7. Long Polling vs SSE vs Web Socket

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0️⃣ Quick Comparison

Feature	Polling	Long Polling	SSE	WebSocket
Real-time	❌	⚠️ Almost	✅	✅
Persistent Connection	❌	⚠️ Temporary	✅	✅
Direction	Client → Server	Client → Server	Server → Client	Bi-directional
HTTP Based	✅	✅	✅	Starts with HTTP
Server Push	❌	⚠️ Sort of	✅	✅
Complexity	Low	Medium	Medium	High
Scalability	Poor	Better	Good	Good

Think of a cricket score app 🏏.

“How does the server push real-time updates to clients?“

1️⃣ Polling

The client repeatedly asks: > “Any new updates?” at a fixed interval.

Example

Client calls: GET /score every 5 seconds.

Client                      Server
 
Request --------------->
 
               Current Score = 100
 
<--------------- Response
 
(wait 5 sec)
 
Request --------------->
 
               Current Score = 100
 
<--------------- Response
 
(wait 5 sec)
 
Request --------------->
 
               Current Score = 120
 
<--------------- Response

Pros

• Simple
• Easy to implement
• Works everywhere

Cons

• Lots of unnecessary requests
• High server load
• Updates are delayed. you get stale data of 5 sec
  • If polling interval = 5 sec Server updated at 10:01 - Client sees at 10:05

System design Use Cases

• Monitoring pages - Dashboard refresh every 1 min

Note: Not suitable for chat apps

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2️⃣ Long Polling

Instead of immediately responding: Server waits until new data arrives.

Pros

• Almost real-time
• Atleast better then Polling

Cons

• HTTP Connection repeatedly opens/closes
• Scalability - Doesn’t scale as well for millions of users - creates http overhead

System design Use Cases

• Notification systems
• Before WebSockets became common

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3️⃣ SSE (Server-Sent Events)

SSE keeps one HTTP connection open. Server continuously pushes updates.

Server seding cricket score data.

data: Score = 100
 
data: Score = 120
 
data: Score = 150

Characteristics

Feature	SSE
Direction	Server → Client
Protocol	HTTP
Persistent Connection	Yes
Auto Reconnect	Yes
Browser Support	Excellent

Pros

• Simpler than WebSockets
• Uses HTTP
• Automatic reconnection
• Lower overhead compair to long polling as connection remain open

Cons

• One-way communication - Client cannot push data through same connection

Interview Use Cases

• Stock prices
• Great for live feeds
• Live sports score
• News feed
• Monitoring dashboard

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4️⃣ WebSockets

WebSocket creates a persistent TCP connection. After connection establishment:

Client <---------> Server

Both sides can send messages anytime.

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Handshake

When the browser wants a WebSocket:

const socket =
    new WebSocket("wss://chat.example.com");

Starts as HTTPS:

GET /chat HTTP/1.1
Host: chat.openai.com
Upgrade: websocket
Connection: Upgrade

Server:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade

The connection becomes: WebSocket Protocol over the same TCP connection.

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Flow

Characteristics

Feature	WebSocket
Direction	Two-way
Persistent	Yes
Real-time	Yes
Protocol Upgrade	Yes

Pros

• Lowest latency
• Bi-Directional
• Efficient - less overhead compair to http based soluation
• Ideal for real-time apps

Cons

• More complex
• Stateful connections - Load balancing becomes harder
• scalability Issue

System Design Use Cases

• Chat applications
• WhatsApp
• Slack
• Multiplayer games
• Trading platforms
• Collaborative editors
• ChatGPT

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System Design Rule

If interviewer asks:

User refreshes every minute

➡️ Polling

Notification service

➡️ Long Polling or SSE

Live cricket score

➡️ SSE Reason:

• Server pushes updates
• Client rarely sends data
• Simpler than WebSocket

WhatsApp / Slack

➡️ WebSocket Reason:

• User sends and receives messages continuously
• Bidirectional communication required

WebSockets are not easier to scale than stateless HTTP. They require maintaining long-lived stateful connections, sticky routing, connection management, and often a shared messaging layer like Redis Pub/Sub or Kafka. However, for large-scale realtime systems they are much more network-efficient than polling because they avoid repeated requests and allow instant bidirectional communication.

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