What are the different techniques for real-time updates (WebSockets vs Server-Sent Events vs long polling)?

Use Cases of Long Polling

• Initial Real-Time Implementations: Many early web apps used long polling for chat messages, live feeds, or notifications before WebSockets were widely available. For example, older chat systems and comment updates on social networks used long polling to push new messages.
• Environments Lacking WebSocket Support: In some legacy systems or restrictive network environments, WebSockets might not be viable. Long polling can serve as a fallback to still get near-real-time behavior using standard HTTP requests.
• Moderate Traffic Apps: Applications that require timely updates but have moderate update frequency (e.g. occasional notifications) can use long polling without significant issues. It’s simpler to implement on typical web servers and might be sufficient for small scale or internal apps.
• Server-Side Simplicity: If server infrastructure cannot maintain thousands of open WebSocket connections, a long polling approach might be simpler to distribute (since it’s just HTTP requests, which can be load-balanced easily). Some server frameworks that don’t support WebSockets natively may stick to long polling.

Pros of Long Polling

• Simple and Works Everywhere: Long polling uses ordinary HTTP, so it works with any browser or server – no special protocols. This simplicity makes it easy to implement with existing web infrastructure (just handle requests and delays on the server side).
• Near Real-Time Effect: Compared to basic periodic polling, long polling greatly reduces latency of updates. The server responds immediately when there’s new data, so the client gets updates almost as fast as with true push. This keeps data timely without constant querying.
• No New Protocols or Ports: Reuses HTTP/HTTPS, so it typically has no issues with firewalls or proxies. If your system can handle HTTP requests, it can do long polling. This is useful in enterprise settings where opening WebSocket ports or using non-HTTP protocols might be disallowed.
• Easy to Implement Incrementally: You can upgrade an existing polling system to long polling with minimal changes – mostly on the server response logic. It doesn't require full-duplex logic, making it straightforward for developers as an interim solution.

Cons of Long Polling

• Higher Overhead than WebSockets: Every long poll response and new request carries the full HTTP headers and overhead repeatedly. The RFC notes that each long poll is a complete HTTP message with headers, which “for small, infrequent messages, the headers can represent a large percentage of the data transmitted”. Thus, long polling is less efficient for high-frequency updates (many redundant HTTP headers).
• Many Open Connections: Each client keeps a connection waiting on the server. Handling a large number of concurrent open HTTP requests can strain server resources (threads or async handlers). Scalability becomes a challenge as user count grows, often requiring load balancers or connection management tweaks.
• Latency on Reconnect: After each response, the client must initiate a new request. There’s a tiny gap (and TCP handshake if not using keep-alive) during reconnect. If events are very frequent, this small latency can add up, making it not truly “instant.” WebSockets/SSE maintain continuous connections and avoid this reconnect cost.
• Complexity in Tuning: Developers must handle timeouts, dropped connections, and possibly phantom requests. Tuning the timeout is tricky: too long and the client might hang unnecessarily; too short and you create needless request churn. Error handling and retry logic add complexity to both client and server implementations compared to SSE or WebSockets.
• Not Truly Bi-Directional: While the client can send requests, these are still one-way HTTP requests. If the client needs to rapidly send data to the server (like in gaming or chat typing events), long polling offers no advantage over normal HTTP posts. It’s primarily optimizing server-to-client delivery.

For more on comparing these approaches, see our system design lesson on Long Polling vs WebSockets vs Server-Sent Events, which delves into their performance in scalable systems.

WebSockets vs SSE vs Long Polling: Which to Choose?

Choosing the right real-time update mechanism depends on your application’s needs and constraints. Here’s a quick comparison of these technologies:

• Communication Direction: WebSockets support bi-directional (client ↔ server) communication, making them ideal for interactive apps (chat, collaborative editing). SSE is uni-directional (server → client) – great for feed updates or notifications where clients just listen. Long Polling is technically half-duplex; the client sends requests, but effectively it’s used for server → client pushes (client → server still uses normal requests).
• Connection Persistence: WebSocket and SSE both keep a single persistent connection open. WebSocket uses a custom protocol after an HTTP upgrade, whereas SSE stays on HTTP. Long polling uses a pseudo-persistent loop of HTTP requests (each one eventually closes and a new one opens). This means WebSockets/SSE generally have less overhead per message than long polling, which repeatedly opens connections.
• Latency and Performance: WebSockets typically offer the lowest latency and overhead for frequent messages, as no HTTP round trips are needed beyond the first handshake. SSE is very low-latency for server-to-client data and has auto-reconnect, but cannot reduce overhead for client-to-server communication (since that still requires separate requests). Long polling has higher latency than the other two for rapid updates, due to the request/response cycle, but for infrequent events it can approximate real-time well.
• Browser/Network Support: WebSockets are widely supported in modern browsers (all major ones for years now). SSE is also supported in modern browsers (except some older IE versions) and can be polyfilled; it benefits from using HTTP so it faces fewer network issues. Long polling works universally anywhere HTTP works. In restrictive networks, SSE or long polling can succeed where WebSocket connections might be blocked.
• Data Format: WebSockets transmit text or binary frames natively. SSE transmits text (UTF-8) event data only. Long polling can send any data in the HTTP response (text or binary as needed), but typically you’d send JSON or text since it’s still an HTTP payload. If binary streaming is required (e.g., real-time video or binary protocol messages), WebSockets have an advantage.
• Use Case Fit: If your application requires two-way interaction or very high-frequency updates (e.g. a trading app with live orders, multiplayer game state), WebSockets are usually the best fit due to their full-duplex nature and efficiency. If the use case is one-way updates like live news feeds, streaming quotes, or notifications where clients don't send much data back, SSE offers a simpler and effective solution. Long polling is suitable as a fallback for environments where WebSockets/SSE aren’t available, or for simpler apps that need basic real-time updates without introducing new protocols. It’s also a stepping stone in understanding real-time mechanics for interview scenarios – demonstrating long polling shows you grasp the fundamentals of server push before discussing more advanced solutions.

In system design interviews, it's wise to mention that WebSockets excel when true real-time bidirectional communication is needed, SSE is great for streaming server-to-client events with less complexity, and long polling can be used to approximate real-time when other options fail or for legacy support. You should also note the trade-offs: for example, mention the overhead of long polling vs the efficiency of WebSockets, or the simplicity of SSE vs the flexibility of WebSockets. Such insights show a grasp of system architecture considerations and can earn you bonus points in an interview.

Conclusion

Real-time update technologies are a must-know in system design interviews. WebSockets, Server-Sent Events, and Long Polling each enable live data flow but in different ways. To recap best practices: use WebSockets for scenarios requiring high-throughput two-way communication (they shine in chats, games, or any interactive multi-user system). Opt for SSE when you only need server→client pushes (great for notifications, live feeds) and want a simpler, HTTP-based solution. Long polling can still serve as a fallback or simpler implementation for low-frequency updates or compatibility, but it’s less efficient for scale.

By understanding these techniques, their pros/cons, and suitable use cases, you’ll be well-prepared to tackle real-time communication questions. In your mock interview practice, try explaining the differences between these approaches and why you’d choose one over the others – it's a common topic that tests your ability to balance trade-offs in a system design. As a technical interview tip, remember to discuss not just what each technology is, but why you would use it in a given scenario (and its implications on system architecture and scalability).

Ready to master system design for your interviews? Explore our Grokking the System Design Interview course on DesignGurus.io. It offers in-depth lessons (including real-time system chapters), expert insights, and practical case studies. By diving into our course, you can build the confidence to design scalable, real-time systems and ace your next interview. Good luck, and happy learning!

FAQs

Q1. How do WebSockets differ from Server-Sent Events (SSE)?

WebSockets open a persistent two-way channel where client and server can send messages at any time. In contrast, SSE (Server-Sent Events) uses a one-way stream from the server to client over HTTP. SSE is easier for simple server-to-client updates (like notifications or feeds), while WebSockets support bidirectional data (better for chat or interactive apps).

Q2. What is the difference between WebSockets and long polling?

With WebSockets, the client and server maintain a continuous two-way connection after an initial handshake, enabling real-time exchange without repeated requests. Long polling emulates push by having the client repeatedly send HTTP requests that the server holds until new data is available. WebSockets have lower latency and less overhead per message, whereas long polling is simpler (uses HTTP) but can be less efficient at scale due to more network overhead.

Q3. When should I use long polling vs WebSockets vs SSE?

Use WebSockets when you need full bidirectional, low-latency communication – for example, in chats, multiplayer games, or collaborative apps where both client and server constantly send updates. Use SSE for simpler one-way streaming updates from server to client (such as live news feeds, stock tickers, or notifications) especially when the client doesn’t need to send data back often. Long polling is useful as a fallback or in legacy scenarios – it can deliver timely updates over HTTP when WebSockets or SSE aren’t available, though it’s less efficient for heavy real-time loads.

Q4. Which technology is best for real-time updates?

It depends on the scenario. WebSockets are often the best choice for real-time updates requiring two-way interaction or very high frequency updates (e.g. instant messaging, trading systems) because they minimize latency and overhead. SSE works well for one-way streams (like live feed updates or server alerts) where the client just listens. Long polling can achieve near-real-time results in basic cases or as a fallback, but it’s generally less efficient than WebSockets or SSE for truly large-scale or high-throughput real-time applications.