Describe several methods of communication between the server and the client.

• REST (Representational State Transfer): The client sends requests (HTTP methods: GET, POST, PUT, DELETE) to server resources represented by URLs. The server responds with data (usually in JSON or XML). The state is not maintained between requests (stateless).

• GraphQL: An alternative to REST. The client requests only the data it needs in a single request. The server responds strictly according to the request. It provides more efficient data transfer, especially for applications with complex data requirements.

• WebSockets: A full-duplex connection between the client and server over a single TCP connection. Both parties can send data at any time without constantly opening/closing connections. Ideal for real-time applications (chats, online games).

  // Example client code for WebSocket
  const ws = new WebSocket('ws://localhost:8080');
  
  ws.onopen = () => {
    console.log('Connection established');
  };
  
  ws.onmessage = (event) => {
    console.log('Received message:', event.data);
  };
  
  ws.onerror = (error) => {
    console.error('WebSocket error:', error);
  };
  
  ws.onclose = () => {
    console.log('Connection closed');
  };
  
  // Sending a message
  ws.send('Hello, server!');
  

• Server-Sent Events (SSE): The server sends streaming updates to the client over a single HTTP connection. The client cannot send data to the server over this connection, only receive. Useful for unidirectional notifications (e.g., news feed updates).

  // Example client code for SSE
  const eventSource = new EventSource('/events');
  
  eventSource.onmessage = (event) => {
    console.log('Received event:', event.data);
  };
  
  eventSource.onerror = (error) => {
    console.error('SSE error:', error);
    eventSource.close();
  };
  

• Polling: The client periodically sends requests to the server to check for updates. Simple but inefficient, as most requests may not contain new data.

• Long Polling: The client sends a request, and the server holds the connection until new data appears or a timeout occurs. Once data is sent or timeout expires, the client immediately sends a new request. More efficient than regular polling but can still create load.

Comparison table:

Method	Directionality	Real-time Data Efficiency	Basic Implementation Simplicity
REST	Client -> Server	Low	High
GraphQL	Client -> Server	Low	Medium
WebSockets	Bidirectional	High	Medium
SSE	Server -> Client	High	Medium
Polling	Client -> Server (frequent)	Low	High
Long Polling	Client -> Server (long)	Medium	Medium