Playwright Architecture — Explained Simply (Step-by-Step)

Playwright Architecture — Explained Simply (Step by Step)

Playwright is a powerful framework for automating browsers and running end-to-end tests. Its architecture may look complex at first, but it becomes very easy to understand when we see how a test travels from our code to the actual browser.

High-Level Idea

From left to right, the data flow looks like this:

• Your test code (in Java, JS/TS, Python, C#, etc.)
• 󠀠→ sends commands over a WebSocket
• 󠀠→ to the Playwright Server (Node.js)
• 󠀠→ which talks via browser protocols (CDP+) to
• 󠀠→ real browser engines like Chrome, Firefox, and WebKit.

Architecture Diagram (Image)

Architecture Diagram (Text Version)

 ┌────────────────────────────┐
 │   Client Libraries         │
 │ (Java, JS/TS, Python, C#)  │
 └──────────────┬─────────────┘
                │  Test Commands (WebSocket)
                ▼
        ┌────────────────────────┐
        │   Playwright Server    │
        │        (Node.js)       │
        └──────────────┬─────────┘
                       │  CDP / Browser Protocols
                       ▼
        ┌──────────────────────────────┐
        │      Browser Engines         │
        │  Chrome | Firefox | WebKit   │
        └──────────────────────────────┘

1️⃣ Client Libraries – Where You Write Tests

On the left side we have client libraries. These let you write Playwright tests in different languages such as:

• JavaScript / TypeScript
• Python
• Java
• C#

Your test code might look like this:

// Example Playwright test (JavaScript/TypeScript)
await page.goto('https://example.com');
await page.click('text=Login');
await page.fill('#username', 'testuser');
await page.fill('#password', 'secret');
await page.click('text=Submit');

At this stage, the code is just instructions. It does not directly control the browser yet. Instead, the client library packages these steps as commands and sends them to the Playwright server.

2️⃣ WebSocket – The Communication Channel

The commands from your test are sent using a WebSocket connection. A WebSocket is a continuous, two-way communication channel between:

• Your test process (client library), and
• The Playwright server (Node.js).

This keeps communication fast and real-time, so your test can send actions and receive results without constantly reconnecting.

3️⃣ Playwright Server (Node.js) – The Brain

In the middle, Playwright runs a Node.js server. This server is the brain of the system:

• It receives commands from client libraries.
• It manages browsers, browser contexts (profiles), and pages (tabs).
• It translates high-level actions like “click this button” into low-level browser instructions.

For example:

• Your test: await page.click('text=Login');
• Server: Finds the element, waits until it is visible and stable, then sends a protocol command to perform a real click in the browser.

4️⃣ CDP+ and Browser Protocols – Talking to Browsers

From the server, Playwright talks to real browsers using special protocols:

• CDP (Chrome DevTools Protocol) for Chromium-based browsers.
• Similar protocols for Firefox and WebKit.

Through these protocols, the Playwright server can:

• Open and close tabs
• Navigate to URLs
• Click elements, type text, and scroll
• Capture screenshots and videos
• Observe network requests and console logs

5️⃣ Browser Engines – Where the Real Action Happens

On the right side we have the actual browser rendering processes:

• Chrome / Edge (Chromium engine)
• Firefox
• WebKit (Safari engine)

These are full browsers. They:

• Load and render your web pages
• Execute HTML, CSS, and JavaScript
• Respond to the simulated user interactions sent by Playwright

This is where your web application is truly tested in real conditions.

Putting It All Together

1. You write tests in Java, JS/TS, Python, or C# using Playwright’s client library.
2. The client library sends test commands over a WebSocket.
3. The Playwright server (Node.js) receives these commands and manages browsers.
4. The server talks to browser engines via CDP and similar protocols.
5. Real browsers (Chrome, Firefox, WebKit) execute the actions and render your web app.

Why This Architecture Is Powerful

• Multi-language support – choose your favorite language.
• Cross-browser – one framework, multiple real browsers.
• Stable tests – protocol-level control and smart waiting.
• Fast communication – thanks to WebSockets.

In short:
Your test code → WebSocket → Playwright (Node.js) → Browser Protocols → Real Browsers.