Real-time Communication Protocols: Comparison and Use Cases

Welcome back to our module on Practical System Design Interviews. In our last lesson, we designed a high-throughput URL shortener. That system was built around a classic request-response model, optimized for a very high ratio of reads to writes. While that pattern is incredibly common, many modern applications—from collaborative tools to financial tickers and live feeds—require the server to push data to the client in real-time, without the client constantly asking for it.

Today, we'll explore the protocols that make this possible. Our goal is to compare real-time communication protocols, specifically WebSockets and long-polling, and their use cases. Understanding the trade-offs between these technologies is crucial for any architect designing interactive systems and is a frequent topic in system design interviews. This knowledge will serve as the direct foundation for our next lesson, where we'll design a scalable chat application.

The Spectrum of Real-Time Communication

The standard HTTP request-response cycle is inefficient for real-time updates. A client would have to repeatedly ask the server, "Is there anything new yet?" This technique, called short polling, generates a lot of network traffic and server load, most of which results in empty responses. It also introduces latency, as an update is only received on the next poll, not when it actually happens.

To solve these issues, engineers developed more sophisticated techniques. Let's explore the three most common ones: long polling, WebSockets, and Server-Sent Events (SSE).

Long Polling: An HTTP-Based Simulation

Long polling is a clever improvement over short polling that uses standard HTTP. Instead of the server responding immediately, it holds the client's request open until it has new data to send.

Here’s the flow:

1. The client sends an HTTP request to the server.
2. The server doesn't have new data, so it holds the connection open, not sending a response yet.
3. When new data becomes available (e.g., another user posts a message), the server sends the data in the response, completing the request.
4. The client receives the data and immediately sends another request, restarting the cycle.
5. If the connection is held for too long (e.g., 30 seconds) without new data, the server sends a timeout response, and the client, again, immediately reconnects.

This approach creates the illusion of a server push while still using the familiar request-response model.

While much better than short polling, long polling isn't perfect. There's a small latency gap between receiving a response and making a new request, during which events can be delayed. It also carries the overhead of HTTP headers with every message.

WebSockets: True Bidirectional Communication

For true, low-latency, two-way communication, we need a different protocol altogether. This is where WebSockets come in.

A WebSocket connection starts with a special HTTP request from the client, asking the server to Upgrade the connection. If the server agrees, the connection is elevated from the HTTP protocol to the WebSocket protocol. This creates a persistent, full-duplex communication channel over a single TCP connection.

• Persistent: The connection stays open for the life of the session, avoiding the overhead of re-establishing connections.
• Full-duplex: Both the client and the server can send messages to each other at any time, independently.
• Low Overhead: After the initial handshake, message frames are very small (as little as 2 bytes), making it highly efficient for frequent messages.

Server-Sent Events (SSE): The One-Way Alternative

Before we do a direct comparison, it's important to know about one more player: Server-Sent Events (SSE). SSE provides a persistent connection where a server can push data to a client. Unlike WebSockets, however, it is unidirectional (server-to-client only).

SSE is built on top of standard HTTP and has some great features, like automatic reconnection. If the client needs to send data to the server, it would do so using a separate, normal HTTP request. This makes SSE an excellent, simple choice for use cases like news feeds, stock tickers, or notification systems where the client is primarily a passive listener.

The Core Comparison: Choosing the Right Tool

The choice between these protocols is a classic system design trade-off. Let's analyze it across several axes that are critical for building scalable systems.

1. Communication Directionality

This is the most fundamental difference.

• Long Polling & SSE: Unidirectional (server-to-client push).
• WebSockets: Bidirectional (client-to-server and server-to-client).

Use Case: If you are building a chat application where users both send and receive messages, you need bidirectional communication, making WebSockets the natural choice. If you are building a dashboard that displays live sports scores, the client only needs to receive updates, making SSE or long polling a simpler and potentially better fit.

2. Latency and Performance

For applications requiring the lowest possible latency, the protocol mechanics matter.

As the article explains, long polling can introduce extra latency for messages that arrive in quick succession. WebSockets, with their persistent connection, deliver each message with consistent, single-transit latency. Furthermore, the minimal framing overhead of WebSockets is significantly more efficient than the full HTTP headers required by each long poll message, a difference that becomes critical at high message volumes.

3. Infrastructure and Scalability

This is where your experience as a backend architect is most relevant. The choice of protocol deeply impacts your system's architecture.

4. Reliability and Failure Modes

Finally, how do these systems break?

• Long Polling failures are often visible as standard HTTP errors (504 Gateway Timeout). Recovery can be simple since the client just needs to issue another HTTP request. However, a server restart can cause a "thundering herd" problem, where all clients try to reconnect simultaneously.
• WebSocket connections can drop silently (e.g., due to a NAT timeout). You must implement an application-level heartbeat (ping/pong) mechanism to detect and handle these dead connections. On reconnection, you also need a strategy (like using event cursors, similar to long polling) to replay any messages the client missed while disconnected.

An Example: Real-Time Auction Service

Let's ground this with a practical example. Imagine designing a real-time auction service.

Here, WebSockets are an excellent choice.

• Bidders need to receive real-time price updates with minimal delay (server-to-client).
• They also need to submit their own bids instantly (client-to-server).
• The architecture reflects the stateful nature of WebSockets. A dedicated pool of WebSocket Servers manages the persistent connections. A Connection Registry service is needed to track which user is connected to which server and subscribed to which auction—this is a classic pattern for managing state in a scalable WebSocket deployment.

Conclusion

We've delved into the critical differences between long polling and WebSockets, with Server-Sent Events as a key alternative. There is no single "best" protocol; the right choice depends entirely on your application's specific requirements.

Key Takeaways:

• Short Polling: Simple but inefficient. Avoid for scalable systems.
• Long Polling: A clever HTTP-based simulation of server push. Good for simple server-to-client updates and for fallbacks, as it works everywhere HTTP does. Its main drawbacks are latency during bursts and header overhead.
• Server-Sent Events (SSE): The modern choice for simple, unidirectional server-to-client push. It's built on HTTP, is lightweight, and has built-in reconnection logic.
• WebSockets: The most powerful option for true bidirectional, low-latency communication. This power comes at the cost of increased complexity in your infrastructure (stateful servers, L4/L7 load balancing) and application logic (heartbeats, manual reconnection, and state synchronization).

As a general rule:

• Need simple server push (feeds, notifications)? Start with SSE.
• Need true two-way interaction (chat, collaboration, gaming)? You need WebSockets.
• Need broad compatibility or a fallback? Long polling is your reliable, if less performant, option.

Now that we have a solid grasp of these protocols and their trade-offs, we are perfectly positioned for our next lesson. We will apply this knowledge to a classic system design interview problem: designing a scalable chat application, which will lean heavily on the capabilities of WebSockets.