Real-time communication used to be a speciality. You needed a dedicated service, a lot of custom infrastructure, and a deep understanding of network protocols just to push a live notification to a browser. Today, the web platform offers two built-in primitives for this: WebSocket and Server-Sent Events (SSE). They do similar things—they let a server send data to a client without the client asking for it—but they're built for different situations.

WebSocket is a two-way, full-duplex communication channel over a single TCP connection. The client can send messages to the server, and the server can send messages to the client at any time. It's the go-to choice for chat apps, collaborative editing, and multiplayer games.

SSE is a one-way, server-to-client only channel over plain HTTP. The client opens a connection, and the server pushes events as they happen. The client cannot send data over the same connection, but it can make separate HTTP requests when needed. SSE is ideal for live dashboards, notification feeds, and any scenario where the client mostly receives updates.

The confusion between them is understandable. Both achieve "real-time" in the browser. But choosing the wrong one leads to overbuilt complexity or frustrating limitations. Let's break down the differences so you can pick the right tool.

The fundamental difference: directionality

The most important distinction is the direction of data flow.

WebSocket is bidirectional. Once the connection is established, both client and server can send data at any time without waiting for a request. This is why it feels like a "socket" in the traditional networking sense—two endpoints with a persistent, symmetric channel.

SSE is unidirectional. The server can send data to the client, but the client cannot send data on the same connection. The client can still send data to the server—it just uses a normal fetch or XMLHttpRequest to a separate endpoint. This makes SSE simpler but limited.

If your application requires the client to frequently send data that triggers real-time updates (like typing a chat message), WebSocket is the natural fit. If the client mostly receives updates (like a stock ticker or a news feed), SSE removes the complexity of a two-way protocol and works over plain HTTP.

If you've built real-time features with Supabase vs Firebase, you've already used abstractions on top of these protocols. Supabase Realtime uses WebSockets, while Firebase's realtime features also use WebSockets. But understanding the underlying protocol helps you make better architecture choices.

How the connections work under the hood

WebSocket starts with an HTTP handshake. The client sends an Upgrade: websocket header, and if the server accepts, the connection upgrades to the WebSocket protocol. From that point on, the connection is no longer HTTP—it's a raw TCP socket with a lightweight framing layer on top. This makes WebSocket very efficient for high-frequency, small messages.

SSE, on the other hand, stays on HTTP. The client makes a regular HTTP request with an Accept: text/event-stream header. The server keeps the connection open and writes data to the response stream in a specific format. The connection is one-way, but because it uses standard HTTP, it benefits from existing infrastructure: HTTP/2 multiplexing, proxies, load balancers, and even CDN support.

This difference matters in practice. WebSocket often requires special server configuration to work behind reverse proxies and load balancers. SSE usually just works with standard HTTP infrastructure, which can significantly simplify deployment.

Use cases where WebSocket shines

WebSocket is the right choice when you need bidirectional, low-latency communication. Chat applications are the classic example: every participant can send messages, and every message must be delivered instantly to all other participants. WebSocket makes this straightforward.

// Client-side WebSocket example
const ws = new WebSocket("wss://example.com/chat");

ws.onopen = () => {
  ws.send(JSON.stringify({ type: "join", room: "general" }));
};

ws.onmessage = (event) => {
  const message = JSON.parse(event.data);
  displayMessage(message);
};

Collaborative editing (like Google Docs) also requires WebSocket. Every keystroke is a small message that must be sent to the server and broadcast to other clients. The bidirectional nature and low overhead are essential.

Multiplayer games, live customer support chat, and real-time collaboration tools all benefit from WebSocket's full-duplex capability. If your application needs to send frequent, small messages in both directions, WebSocket is the clear winner.

Use cases where SSE is simpler and better

SSE is the right choice when the server pushes updates but the client rarely sends data. Live dashboards, news feeds, notification streams, and stock tickers are perfect examples.

// Client-side SSE example
const eventSource = new EventSource("/events");

eventSource.onmessage = (event) => {
  const data = JSON.parse(event.data);
  updateDashboard(data);
};

eventSource.addEventListener("stock-update", (event) => {
  const data = JSON.parse(event.data);
  updateStockPrice(data);
});

Because SSE uses standard HTTP, it's simpler to implement on the server. You just write to the response stream and let the browser handle reconnection automatically. SSE also supports named events, so you can send different types of updates on the same connection.

SSE is often the better choice for applications that only need server-to-client pushes. It avoids the complexity of managing bidirectional state and works seamlessly with existing HTTP infrastructure.

Reconnection and error handling

One area where SSE has a built-in advantage is automatic reconnection. The browser's EventSource API automatically attempts to reconnect if the connection drops, with an exponential backoff. It also includes a Last-Event-ID header that allows the server to resume from where the client left off.

WebSocket, in contrast, does not automatically reconnect. If the connection drops, you have to implement reconnection logic yourself. This can be a significant amount of code, especially if you need to handle backoff, message deduplication, and session resumption.

For applications that require resilient real-time updates, SSE's automatic reconnection is a huge win. For WebSocket, you'll need to write a reconnection wrapper or use a library that handles it for you.

If you've dealt with TypeScript error handling in try catch blocks, you know that robust error handling is essential for any network communication. SSE's built-in reconnection reduces the error surface, while WebSocket requires more defensive programming.

Performance and scalability

WebSocket is generally more efficient for high-frequency, small messages. The framing overhead is minimal—just a few bytes per message. SSE has slightly higher overhead because it uses HTTP/1.1 streaming or HTTP/2 multiplexing, which adds headers and newline-based formatting.

However, for most real-time applications, the difference is negligible. The real scalability bottleneck is the number of open connections on the server. Both protocols require the server to maintain a persistent connection per client, which can exhaust file descriptors and memory on traditional server architectures.

Modern runtimes like Bun and Deno have improved this by using asynchronous I/O and efficient connection pooling, but the fundamental limitation remains. If you're building at massive scale, you'll likely need a dedicated real-time infrastructure layer regardless of protocol.

If you've been considering Bun vs Node.js performance, you'll know that Bun's faster startup and better concurrency can help with WebSocket-heavy workloads.

A real-world story: choosing SSE over WebSocket for a notification system

I once built a notification system for a project management app. Users needed to see real-time updates when tasks were assigned, comments were added, or deadlines approached. The original plan was to use WebSocket for all real-time communication, but after analyzing the actual data flow, we realized that 95% of the traffic was server-to-client. The client rarely sent anything other than the initial subscription.

We switched to SSE. The server could push notifications, task updates, and mention alerts over a simple HTTP stream. The client used EventSource, which automatically reconnected on network drops and handled backoff. The implementation was significantly simpler—no WebSocket handshake, no custom reconnection logic, no need to manage bidirectional state.

The only thing we lost was the ability to send messages from the client over the same connection, but we didn't need that. For user actions like marking a notification as read, we used a regular fetch call to a REST endpoint. The result was a cleaner architecture and fewer lines of code to maintain.

That experience taught me to ask a simple question before choosing a protocol: does the client need to send data over the same persistent connection, or can it use separate HTTP requests? If the answer is separate HTTP requests are fine, SSE is probably the simpler choice.

When WebSocket is the only viable option

There are cases where SSE cannot replace WebSocket. If your application requires true bidirectional communication with low latency—chat, multiplayer games, collaborative editing—you cannot use SSE because it's one-way only. WebSocket is the only built-in browser API that provides full-duplex communication.

If you need to send binary data efficiently, WebSocket supports binary frames natively. SSE is text-based and requires encoding binary data as Base64, which adds overhead.

If you're building a protocol that requires client-initiated messages to trigger immediate server responses without an HTTP round trip, WebSocket is the answer. The client can send a message, and the server can respond on the same connection without waiting for a new HTTP request.

When SSE is the pragmatic winner

SSE should be your default for server-to-client push notifications unless you have a specific reason to need bidirectional communication. It's simpler, more robust, and easier to deploy behind standard infrastructure. The browser handles reconnection, event filtering, and backoff for you.

For live dashboards, feed updates, notification streams, and real-time analytics, SSE is often the right tool. If you're using Next.js, SSE can be implemented in an API route with just a few lines of code, whereas WebSocket often requires a separate server process or a platform that supports WebSocket upgrades.

The infrastructure reality

WebSocket requires your server to handle the initial HTTP upgrade, maintain the TCP connection, and manage the WebSocket framing. Many serverless platforms (like Vercel and Netlify) do not support WebSocket natively or require add-ons. SSE, being plain HTTP, works everywhere.

If you're deploying on a platform that doesn't support WebSocket, or you want to avoid managing persistent connections, SSE is the pragmatic choice. You can still achieve real-time updates without the infrastructure complexity.

Wrapping up

WebSocket and SSE are both excellent tools for real-time communication. The choice depends on the direction of your data flow and your deployment constraints.

Use WebSocket when the client needs to send data and receive data over the same persistent connection, or when you need binary messages and minimal overhead.
Use SSE when the server pushes updates to the client, and the client can use separate HTTP requests for its own actions.

If you're building a chat app or a multiplayer game, WebSocket is the clear winner. If you're building a live dashboard or a notification feed, SSE will save you a lot of complexity.

The next time you're planning a real-time feature, ask yourself: does the client need to send messages back over the same channel? If the answer is no, consider SSE—it might be the simpler, more maintainable solution.

Need help choosing the right real-time architecture for your application? Red Surge Technology designs and builds real-time systems that scale. Get in touch to discuss your project.

WebSocket vs SSE: Which Real-Time Communication Protocol Fits Your Project?