Module 8 — APIs

WebSockets & SSE

Real-time communication patterns. Push updates to clients without them asking.

1The Communication Analogy

💡 Simple Analogy

HTTP Polling: Kid asking "Are we there yet?" every 5 seconds.

Long Polling: Kid asks, parent waits until they have news to respond.

SSE: Radio broadcast—parent announces updates, kid just listens.

WebSockets: Phone call—both can talk anytime.

2Comparison

HTTP Polling

Client repeatedly requests at intervals

Direction: Client → Server (repeated)

Pros: Simple, works everywhere

Cons: Wasteful, high latency

Use case: Non-critical updates, legacy systems

Long Polling

Server holds request until data available

Direction: Client → Server (held open)

Pros: Lower latency than polling

Cons: Connection overhead, complex error handling

Use case: Chat before WebSockets

Server-Sent Events (SSE)

Server pushes events over HTTP

Direction: Server → Client only

Pros: Simple, auto-reconnect, works with HTTP/2

Cons: One direction only, limited browser connections

Use case: Notifications, live feeds, stock prices

WebSockets

Full-duplex persistent connection

Direction: Client ↔ Server (bi-directional)

Pros: True real-time, low latency

Cons: More complex, connection management

Use case: Chat, gaming, collaboration tools

3Quick Reference

Feature	Polling	SSE	WebSocket
Direction	Client→Server	Server→Client	Bi-directional
Protocol	HTTP	HTTP	ws://
Reconnect	Manual	Automatic	Manual
Complexity	Low	Low	High

4When to Use What

Stock prices, news feed

SSE

Server-to-client, auto-reconnect

Chat application

WebSocket

Bi-directional messaging needed

Collaborative editing

WebSocket

Real-time sync both ways

Notification system

SSE

Simple push, no client messages

Dashboard refresh

Polling or SSE

Depends on update frequency

5Key Takeaways

1Polling: simple but wasteful. Use for non-critical updates.

2SSE: server-to-client push. Great for notifications, feeds.

3WebSockets: bi-directional real-time. Use for chat, gaming.

4SSE is simpler than WebSockets for one-way updates.

5WebSockets have connection management overhead.

6In interviews: discuss trade-offs for the specific use case.