Skip to content

Server-Sent Events

Quick start

use rust_web_server::sse::{Sse, SseEvent};

fn handle_updates(_req: &Request, _conn: &ConnectionInfo) -> Response {
    Sse::new()
        .event("connected", "ready")
        .push(SseEvent::data(r#"{"count":1}"#).id("1").event_type("update"))
        .push(SseEvent::data(r#"{"count":2}"#).id("2").event_type("update"))
        .comment("keep-alive")
        .into_response()
}

`Sse` builder

Sse::new() creates an empty builder. Every method takes self and returns Self for chaining. Call .into_response() at the end to get a Response.

Method	Wire output
`.event(event_type, data)`	`event: <type>\ndata: <data>\n\n`
`.data(data)`	`data: <data>\n\n` (no `event:` field — client sees `"message"`)
`.push(SseEvent)`	fully configured event
`.retry(ms)`	`retry: <ms>\n\n` — overrides client reconnect timer
`.comment(text)`	`: <text>\n\n` — ignored by browsers, used as keep-alive ping

.into_response() sets:

200 OK
Content-Type: text/event-stream
Cache-Control: no-cache
X-Accel-Buffering: no — disables nginx proxy buffering

`SseEvent`

Build with SseEvent::data(payload) and chain optional setters:

let event = SseEvent::data(r#"{"token":"Hello"}"#)
    .id("42")
    .event_type("token")
    .retry(3000);

Field	Type	Meaning
`data`	`String` (required)	Payload; multi-line strings split into multiple `data:` lines
`id`	`Option<String>`	Event ID; client sends `Last-Event-ID` on reconnect
`event_type`	`Option<String>`	Event type; client listens with `addEventListener("type", fn)`
`retry_ms`	`Option<u32>`	Reconnect delay in milliseconds

Multi-line data

Newlines in the data string are split into separate data: lines per the SSE specification. The client concatenates them with \n:

SseEvent::data("line one\nline two\nline three")
// Wire: data: line one\ndata: line two\ndata: line three\n\n

Use case: streaming AI tokens

For live token-by-token output where events arrive over time, write the SSE headers and raw event lines directly to the TCP stream. The buffered Sse builder is for pre-known event sequences; live streaming requires a custom accept loop:

// 1. Emit SSE headers
response.headers.push(Header::new("Content-Type", "text/event-stream"));
response.headers.push(Header::new("Cache-Control", "no-cache"));
response.headers.push(Header::new("X-Accel-Buffering", "no"));
write_headers(&mut stream, &response);

// 2. Loop over model output and push each token
for token in model_stream {
    let event = SseEvent::data(&token).encode();
    stream.write_all(&event)?;
    stream.flush()?;
}

Client-side JavaScript

const source = new EventSource("/updates");

// Default handler — fires for events with no `event:` type field
source.onmessage = (e) => {
    console.log("message:", e.data);
};

// Named event handler
source.addEventListener("update", (e) => {
    const payload = JSON.parse(e.data);
    console.log("update count:", payload.count);
});

// Reconnection — browser retries automatically; Last-Event-ID is sent
source.onerror = () => console.warn("SSE connection lost, retrying…");