duration: '2m', target: 200 },  // Stay at 200 users
    { duration: '1m', target: 0 },    // Ramp down
  ],
  thresholds: {
    http_req_duration: ['p(95)<1000'], // 95% of requests under 1 second
    http_req_failed: ['rate<0.05'],    // Less than 5% errors
  },
};

export default function () {
  const uniqueEmail = user_${randomString(8)}@example.com;
 
  const payload = JSON.stringify({
    fullName: 'Load Test User',
    email: uniqueEmail,
    password: 'SecurePass123!',
  });

  const response = http.post('https://api.example.com/register', payload, {
    headers: { 'Content-Type': 'application/json' },
  });

  check(response, {
    'registration successful': (r) => r.status === 201,
    'response time acceptable': (r) => r.timings.duration < 1000,
  });

  sleep(Math.random() * 3); // Random delay between 0-3 seconds
}

The registration API handles 200 concurrent users with acceptable response times and low error rates. It simulates a product launch where many people sign up simultaneously.

What Each Test Detects

Problem

Playwright Detects?

k6 Detects?

Submit button missing

✅ Yes

❌ No

Password validation is broken in the UI

✅ Yes

❌ No

API returns the wrong status code

✅ Yes (indirectly)

✅ Yes

Database connection pool exhausted

❌ No

✅ Yes

Response time spikes to 10 seconds

❌ No

✅ Yes

Email service fails at 100 signups/minute

❌ No

✅ Yes

The success message doesn't appear

✅ Yes

❌ No

Resource Requirements: A Practical Difference

These tools demand vastly different computing resources, which affects where and when you run them.

Playwright Resource Usage

Each Playwright test launches a real browser. Browsers consume significant memory and CPU.

Resource Requirements: A Practical Difference

These tools demand vastly different computing resources, which affects where and when you run them.

Playwright Resource Usage

Each Playwright test launches a real browser. Browsers consume significant memory and CPU.

Parallel Tests

Approximate RAM

CPU Cores Needed

1

200–400 MB

1

2

800 MB – 1.5 GB

2–4

10

2–4 GB

4–8

20

4–8 GB

8+

Running 50 parallel Playwright tests requires a substantial CI machine or distributed infrastructure.

k6 Resource Usage

k6 operates without browsers. It generates HTTP traffic directly, requiring minimal resources.

Virtual Users

Approximate RAM

CPU Cores Needed

100

50–100 MB

1

1,000

200–400 MB

1–2

10,000

1–2 GB

2–4

50,000

4–8 GB

4–8

A single laptop can simulate thousands of users with k6. Simulating the same load with Playwright would require a data center.

Practical Implication

  • Run Playwright tests on every commit (limited parallelism)

  • Run k6 tests nightly or before releases (high load simulation)

  • Use Playwright in CI/CD pipelines with reasonable timeouts

  • Use k6 for scheduled performance regression checks

When You Need Playwright

Choose Playwright when you need to verify user-facing functionality works correctly.

Scenarios for Playwright

Critical User Journeys

  • Checkout and payment flows

  • User registration and login

  • Search and filter functionality

  • Form submissions with validation

Visual Correctness

  • Elements appear in correct positions

  • Responsive layouts work across viewports

  • Dynamic content renders properly

  • Error states display correctly

JavaScript-Dependent Features

  • Single-page application navigation

  • Interactive components (modals, dropdowns)

  • Real-time updates (notifications, live data)

  • Client-side form validation

Cross-Browser Verification

  • Features work in Chrome, Firefox, and Safari

  • Mobile browser compatibility

  • Different viewport behaviors

When You Need k6

Choose k6 when you need to verify your system handles expected (or unexpected) traffic volumes.

Scenarios for k6

Capacity Planning

  • How many users can the system support?

  • At what point do response times degrade?

  • Where are the bottlenecks?

Performance Regression

  • Did the latest deployment slow down the API?

  • Are database queries still efficient?

  • Has third-party integration latency changed?

Stress Testing

  • What happens during traffic spikes?

  • Does the system recover after overload?

  • Which component fails first?

SLA Validation

  • Do 95% of requests complete under 500ms?

  • Is uptime maintained under load?

  • Are error rates within acceptable limits?

Using Both Tools Together

Mature testing strategies combine both tools. Here's how they fit into a typical workflow.

CI/CD Pipeline Integration

yaml
# Example GitHub Actions workflow
name: Complete Test Suite

on: [push, pull_request]

jobs:
  # Fast feedback on every commit
  playwright-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Install dependencies
        run: npm ci
      - name: Run Playwright tests
        run: npx playwright test
      - name: Upload test results
        uses: actions/upload-artifact@v4
        with:
          name: playwright-report
          path: playwright-report/

  # Performance check on main branch only
  k6-load-tests:
    runs-on: ubuntu-latest
    if: github.ref == 'refs/heads/main'
    steps:
      - uses: actions/checkout@v4
      - name: Install k6
        run: |
          sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys C5AD17C747E3415A3642D57D77C6C491D6AC1D69
          echo "deb https://dl.k6.io/deb stable main" | sudo tee /etc/apt/sources.list.d/k6.list
          sudo apt-get update
          sudo apt-get install k6
      - name: Run load tests
        run: k6 run ./load-tests/api-performance.js

Testing Schedule Recommendation

Test Type

Tool

Frequency

Duration

Smoke tests

Playwright

Every commit

2–5 minutes

Full E2E suite

Playwright

Every PR

10–20 minutes

API load test

k6

Nightly

10–30 minutes

Stress test

k6

Weekly

30–60 minutes

Soak test

k6

Before releases

2–4 hours

Shared Test Data Strategy

Keep test data consistent across both tools:

javascript
// shared/test-data.js
export const testUser = {
  name: 'Test User',
  email: '[email protected]',
  password: 'TestPass123!',
};

export const testProduct = {
  id: 'prod_12345',
  name: 'Test Product',
  price: 29.99,
};

Both Playwright and k6 can import this data, ensuring tests validate the same scenarios.

Common Mistakes Teams Make

Mistake 1: Using Playwright for Load Testing

Some teams try to run many Playwright tests in parallel to simulate load. This approach fails for several reasons:

  • Browsers consume too many resources

  • Tests include UI rendering overhead

  • Results don't reflect pure API performance

  • Infrastructure costs become prohibitive

Solution: Use Playwright for functional verification and k6 for load simulation.

Mistake 2: Skipping UI Tests Because APIs Pass

A passing k6 test means the API works under load. It does not mean users can access that API through the UI.

Example: The payment API handles 1,000 requests per second perfectly. But a JavaScript error prevents the "Pay Now" button from appearing. k6 never catches this.

Solution: Both tools serve essential roles. Neither replaces the other.

Mistake 3: Running Load Tests Too Rarely

Teams often run k6 only before major releases. Performance regressions introduced weeks earlier go unnoticed until production users complain.

Solution: Run baseline load tests nightly. Catch regressions early when fixes are simpler.

Mistake 4: Testing Only Happy Paths

Both tools should verify error handling:

javascript
// Playwright: Test error states
test('shows error for invalid email', async ({ page }) => {
  await page.goto('/register');
  await page.getByLabel('Email').fill('not-an-email');
  await page.getByRole('button', { name: 'Submit' }).click();
  await expect(page.getByText('Please enter a valid email')).toBeVisible();
});

// k6: Test API error handling under load
export default function () {
  // Intentionally send invalid data
  const response = http.post('https://api.example.com/register',
    JSON.stringify({ email: 'invalid' }),
    { headers: { 'Content-Type': 'application/json' } }
  );
 
  check(response, {
    'returns 400 for invalid data': (r) => r.status === 400,
    'error response is fast': (r) => r.timings.duration < 200,
  });
}

Choosing Your Starting Point

If you have neither tool in place, start based on your most pressing problem.

Start with Playwright If:

  • Users report broken features after deployments

  • Manual testing consumes too much time

  • Bugs reach production that testing should catch

  • You need cross-browser verification

  • Your application relies heavily on JavaScript

Start with k6 If:

  • Users complain about slow response times

  • The application crashes during traffic spikes

  • You're preparing for a launch or marketing campaign

  • SLAs require performance guarantees

  • Previous outages were capacity-related

Eventually, Add Both

Most production applications benefit from both tools. Start where the pain is greatest, then expand coverage as resources allow.

Conclusion

Playwright and k6 protect different aspects of your application. Playwright ensures features work correctly for users. k6 ensures infrastructure handles the expected load. Skipping either creates blind spots that eventually cause production incidents.

Teams that master both tools ship with greater confidence. They detect UI bugs before users encounter them. They identify performance bottlenecks before traffic spikes expose them. The investment in both testing layers pays dividends through fewer incidents, faster releases, and happier users.

FAQs


1. Does k6 support Playwright?

Yes, but not directly. k6 browser module uses a Playwright-compatible API, meaning if you know Playwright, you'll find k6's syntax very similar. However, k6 doesn't run Playwright itself, it has its own browser automation built on Chromium DevTools Protocol (CDP). Grafana Labs describes k6 browser as having "rough compatibility" with Playwright. You can migrate Playwright scripts to k6 with minor adjustments, and the k6-jslib-testing library provides Playwright-like assertions. The syntax is intentionally similar, so teams don't need to learn a completely new API.

2. Why is k6 better than JMeter?

k6 outperforms JMeter in several areas for modern development workflows. First, resource efficiency: k6 uses Go goroutines instead of JVM threads, consuming roughly 100KB per virtual user versus JMeter's 1MB default, meaning one k6 instance can simulate tens of thousands of users while JMeter maxes out at a few thousand. Second, developer experience: k6 scripts are written in JavaScript and version-controlled with Git, while JMeter's XML test plans are difficult to review and collaborate on. Third, CI/CD integration: k6's CLI-first design fits naturally into pipelines, whereas JMeter requires additional configuration. However, JMeter still wins for legacy protocol support (FTP, JDBC, LDAP, SMTP) and teams preferring GUI-based test creation.

3. Is k6 owned by Grafana?

Yes. Grafana Labs acquired k6 in 2021. k6 was originally developed by Load Impact (founded in Sweden), which later rebranded to k6. The acquisition integrated k6 into Grafana's observability ecosystem, enabling native connections to Grafana dashboards, Prometheus, and InfluxDB for real-time metrics visualization. k6 remains open-source under Grafana Labs' stewardship.

4. Is k6 written in JavaScript?

Partially. You write k6 test scripts in JavaScript (ES6+), but the k6 engine itself is written in Go. This hybrid approach gives you the developer-friendly scripting of JavaScript while leveraging Go's performance for executing virtual users efficiently. k6 uses Sobek (formerly goja), a JavaScript runtime written in Go, rather than Node.js, which means some npm packages won't work directly in k6 scripts.

5. Should I use Playwright or k6 for API testing?

Both can test APIs, but they serve different purposes. Use Playwright when you need to test APIs as part of end-to-end browser flows (intercepting network requests, mocking responses). Use k6 when you need to stress-test API endpoints with thousands of concurrent requests and measure performance metrics like p95 response times. Most teams use both: Playwright for functional API validation in E2E tests, k6 for dedicated API performance testing.

6. Can Playwright and k6 work together?

Yes, as complementary tools in your testing strategy. Use Playwright for detailed functional testing (does the checkout flow work correctly?), then use k6 for load testing (can 1,000 users checkout simultaneously?). Some teams even share page object patterns between both frameworks. For browser-based load testing specifically, k6 browser lets you reuse Playwright-like syntax while leveraging k6's load generation capabilities.

7. How many virtual users can k6 simulate compared to Playwright?

k6 can simulate 30,000-40,000 virtual users for protocol-level (HTTP) testing on a single machine. For browser-based testing, expect 5-20 concurrent browser sessions depending on your hardware. Playwright, being designed for functional testing, typically runs 1-10 parallel browser instances. The difference is architectural: k6 optimizes for load generation, Playwright optimizes for test reliability and debugging.






Share this post

Experience AI-Powered Testing in Action with Supatest AI

Loading...
Copyright © 2026, Supatest Blog. All rights reserved

Published with

Typeflo