# Streams: SSE and WebSocket

> Streams are relayed without holding: each event arrives when the upstream emits it. Product side: API gateway; operations side: SSE and WebSocket streams.

Guide - Since version 0.2.0 - Flexway v1.6.3 - https://flexway.io/en/docs/developer-streams/

> Streams are relayed **without holding**: each event arrives when the upstream emits it.
>
> Product side: [API gateway](https://flexway.io/en/docs/product-api-gateway/); operations side: [SSE and WebSocket streams](https://flexway.io/en/docs/operations-examples-rest/#sse-and-websocket-streams).

## Server-sent events

```bash
curl -N -H "Authorization: Bearer $TOKEN" -H 'Accept: text/event-stream' https://api.example.com/events
```

| Rule | Detail |
| --- | --- |
| No total deadline | An open stream is not cut by the operation's budget |
| Silence ends it | Past the operator's read timeout without a byte, the stream closes: have the upstream send a heartbeat (`: ping`) more often |
| Rate | Counted at opening |

A browser's `EventSource` cannot set `Authorization`. Read the stream with `fetch`:

```javascript
const res = await fetch("https://api.example.com/events", {
  headers: { Authorization: `Bearer ${token}`, Accept: "text/event-stream" },
});
const reader = res.body.pipeThrough(new TextDecoderStream()).getReader();
for (;;) {
  const { value, done } = await reader.read();
  if (done) break;
  handle(value); // split on the blank line that ends an event
}
```

## WebSocket

The upgrade is a request like any other: identity and rate are checked **at opening**, then the
connection outlives the operation's budget.

```javascript
// A browser sets no header on a WebSocket: the operator decides how the identity arrives (a cookie
// the contract declares, an edge that authenticates). A server-side client sets Authorization:
const ws = new WebSocket("wss://api.example.com/ws", { headers: { Authorization: `Bearer ${token}` } }); // Node (ws)
```

A broker's event channels (acknowledgements, resumption) are described apart:
[asynchronous channels](https://flexway.io/en/docs/developer-async/).