Key Takeaways
- REST: Request-response over HTTP. Client asks, server answers, connection closes. Best for CRUD operations and standard APIs.
- WebSocket: Persistent bidirectional connection. Both sides can send messages at any time. Best for real-time, low-latency applications.
- SSE: One-directional server-to-client streaming over HTTP. Best for live updates, notifications, and AI token streaming.
- Rule of thumb: Start with REST. Add WebSocket or SSE when you need real-time that polling cannot handle efficiently.
The difference between REST and WebSocket comes down to one question: does the server ever need to send data to the client without the client asking first?
If the answer is no — the client asks, the server responds — REST is the right tool. If the answer is yes — the server needs to push updates, notify clients of changes, or participate in a continuous low-latency exchange — WebSocket (or Server-Sent Events) is the right tool.
These are not competing technologies; they are tools for different communication patterns. Most real applications use both: REST for data retrieval and operations, WebSocket or SSE for real-time features.
How REST APIs Work
REST uses HTTP's request-response model: the client initiates a request, the server handles it and sends a response, and the connection is closed (or reused briefly via keep-alive, but logically it is a one-shot exchange).
Every REST request is independent. The server does not maintain ongoing state about what a particular client is doing between requests (that is what authentication tokens and session handling are for). This statelessness makes REST easy to scale — any server instance can handle any request without knowing about prior requests.
REST is the protocol of the web. Every time your browser loads a page, fetches an image, or submits a form, it is making HTTP requests. The simplicity and universality of this model is why REST became the dominant API design.
The limitation: REST cannot push. The server can only respond when the client asks. If you want to show a user a new chat message the moment it arrives, REST alone requires the client to repeatedly ask "are there new messages?" (polling) — which is inefficient, introduces latency, and wastes resources.
How WebSockets Work
WebSocket establishes a persistent, full-duplex connection between client and server. Once the connection is open, both sides can send messages at any time without initiating a new request.
The WebSocket protocol starts with an HTTP handshake — the client sends a standard HTTP request with an Upgrade: websocket header, the server accepts, and the connection upgrades from HTTP to the WebSocket protocol. From that point, the connection stays open until explicitly closed.
With an open WebSocket connection:
- The server can push a new chat message to the client the instant it arrives
- The client can send a message without HTTP overhead
- Both sides communicate over a single persistent connection with very low latency
- There is no per-message HTTP header overhead (REST headers add bytes to every request)
The trade-off: WebSocket connections are stateful and require the server to maintain open connections for every connected client. This creates different scaling challenges than stateless REST. A server handling 100,000 simultaneous WebSocket connections faces very different infrastructure considerations than one handling 100,000 REST requests per minute.
Key Differences
- Communication direction: REST is client-to-server only (client always initiates). WebSocket is bidirectional (either side can initiate).
- Connection lifetime: REST connections are short-lived (per request). WebSocket connections persist until closed.
- Overhead: REST has HTTP header overhead on every message. WebSocket has high initial handshake cost but very low per-message overhead.
- Caching: REST responses can be cached (GET requests). WebSocket messages cannot.
- State: REST is stateless. WebSocket requires server-side connection state management.
- Tooling: REST has extremely mature tooling, documentation, testing, and ecosystem support. WebSocket tooling is good but requires more careful implementation.
When to Use REST
REST is the right choice for the vast majority of API interactions. Use REST for:
- CRUD operations — create, read, update, delete data
- Loading page content, fetching user profiles, querying databases
- Form submissions and data processing
- File uploads and downloads
- Any interaction that follows the request-response pattern
- Public APIs that external developers will integrate
- Anything that benefits from HTTP caching, logging, and standard tooling
If polling is fast enough for your use case — checking for updates every few seconds is acceptable — REST with polling is often simpler than implementing WebSocket. The added complexity of WebSocket is only worth it when polling creates unacceptable latency or resource consumption.
When to Use WebSocket
Use WebSocket when you need true real-time, bidirectional, low-latency communication.
- Chat applications: Messages need to appear instantly. Polling with REST introduces latency; WebSocket delivers messages within milliseconds.
- Multiplayer games: Position updates, game state, and player actions need to be shared with sub-100ms latency. REST cannot deliver this.
- Collaborative editing: Google Docs-style concurrent editing requires real-time sync of every keystroke. WebSocket handles this; polling does not.
- Live trading platforms: Stock prices changing dozens of times per second need to be pushed to clients without per-update request overhead.
- Operational dashboards: Monitoring screens showing live metrics need server pushes, not client polling.
Server-Sent Events: The Third Option
Server-Sent Events (SSE) is a middle ground: one-directional real-time communication from server to client, over standard HTTP. The client makes a single HTTP request and keeps the connection open; the server sends data whenever it has something to push.
SSE is simpler than WebSocket because it uses standard HTTP (no protocol upgrade), works through more firewalls and proxies, and is natively supported in browsers via the EventSource API. The limitation: the client cannot send messages back over the SSE connection (it can make separate REST requests for that).
SSE is ideal for: notification systems, live dashboards, activity feeds, and streaming text responses — all cases where the server needs to push data but two-way communication on the same connection is not required.
AI Streaming: REST, WebSocket, or SSE?
AI streaming responses — the word-by-word text generation you see in ChatGPT and Claude — use Server-Sent Events (SSE) over HTTPS, not WebSocket. This is a common point of confusion.
When you send a message to an AI chatbot, the client makes a standard POST request with your prompt. The server starts generating tokens and streams them back using SSE format — each token is sent as a new event over the persistent HTTP connection. The client receives tokens as they arrive and renders them progressively.
This is one-directional (server to client for the response), which makes SSE the appropriate choice. WebSocket would add complexity without adding benefit for this specific pattern. Understanding this is directly relevant if you are building AI-powered applications: use the AI SDK's streaming utilities, which handle the SSE response format correctly.