AJAX - Long Polling and Server Push Techniques

1. Why Do We Need Long Polling or Server Push?

Traditional AJAX works like this:

• Client sends a request

• Server responds immediately

• Connection closes

This is fine for static data, but not suitable for real-time updates like:

• Chat messages

• Notifications

• Live scores

• Order status updates

To handle near real-time data before WebSockets, developers used polling techniques.

2. What Is Polling? (Basic Idea)

Polling means:

The client repeatedly asks the server: “Any new data?”

There are two main types:

1. Short Polling

2. Long Polling

3. Short Polling (For Understanding)

In short polling:

• Client sends requests at fixed intervals (e.g., every 5 seconds)

• Server responds immediately (even if no data)

Example:

Client → Request → Server → “No update”
(wait 5 sec)
Client → Request → Server → “No update”

❌ Wastes bandwidth
❌ Server load increases
❌ Delayed updates

4. What Is Long Polling?

Long polling is an improved polling technique where:

• Client sends a request

• Server keeps the request open

• Responds only when new data is available

• Client immediately sends another request after receiving response

This creates an illusion of server push.

5. Long Polling Flow (Step by Step)

Step 1: Client sends AJAX request

fetch("/notifications");

Step 2: Server holds the request

• No data yet

• Connection stays open

Step 3: New data arrives on server

• Server sends response immediately

Step 4: Client processes data and re-requests

• Another long poll starts

Client ─────────────► Server (wait)
Client ◄──── Data ─── Server
Client ─────────────► Server (wait again)

6. Why Is It Called “Server Push”?

Although the client initiates the request:

• The server decides when to respond

• Data is sent instantly when available

Hence, it behaves like server push.

7. Key Characteristics of Long Polling

8. Example Use Case (Easy to Visualize)

Chat Application

• User A sends message

• Server immediately responds to waiting long-poll request

• User B receives message instantly

• New long-poll request is started

This avoids constant “Are there messages?” requests.

9. Long Polling vs Short Polling (Exam Table)

10. Limitations of Long Polling (Very Important)

❌ Each update still requires a new HTTP request
❌ Server resources held while waiting
❌ Not scalable for very large users
❌ Complex server-side logic

This is why modern apps prefer WebSockets or Server-Sent Events (SSE).

11. Long Polling vs Modern Server Push

12. Exam-Friendly One-Line Definitions

• Long Polling:

  A technique where the server delays its response to an AJAX request until new data is available, enabling near real-time updates.

• Server Push (Simulated):

  A method where the server sends data to the client as soon as it becomes available without continuous client polling.

13. Why Students Must Learn This

• Common AJAX theory question

• Helps understand real-time web evolution

• Explains how apps worked before WebSockets

• Appears in difference-based exam questions

14. Key Line to Remember for Exams

Long polling reduces unnecessary requests by holding the connection open until data is ready, unlike short polling which checks repeatedly.