Schreiben Sie einen grundlegenden Push-Service mit WebSockets.
Sure, here's a basic example of a WebSocket-based push service that can send push notifications to clients:

Server-side (Node.js and WebSocket library):
```
const WebSocket = require('ws');

// create a new WebSocket server on port 8080
const wss = new WebSocket.Server({ port: 8080 });

// array to store connected clients
const clients = [];

// when a client connects to the server
wss.on('connection', (ws) => {
  // add the client to the list of connected clients
  clients.push(ws);

  // when a message is received from a client
  ws.on('message', (message) => {
    console.log(`Received message: ${message}`);

    // send the message to all connected clients
    clients.forEach((client) => {
      if (client.readyState === WebSocket.OPEN) {
        client.send(message);
      }
    });
  });

  // when a client disconnects
  ws.on('close', () => {
    // remove the client from the list of connected clients
    clients.splice(clients.indexOf(ws), 1);
  });
});
```

Client-side (JavaScript):
```
const ws = new WebSocket('ws://localhost:8080');

// when the WebSocket connection is established
ws.addEventListener('open', (event) => {
  console.log('WebSocket connection established');
});

// when a message is received from the server
ws.addEventListener('message', (event) => {
  console.log(`Received message: ${event.data}`);
});

// send a message to the server
ws.send('Hello, server!');
```

This is a very basic example, but it should give you an idea of how a WebSocket-based push service can be implemented. In this example, the server sends any message it receives from a client to all connected clients, and the clients simply log any messages they receive from the server. In a real-world push service, you would likely want to customize the behavior based on your specific use case.