Overview
Long polling, WebSockets, and Server-Sent Events are three techniques for pushing live updates from a server to a browser without the client repeatedly refreshing the page. WebSockets open a single full-duplex socket for two-way messaging, while Server-Sent Events keep one HTTP connection open for one-way server-to-client streaming; long polling predates both and fakes real-time updates by re-issuing HTTP requests. Picking the right one depends on whether you need bidirectional traffic, binary data, or just simple broadcast updates.
Comparison Diagram
Comparison Table
| Aspect | Long Polling | WebSockets | Server-Sent Events |
|---|---|---|---|
| Connection setup | Client sends a normal HTTP GET; server holds it open until data is ready or a timeout, then the client immediately re-requests | Client sends an HTTP Upgrade request; the connection switches to a persistent ws:// socket | Client opens a single HTTP GET with Accept: text/event-stream; server keeps that response stream open |
| Underlying protocol | Plain HTTP request/response, repeated | Dedicated ws/wss protocol after the handshake | Standard HTTP with a chunked/streaming response |
| Data direction | Bidirectional, but each direction needs its own HTTP exchange | Full-duplex over one socket; either side can send anytime | Server to client only |
| Client-to-server messages | Sent as the next poll request | Sent directly over the open socket | Requires a separate ordinary HTTP request, e.g. fetch |
| Update latency | Near real-time, bounded by the request/re-request round trip | Lowest; messages are pushed the instant they’re sent | Low; comparable to WebSockets for server-to-client pushes |
| Reconnection handling | Manual: app code re-issues the request after every response or timeout | Manual: app must implement its own reconnect/backoff | Automatic: built into the EventSource API with Last-Event-ID resume |
| Server resource footprint | Many short-lived held requests; can spike thread/connection usage under load | One long-lived socket per client; efficient per message but needs socket-aware infra | One long-lived HTTP connection per client; works with standard HTTP servers and proxies |
| Typical use cases | Legacy fallback where WebSockets aren’t available, low-frequency updates | Chat, multiplayer games, collaborative editing | Live feeds, notifications, stock tickers, dashboards |
Key Differences
- Only WebSockets support true bidirectional messaging over a single connection; Long Polling and SSE both need a separate request for client-to-server data.
- Server-Sent Events ship built-in reconnection and event IDs via the EventSource API, while WebSockets and Long Polling require hand-rolled reconnect logic.
- Long Polling reuses plain HTTP semantics with no protocol upgrade, making it the easiest to route through legacy infrastructure but the least efficient under frequent updates.
- WebSockets can carry binary frames, whereas SSE is restricted to UTF-8 text events.
When to Use Each
Long Polling
- Legacy or Restrictive Networks: Since it reuses plain HTTP request/response with no protocol upgrade, long polling passes through proxies and firewalls that may block WebSocket handshakes.
- Infrequent Updates: When updates are rare, the round-trip delay of re-issuing requests is an acceptable tradeoff against building persistent-connection infrastructure.
- Fallback for Unsupported Clients: It serves as a simple degrade path when a client or network can’t sustain a WebSocket or SSE connection.
WebSockets
- Chat and Multiplayer Games: Full-duplex messaging over one socket lets either side push data the instant it’s ready, which one-way SSE can’t do for client input.
- Collaborative Editing: Frequent, low-latency updates in both directions fit a single persistent socket better than repeated HTTP exchanges.
- Binary Data Transfer: Only WebSockets carry binary frames, whereas SSE is restricted to UTF-8 text events.
Server-Sent Events
- One-Way Live Feeds: Notifications, stock tickers, and dashboards only need server-to-client pushes, which is exactly what SSE provides without WebSocket’s added complexity.
- Automatic Reconnection Needed: The EventSource API’s built-in reconnect and Last-Event-ID resume avoid hand-rolling the reconnect logic WebSockets and long polling require.
- Standard HTTP Infrastructure: Running over ordinary HTTP means SSE works with existing proxies and servers without needing socket-aware infrastructure.