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+---
+name: Set per-socket contextual data on a WebSocket
+---
+
+When building a WebSocket server, it's typically necessary to store some identifying information or context associated with each connected client.
+
+With [Bun.serve()](/docs/api/websockets#contextual-data), this "contextual data" is set when the connection is initially upgraded by passing a `data` parameter in the `server.upgrade()` call.
+
+```ts
+Bun.serve<{ socketId: number }>({
+  fetch(req, server) {
+    const success = server.upgrade(req, {
+      data: {
+        socketId: Math.random(),
+      },
+    });
+    if (success) return undefined;
+
+    // handle HTTP request normally
+    // ...
+  },
+  websocket: {
+    // define websocket handlers
+    async message(ws, message) {
+      // the contextual dta is available as the `data` property
+      // on the WebSocket instance
+      console.log(`Received ${message} from ${ws.data.socketId}}`);
+    },
+  },
+});
+```
+
+---
+
+It's common to read cookies/headers from the incoming request to identify the connecting client.
+
+```ts
+type WebSocketData = {
+  createdAt: number;
+  token: string;
+  userId: string;
+};
+
+// TypeScript: specify the type of `data`
+Bun.serve<WebSocketData>({
+  async fetch(req, server) {
+    // use a library to parse cookies
+    const cookies = parseCookies(req.headers.get("Cookie"));
+    const token = cookies["X-Token"];
+    const user = await getUserFromToken(ws.data.authToken);
+
+    const upgraded = server.upgrade(req, {
+      data: {
+        createdAt: Date.now(),
+        token: cookies["X-Token"],
+        userId: user.id,
+      },
+    });
+
+    if (upgraded) return undefined;
+  },
+  websocket: {
+    async message(ws, message) {
+      // save the message to a database
+      await saveMessageToDatabase({
+        message: String(message),
+        userId: ws.data.userId,
+      });
+    },
+  },
+});
+```