Tell me about SSE (Server-Sent Events) and compare it with WebSocket and polling.

Server-Sent Events (SSE) is a technology that allows a server to send a stream of updates to a client over a unidirectional HTTP connection. The client opens a connection to the server and listens for events that the server sends in real time.

Features of SSE:

• Unidirectional: server → client.
• Uses standard HTTP, easily passes through proxies and firewalls.
• Automatically reconnects when the connection is lost.
• Message format is simple, text-based (usually in the format "event: ...\ndata: ...\n\n").

Comparison with WebSocket:

• WebSocket is a bidirectional protocol, allowing both client and server to exchange messages in real time.
• WebSocket is more efficient for interactive applications with frequent data exchange in both directions.
• SSE is simpler to implement for scenarios where the server just sends updates (e.g., news feeds, notifications).

Comparison with Polling:

• Polling involves the client periodically making requests to the server to check for new data.
• SSE is more efficient as the server sends data immediately, without delays and unnecessary requests.
• Polling creates additional load on the server and network due to frequent requests.

Example of using SSE on the client:

const evtSource = new EventSource('/events');
evtSource.onmessage = function(event) {
  console.log('New message:', event.data);
};