added web socker doc for nginx

Web-Servers/Nginx/12-WebSocket.md

@@ -0,0 +1,144 @@
+# WebSocket and Nginx Reverse Proxy Configuration Guide
+
+## 1. Introduction
+
+**WebSocket** is a **bi-directional, full-duplex communication protocol** that operates over a **single TCP connection**. Unlike traditional HTTP, where the client sends a request and the server responds, WebSocket allows both client and server to send messages to each other at any time during the session.
+
+This makes it highly suitable for **real-time applications** such as chat systems, live dashboards, and streaming services.
+
+---
+
+## 2. How WebSocket Works
+
+### Traditional HTTP
+
+In HTTP:
+
+* The client sends a request.
+* The server sends a response.
+* The connection is then closed (unless using persistent connections).
+
+### WebSocket
+
+In WebSocket:
+
+* The client initiates an **HTTP request** with special headers to **upgrade** the connection from HTTP to WebSocket.
+* The server accepts the upgrade and switches protocols.
+* Once established, both sides can exchange data freely over a persistent TCP connection.
+
+---
+
+## 3. Establishing a WebSocket Connection
+
+### Client → Server (Upgrade Request)
+
+The WebSocket connection starts as a standard HTTP request with additional headers:
+
+```http
+GET /chat HTTP/1.1
+Host: example.com
+Upgrade: websocket
+Connection: Upgrade
+Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
+Sec-WebSocket-Version: 13
+```
+
+### Server → Client (Upgrade Response)
+
+If the server supports WebSocket, it responds with:
+
+```http
+HTTP/1.1 101 Switching Protocols
+Upgrade: websocket
+Connection: Upgrade
+Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
+```
+
+---
+
+## 4. Header Explanation
+
+* **Upgrade:**
+  Indicates the protocol the client wants to switch to (in this case, `websocket`).
+
+* **Connection:**
+  Must include the value `Upgrade` to signal the intention to upgrade the connection.
+
+* **Sec-WebSocket-Key:**
+  A randomly generated Base64-encoded value (16 bytes) sent by the client.
+  It ensures a unique connection and prevents cross-protocol attacks.
+
+* **Sec-WebSocket-Version:**
+  Specifies the WebSocket protocol version the client supports.
+  Most servers use **version 13**.
+  If not supported, the server responds with:
+
+  ```http
+  HTTP/1.1 426 Upgrade Required
+  Sec-WebSocket-Version: 13
+  ```
+
+* **Sec-WebSocket-Accept:**
+  Returned by the server as proof that it successfully accepted the handshake.
+  It’s derived from the client’s `Sec-WebSocket-Key` using a predefined GUID.
+
+---
+
+## 5. HTTP Version Requirement
+
+WebSocket requires **HTTP/1.1 or higher** because:
+
+* **HTTP/1.0** opens a new TCP connection for each request, leading to high overhead.
+* **HTTP/1.1** introduced **persistent connections (Keep-Alive)**, allowing multiple requests to share a single TCP connection.
+
+For example:
+
+* In HTTP/1.0, downloading 4 images would open 4 separate TCP connections.
+* In HTTP/1.1, one TCP connection can handle all 4 requests.
+
+WebSocket leverages this concept to maintain a **single long-lived TCP connection** between the client and server.
+
+---
+
+## 6. Enabling WebSocket Support in Nginx
+
+When using Nginx as a **reverse proxy**, the connection upgrade must be explicitly configured.
+
+### Example Configuration
+
+```nginx
+location / {
+    proxy_pass http://backend:8080;
+    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Connection "upgrade";
+    proxy_set_header Host $host;
+    proxy_http_version 1.1;
+}
+```
+
+### Explanation
+
+* **proxy_pass:**
+  Forwards the request to the backend service.
+
+* **proxy_set_header Upgrade $http_upgrade;**
+  Passes the `Upgrade` header from the client to the backend.
+
+* **proxy_set_header Connection "upgrade";**
+  Ensures Nginx forwards the `Connection: Upgrade` header required for WebSocket.
+
+* **proxy_http_version 1.1;**
+  Enables HTTP/1.1 support in proxying, required for WebSocket functionality.
+
+---
+
+## 7. Summary
+
+| Feature               | Description                                                                         |
+| --------------------- | ----------------------------------------------------------------------------------- |
+| Protocol              | Bi-directional, full-duplex                                                         |
+| Base Protocol         | HTTP/1.1                                                                            |
+| Upgrade Mechanism     | HTTP 101 Switching Protocols                                                        |
+| Persistent Connection | Yes                                                                                 |
+| Key Headers           | Upgrade, Connection, Sec-WebSocket-Key, Sec-WebSocket-Version, Sec-WebSocket-Accept |
+| Nginx Requirement     | Must enable `proxy_http_version 1.1` and pass upgrade headers                       |