HTTP Short Polling vs Long Polling vs WebSockets vs SSE

Modern web apps need ways to communicate in near real time—whether it’s chat messages, notifications, or live updates—and HTTP short polling, long polling, Server-Sent Events (SSE), and WebSockets are the main techniques used to make that happen, each with different trade-offs in how they communicate, scale, and perform.

1. HTTP Short Polling

The client repeatedly sends requests at regular intervals (e.g., every 5 seconds) asking “any updates?”

Use case: Simple status dashboards, weather widgets, low-frequency updates

Client                    Server
  |                          |
  |-------- Request -------->|
  |<------- Response --------|
  |                          |
  | (wait 5 seconds)         |
  |                          |
  |-------- Request -------->|
  |<------- Response --------|
  |                          |
  | (wait 5 seconds)         |
  |                          |
  |-------- Request -------->|
  |<------- Response --------|

Pros:

• Extremely simple to implement (just a loop Interval)
• Works with any HTTP client/server
• No special infrastructure needed
• Easy to debug and monitor
• Works through all proxies and firewalls
• Stateless - no connection management

Cons:

• High latency (up to polling interval)
• Extremely wasteful bandwidth (constant requests)
• High server load (N clients = N requests/interval)
• Battery drain on mobile devices
• Can overwhelm servers during traffic spikes

2. HTTP Long Polling

The client sends a request, and the server holds it open until new data is available, then responds. The client immediately makes a new request.

Use case: Chat apps without WebSocket support, notification systems

Client                    Server
  |                          |
  |-------- Request -------->|
  |                          | (holds connection open)
  |                          | (waiting for data...)
  |                          |
  |                          | (data arrives!)
  |<------- Response --------|
  |                          |
  |-------- Request -------->| (immediate reconnect)
  |                          | (holds connection open)
  |                          | (waiting for data...)

Pros:

• Lower latency than short polling
• Works over standard HTTP/HTTPS
• Compatible with most proxies
• Reasonable for moderate update frequencies
• Easier than WebSockets to implement
• No special client libraries needed

Cons:

• Still creates many connections over time
• Connection overhead (TCP handshake for each cycle)
• Timeout management complexity
• Load balancer complications (sticky sessions)
• Resource intensive on server (one thread/connection)
• Not truly bidirectional
• Difficult to scale to thousands of clients
• Proxy timeout issues (30-60 second limits)

3. WebSockets

A persistent, bidirectional connection established via HTTP upgrade. Both client and server can send messages anytime.

Use case: Real-time gaming, collaborative editing (Google Docs), live trading, chat applications, multiplayer experiences

Client                    Server
  |                          |
  |--- HTTP Upgrade Req ---->|
  |<-- 101 Switching Proto --|
  |                          |
  |<=====Persistent TCP=====|
  |                          |
  |<------ Message ----------|
  |------- Message --------->|
  |<------ Message ----------|
  |------- Message --------->|
  |                          |
  | (connection stays open)  |

Pros:

• True real-time bidirectional communication
• Very low latency (no polling delay)
• Minimal overhead after handshake (2-byte frames)
• Efficient for high-frequency messages
• Single persistent connection
• Supports binary data
• Better battery life on mobile
• Scales well (one connection per client)

Cons:

• More complex to implement
• Requires WebSocket-compatible server
• Stateful (connection management needed)
• Doesn’t work through all proxies/firewalls
• No automatic reconnection (must implement)
• Load balancing complexity (sticky sessions)
• Harder to debug than HTTP
• Incompatible with HTTP caching
• Can be blocked by corporate networks
• Need fallback mechanisms for compatibility

4. Server-Sent Events (SSE)

The server pushes updates to the client over a single long-lived HTTP connection. Unidirectional (server → client only).

Use case: Live news feeds, social media updates, stock tickers, server progress updates, notifications, real-time analytics dashboards

Client                    Server
  |                          |
  |-------- Request -------->|
  |<-- Content-Type: --------|
  |    text/event-stream     |
  |                          |
  |<====== Stream Open ======|
  |                          |
  |<------ Event Data -------|
  |<------ Event Data -------|
  |                          |
  | (connection stays open)  |
  |                          |
  |<------ Event Data -------|

Pros:

• Simple to implement (built into browsers)
• Automatic reconnection with retry logic
• Event IDs for resuming from last received
• Works over standard HTTP/HTTPS
• Text-based, easy to debug
• Multiplexed over HTTP/2
• UTF-8 encoding built-in
• Named events for message routing
• Lower overhead than long polling
• Standard browser EventSource API

Cons:

• Unidirectional only (server → client)
• Limited to 6 concurrent connections per domain (HTTP/1.1)
• Text only - no binary support
• Limited browser support for custom headers
• Not suitable for bidirectional needs
• IE/Edge legacy support requires polyfills
• Connection limits can be restrictive

Decision Flowchart

Need bidirectional?
    ├─ YES → WebSockets
    └─ NO → Server push only?
              ├─ YES → SSE (simple) or WebSockets (complex)
              └─ NO → Updates infrequent?
                        ├─ YES → Short Polling
                        └─ NO → Long Polling or SSE