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To stream Server-Sent Events (SSE) from FastAPI to React 19, return FastAPI’s EventSourceResponse, send properly framed events, and consume them with the browser’s EventSource API or a Fetch stream. Preventing buffering also depends on every proxy and hosting layer between your app and the browser—not just the endpoint.
How SSE works between FastAPI and React
SSE is a one-way HTTP text stream from server to browser. The response uses text/event-stream; each event is a record ending with a blank line. Records can include data, event, id, and retry fields. FastAPI’s SSE tutorial and SSE API reference document its native support.
React 19 does not provide the transport or parse SSE. The browser receives the stream; your React component turns received event data into UI state. This is different from React’s server-rendering features that stream HTML, as discussed in the React 19 release material and React 19.3 release material.
How to stream SSE from FastAPI to React
1. Define the event contract
Choose event names and a small, serializable payload schema. Add a stable event ID if clients need to resume after reconnecting, and define how a finite stream signals completion. FastAPI’s ServerSentEvent supports event metadata such as event, id, retry, and comment. Its data field is JSON-encoded; use raw_data for preformatted text that must not be JSON-encoded.
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2. Emit events with FastAPI
FastAPI’s native SSE support was added in version 0.135.0. Check the version pinned in your project before using this API; older installations may need a different response implementation.
from collections.abc import AsyncIterable
from fastapi import FastAPI
from fastapi.sse import EventSourceResponse, ServerSentEvent
app = FastAPI()
@app.get("/events", response_class=EventSourceResponse)
async def events() -> AsyncIterable[ServerSentEvent]:
yield ServerSentEvent(data={"status": "started"}, event="status", id="1")
yield ServerSentEvent(data={"progress": 50}, event="progress", id="2")For straightforward payloads, FastAPI’s tutorial also shows yielding typed model instances for serialization into data. Use an async generator when awaiting I/O or producing asynchronous events. Avoid blocking the event loop with synchronous work; FastAPI documents that synchronous generator endpoints are run appropriately. Complete authorization and input validation before starting the stream, because the response has begun once streaming starts.
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3. Receive events with React 19
For a conventional GET feed, native EventSource handles SSE framing and named-event dispatch. Close it when the component unmounts so rerenders or navigation do not leave a connection open.
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import { useEffect, useState } from "react";
export function LiveStatus() {
const [events, setEvents] = useState([]);
useEffect(() => {
const source = new EventSource("/events");
source.addEventListener("progress", (event) => {
setEvents((current) => [...current, JSON.parse(event.data)]);
});
source.onerror = () => {
// Show a reconnecting or failed state as appropriate.
};
return () => source.close();
}, []);
return <pre>{JSON.stringify(events, null, 2)}</pre>;
}Use the event name configured by the server in addEventListener; unnamed messages use the default message event. Parse JSON only when your event contract says the payload is JSON.
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Should you use EventSource or fetch for SSE?
| Consideration | Native EventSource | Fetch with a readable body |
|---|---|---|
| SSE parsing | The browser dispatches messages and named events. | Your application parses the event framing and fields. |
| Request shape | URL-based event-stream connection. | Supports custom method, headers, body, and cancellation. |
| Reconnection | Provides native event-stream reconnection behavior. | Your application implements retry and resume policy. |
| Best fit | Simple one-way GET event feeds. | POST streams or requests needing custom headers or lifecycle. |
| Main caution | Plan authorization, CORS, and connection lifecycle. | Decode arbitrary chunk boundaries correctly; do not buffer the whole response. |
For a standard GET, prefer EventSource unless the request requires Fetch capabilities. MDN documents EventSource and incremental Fetch response reading.
Use Fetch when the request needs more control
Fetch returns a readable response body that you can decode incrementally. Check the HTTP status and body before reading, and use an AbortController so cleanup or user cancellation stops the request.
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const controller = new AbortController();
const response = await fetch("/events", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify({ topic: "build" }),
signal: controller.signal,
});
if (!response.ok || !response.body) throw new Error("Stream request failed");
const reader = response.body
.pipeThrough(new TextDecoderStream())
.getReader();
let buffer = "";
while (true) {
const { value = "", done } = await reader.read();
buffer += value;
// Parse complete SSE records; retain the incomplete suffix in buffer.
if (done) break;
}This is a reader outline, not a complete SSE parser. Network chunks can split lines or events at arbitrary points. A production parser must handle LF and CRLF delimiters, multiple data: lines, comments, named events, IDs, retry fields, and any final unterminated buffer.
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FastAPI documents a comment ping every 15 seconds when there are no messages, and defaults of Cache-Control: no-cache and X-Accel-Buffering: no to help with common proxy issues. These settings do not guarantee that every intermediary will pass chunks through promptly. Check the actual reverse proxy, ingress, CDN, load balancer, compression middleware, and hosting platform configuration.
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- Test a delayed sequence of events in the deployed environment and compare when the server produces each event with when the browser receives it.
- Log production and receipt timestamps to distinguish server-side or intermediary buffering from client-side display cadence.
- Inspect each intermediary’s buffering and compression settings rather than assuming the FastAPI response headers settle the behavior.
A stream can be yielded promptly by the server while React batches state updates; conversely, a proxy can aggregate bytes before the browser sees them. Endpoint code alone cannot establish that a deployed stream is unbuffered.
How reconnects, resume, and cleanup work
When an event includes an id, a reconnecting native EventSource sends the last event ID in the Last-Event-ID header. Resuming requires server logic that can use that ID to find retained or replayable events; the header does not store the event history. FastAPI describes this pattern in its SSE tutorial.
Quick Recap
- For
EventSource, callclose()in the React effect cleanup and avoid opening duplicate connections during rerenders. - For Fetch, abort the active request with its
AbortControlleron cleanup or cancellation. - For either client, decide how the UI represents errors and completion. With a custom Fetch parser, define retry, resume, and duplicate-event handling yourself; native EventSource reconnection behavior does not automatically apply.
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