Server-Sent Events in C++: Real-time Without Magic and Extra Libraries

Why a regular HTTP request is like a letter by mail, and SSE is a real-time telegram

Imagine: you are waiting for a notification about a new comment on a website. You refresh the page every 5 seconds. The server returns a bunch of HTML, CSS, images each time — and only then, at the very bottom, you see: "No new comments." Sound familiar? This is called polling. It works, but it consumes traffic and loads the server.

Now imagine another scenario: you open a page, and a second later a message appears on it: "Vasily left a comment: 'Great article!'" Without reloading, without extra requests. The server itself "pushes" data to you. This is Server-Sent Events (SSE) — a technology that allows the server to send a stream of events to the client over a regular HTTP connection.

In this article, we will break down how to implement SSE in pure C++ without using heavy frameworks. You will learn why Server-Sent Events in C++ is an excellent choice for monitoring, stock quotes, chat notifications, and any tasks requiring one-way real-time data transfer.

How do Server-Sent Events work? Protocol breakdown

SSE is part of the HTML5 specification. The client opens a regular HTTP connection and tells the server: "I am ready to listen for events." In response, the server does not close the connection but starts sending data in a special text/event-stream format.

The message format is extremely simple:

data: This is the event text\\n\\n

Each event starts with data: and ends with a double newline. You can add id: for identification, event: for the event type, and retry: to specify the reconnection time in milliseconds.

Example of a full stream:

id: 42\nevent: new_message\ndata: {"user": "Vasily", "text": "Hello!"}\n\nid: 43\nevent: ping\ndata: Server is alive\n\n

The client (browser or any application) processes these lines and generates JavaScript events. Everything ingenious is simple.

SSE vs WebSocket: when to choose what?

Many immediately think of WebSocket when it comes to real-time. Let's compare:

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• WebSocket — two-way communication. The client and server can initiate an exchange. Requires a separate protocol (ws://), passing through proxies, often blocked by corporate firewalls.
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• SSE — one-way communication (server → client). Works over regular HTTP. Automatically reconnects on disconnection. Easily passes through any proxies.
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If your task is to send notifications to the client, rather than wait for commands from it, Server-Sent Events in C++ is an elegant and simple solution. No need to drag in heavy libraries, no need to write complex handlers.

Writing an SSE server in C++ from scratch

For implementation, we need a minimum: sockets and HTTP handling. In real projects, Boost.Beast or libmicrohttpd are used, but to understand the essence, we will write a simple server in pure C++ using POSIX sockets.

Our server will:

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• Accept an HTTP request from the client
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• Determine that the client wants SSE (by the Accept: text/event-stream header)
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• Send a stream of events with the current time every 2 seconds
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Step 1: Create a socket and listen on a port

#include <iostream>\n#include <string>\n#include <cstring>\n#include <unistd.h>\n#include <arpa/inet.h>\n#include <sys/socket.h>\n#include <thread>\n#include <chrono>\n#include <sstream>\n\nvoid handle_client(int client_socket) {\n    // Buffer for the request\n    char buffer[4096] = {0};\n    read(client_socket, buffer, 4096);\n    \n    // Check that this is a GE