Real-time: SSE & WebSockets

Server-Sent Events (any engine)

SSE works on every engine, straight from a normal route:

app.get("/events", ctx -> {
    try (SseEmitter sse = ctx.sse()) {
        sse.send("plain data");
        sse.send("update", "{\"n\":1}");          // named event
        sse.send("update", "42", "{\"n\":2}");    // with id (client resume)
        sse.comment("keep-alive");
    }
});

The client consumes it with a standard EventSource. Thanks to virtual threads, holding thousands of open SSE connections is cheap.

WebSockets (Jetty engine)

The WebSocket API lives in core (WsHandler, WsSession); the implementation lives in the engine adapter. Today that's ligero-server-jetty — the JDK engine cannot upgrade protocols and fails at startup with a clear message if WebSocket routes exist.

runtimeOnly 'com.ligero:ligero-server-jetty:0.2.0-SNAPSHOT' // instead of ligero-server-jdk

app.websocket("/chat", new WsHandler() {
    @Override public void onConnect(WsSession session) {
        session.attributes().put("joined", Instant.now());
    }

    @Override public void onMessage(WsSession session, String message) {
        session.send("echo: " + message);
    }

    @Override public void onClose(WsSession session, int statusCode, String reason) { }

    @Override public void onError(WsSession session, Throwable error) { }
});

HTTP routes and WebSockets share the same port; non-upgrade requests flow through the normal pipeline untouched.

Which one do I need?

	SSE	WebSocket
Direction	server → client	bidirectional
Transport	plain HTTP	protocol upgrade
Engine support	all	Jetty adapter
Typical use	feeds, notifications, progress	chat, games, collaborative editing