Server-Sent Events (SSE)

Server-Sent Events (SSE) is a lightweight standard for pushing a one-way stream of updates from a server to a client over a single, long-lived HTTP connection. The client opens a connection with the browser's `EventSource` API (or an equivalent library) and the server keeps it open, emitting text events as `data:` lines whenever something new happens. Unlike WebSockets, SSE is unidirectional — server to client only — which makes it simpler to run: it rides on ordinary HTTP, reconnects automatically, and passes through most proxies and load balancers without special handling. For AI SaaS builders this is the workhorse behind token-by-token streaming: most LLM APIs stream their responses as SSE so users see text appear as it's generated instead of waiting for the whole completion. It's also a clean fit for live dashboards, notifications, and progress bars. The catch: it only carries text and browsers cap the number of open connections, so it's not for high-frequency bidirectional traffic.