Users expect real-time updates in modern applications. Notifications should arrive instantly. Chat messages should appear without refreshing. Dashboards should update automatically. Behind the scenes, there are two main technologies making this possible, and most developers have heard of one while the other remains underutilized despite being simpler and more efficient for many use cases.

To understand these technologies, you first need to understand the problem with traditional web communication. Standard HTTP requests follow a simple pattern: the client sends a request, the server sends back a response, and the connection closes. This works perfectly for traditional applications where the client initiates all communication.

But for real-time features, this pattern breaks down. What happens when the server has new data to send to the client but the client hasn't asked for it? The server can't initiate a connection. You need a way for the server to push data to the client whenever updates become available. That's where modern real-time technologies come in.

The first approach is Server-Sent Events, often abbreviated as SSE. This technology is beautifully simple. The client opens a connection to the server and keeps it open. The server sends data over this connection whenever updates are available. The connection stays open until either side decides to close it.

The strength of Server-Sent Events is its simplicity. The client API is straightforward — you just create a connection and listen for messages. The whole thing works over plain HTTP with no special protocols or server requirements. If the connection drops, the browser automatically tries to reconnect. You get all of this functionality for free.

The weakness of Server-Sent Events is that communication is one-way only. The client can receive data from the server, but it can't send data back over the same connection. If the client needs to send data to the server, it must make a separate standard HTTP request. This is fine for many use cases but limiting for others.

Additionally, browsers limit the number of simultaneous connections to the same domain. If you open too many Server-Sent Events connections, some will be queued or blocked. And the naive implementation that checks for updates every second is inefficient and creates unnecessary server load.

The second approach is WebSockets. Unlike Server-Sent Events, WebSockets provide full bidirectional communication. The client and server can both send messages at any time over a persistent connection. It's like a telephone call where both parties can speak whenever they want, compared to a mail system where messages go one way at a time.

The strengths of WebSockets are significant for the right use cases. Communication is bidirectional, enabling interactive features that Server-Sent Events cannot support. Latency is extremely low because there's no polling overhead — data is pushed immediately when available. Once the connection is established, each message has minimal overhead.

But these benefits come with costs. WebSockets are more complex to set up. You need a dedicated WebSocket server that runs separately from your HTTP server. The server must maintain state for each connected client, which makes horizontal scaling tricky — if you have multiple server instances, you need a way to coordinate messages between them using a message broker.

So how do you choose between these technologies? The decision comes down to your specific use case.

Choose Server-Sent Events when you need only one-way communication from server to client. This is perfect for notifications, live score updates, news feeds, and stock tickers. Choose it when simplicity is your priority and you want to avoid managing a separate server. Choose it when you're already using standard HTTP infrastructure and don't want to introduce new protocols.

Choose WebSockets when you need bidirectional communication. This is necessary for chat applications, collaborative editing tools where multiple users edit the same document, multiplayer games, and any application where the client needs to send frequent messages to the server. Choose WebSockets when low latency is absolutely critical and you can invest in the necessary infrastructure. Choose them when you're building a complex interactive application that justifies the additional complexity.

For practical implementation in modern applications, there are excellent managed solutions that handle the complexity for you. These services manage connection handling, automatic reconnection, fallbacks for older browsers, and horizontal scaling. You simply broadcast events from your backend and listen for them on the frontend.

Real-time communication transforms user experience in ways that traditional request-response cannot match. Choosing the right technology for your specific use case matters, but implementing either approach well will make your users feel the difference immediately. Most applications don't actually need WebSockets — Server-Sent Events are simpler, more efficient, and perfectly adequate for notifications and live updates.