Long Polling vs Server-Sent Events (SSE)

Introduction

Many modern applications require real-time communication between clients and servers.

Examples include:

The traditional HTTP request-response model was not designed for continuous real-time communication.

Standard HTTP works like this:

Client → Request → Server
Server → Response → Client
Connection closes

If the client wants updates, it must repeatedly send requests.

This leads to inefficient patterns like polling.

To solve this, several techniques evolved:

1. Polling
2. Long Polling
3. Server-Sent Events (SSE)
4. WebSockets

This article focuses on Long Polling and SSE, which are widely used alternatives to full-duplex WebSockets.

The Problem with Traditional Polling

Traditional polling repeatedly checks the server for updates.

Example:

Problems with polling:

Example:

If a client polls every 2 seconds, most requests return no data.

This inefficiency led to Long Polling.

Long Polling

What is Long Polling?

Long Polling is a technique where the client sends a request and the server keeps the connection open until new data becomes available.

Instead of responding immediately, the server waits for updates.

Workflow:

This significantly reduces unnecessary requests.

Long Polling Architecture

Components:

Long Polling Request Lifecycle

1. Client sends request to server
2. Server keeps request open
3. If event occurs → server responds
4. Client immediately sends a new request

Diagram:

Advantages of Long Polling

Limitations of Long Polling

For large systems, this approach becomes expensive.

Server-Sent Events (SSE)

What is SSE?

Server-Sent Events allow the server to push updates continuously to the client over a single HTTP connection.

Unlike Long Polling:

• Connection remains open
• Server streams events
• Client receives updates continuously

Workflow:

The connection stays alive indefinitely.

SSE Architecture

The server maintains a persistent HTTP connection with the client.

SSE Communication Flow

The server sends event streams.

SSE Message Format

SSE uses a simple text format.

Example:

event: message
data: Hello World

event: notification
data: New message received

Fields:

SSE Client Example (JavaScript)

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

The browser automatically maintains the connection.

Advantages of SSE

Limitations of SSE

Long Polling vs SSE

Comparison:

Architecture Comparison

Long Polling:

Repeated connections.

SSE:

Single connection.

Scaling Long Polling

To scale long polling:

Example architecture:

Scaling SSE

SSE scaling strategies:

Architecture:

Real-World Use Cases

Long Polling:

SSE:

SSE vs WebSockets

Although SSE is powerful, WebSockets offer full-duplex communication.

Choosing the Right Approach

Decision table:

Summary

Real-time communication is essential for modern applications.

Long Polling and Server-Sent Events provide efficient alternatives to traditional polling.

Key takeaways:

Compared to polling, both approaches:

• Reduce latency
• Lower server load
• Enable real-time data delivery

SSE is generally preferred when unidirectional streaming updates are needed, while Long Polling remains useful for legacy systems or environments without SSE support.