📝 Add docs for Server Sent Events

docs/en/docs/tutorial/server-sent-events.md

@@ -0,0 +1,114 @@
+# Server-Sent Events (SSE) { #server-sent-events-sse }
+
+You can stream data to the client using **Server-Sent Events** (SSE).
+
+This is similar to [Stream JSON Lines](stream-json-lines.md){.internal-link target=_blank}, but uses the `text/event-stream` format, which is supported natively by browsers with the <a href="https://developer.mozilla.org/en-US/docs/Web/API/EventSource" class="external-link" target="_blank">`EventSource` API</a>.
+
+## What are Server-Sent Events? { #what-are-server-sent-events }
+
+SSE is a standard for streaming data from the server to the client over HTTP.
+
+Each event is a small text block with "fields" like `data`, `event`, `id`, and `retry`, separated by blank lines.
+
+It looks like this:
+
+```
+data: {"name": "Portal Gun", "price": 999.99}
+
+data: {"name": "Plumbus", "price": 32.99}
+
+```
+
+SSE is commonly used for AI chat streaming, live notifications, logs and observability, and other cases where the server pushes updates to the client.
+
+/// tip
+
+If you want to stream binary data, for example video or audio, check the advanced guide: [Stream Data](../advanced/stream-data.md){.internal-link target=_blank}.
+
+///
+
+## Stream SSE with FastAPI { #stream-sse-with-fastapi }
+
+To stream SSE with FastAPI, use `yield` in your *path operation function* and set `response_class=EventSourceResponse`.
+
+Import `EventSourceResponse` from `fastapi.sse`:
+
+{* ../../docs_src/server_sent_events/tutorial001_py310.py ln[1:25] hl[4,22] *}
+
+Each yielded item is encoded as JSON and sent in the `data:` field of an SSE event.
+
+If you declare the return type as `AsyncIterable[Item]`, FastAPI will use it to **validate**, **document**, and **serialize** the data using Pydantic.
+
+{* ../../docs_src/server_sent_events/tutorial001_py310.py ln[1:25] hl[10:12,23] *}
+
+/// tip
+
+As Pydantic will serialize it in the **Rust** side, you will get much higher **performance** than if you don't declare a return type.
+
+///
+
+### Non-async *path operation functions* { #non-async-path-operation-functions }
+
+You can also use regular `def` functions (without `async`), and use `yield` the same way.
+
+FastAPI will make sure it's run correctly so that it doesn't block the event loop.
+
+As in this case the function is not async, the right return type would be `Iterable[Item]`:
+
+{* ../../docs_src/server_sent_events/tutorial001_py310.py ln[28:31] hl[29] *}
+
+### No Return Type { #no-return-type }
+
+You can also omit the return type. FastAPI will use the [`jsonable_encoder`](./encoder.md){.internal-link target=_blank} to convert the data and send it.
+
+{* ../../docs_src/server_sent_events/tutorial001_py310.py ln[34:37] hl[35] *}
+
+## `ServerSentEvent` { #serversentevent }
+
+If you need to set SSE fields like `event`, `id`, `retry`, or `comment`, you can yield `ServerSentEvent` objects instead of plain data.
+
+Import `ServerSentEvent` from `fastapi.sse`:
+
+{* ../../docs_src/server_sent_events/tutorial002_py310.py hl[4,26] *}
+
+The `data` field is always encoded as JSON. You can pass any value that can be serialized as JSON, including Pydantic models.
+
+## Raw Data { #raw-data }
+
+If you need to send data **without** JSON encoding, use `raw_data` instead of `data`.
+
+This is useful for sending pre-formatted text, log lines, or special <dfn title="A value used to indicate a special condition or state">"sentinel"</dfn> values like `[DONE]`.
+
+{* ../../docs_src/server_sent_events/tutorial003_py310.py hl[17] *}
+
+/// note
+
+`data` and `raw_data` are mutually exclusive. You can only set one of them on each `ServerSentEvent`.
+
+///
+
+## Resuming with `Last-Event-ID` { #resuming-with-last-event-id }
+
+When a browser reconnects after a connection drop, it sends the last received `id` in the `Last-Event-ID` header.
+
+You can read it as a header parameter and use it to resume the stream from where the client left off:
+
+{* ../../docs_src/server_sent_events/tutorial004_py310.py hl[25,27,31] *}
+
+## SSE with POST { #sse-with-post }
+
+SSE works with **any HTTP method**, not just `GET`.
+
+This is useful for protocols like <a href="https://modelcontextprotocol.io" class="external-link" target="_blank">MCP</a> that stream SSE over `POST`:
+
+{* ../../docs_src/server_sent_events/tutorial005_py310.py hl[14] *}
+
+## Technical Details { #technical-details }
+
+FastAPI implements some SSE best practices out of the box.
+
+* Send a **"keep alive" comment** every 15 seconds when there hasn't been any message, to prevent some proxies from closing the connection, as suggested in the <a href="https://html.spec.whatwg.org/multipage/server-sent-events.html#authoring-notes" class="external-link" target="_blank">HTML specification: Server-Sent Events</a>.
+* Set the `Cache-Control: no-cache` header to **prevent caching** of the stream.
+* Set a special header `X-Accel-Buffering: no` to **prevent buffering** in some proxies like Nginx.
+
+You don't have to do anything about it, it works out of the box. 🤓

docs/en/docs/tutorial/stream-json-lines.md

@@ -100,6 +100,6 @@ You can also omit the return type. FastAPI will then use the [`jsonable_encoder`
 
 {* ../../docs_src/stream_json_lines/tutorial001_py310.py ln[33:36] hl[34] *}
 
-## Server Sent Events (SSE) { #server-sent-events-sse }
+## Server-Sent Events (SSE) { #server-sent-events-sse }
 
-A future version of FastAPI will also have first-class support for Server Sent Events (SSE), which are quite similar, but with a couple of extra details. 🤓
+FastAPI also has first-class support for Server-Sent Events (SSE), which are quite similar but with a couple of extra details. You can learn about them in the next chapter: [Server-Sent Events (SSE)](server-sent-events.md){.internal-link target=_blank}. 🤓

docs/en/mkdocs.yml

@@ -155,6 +155,7 @@ nav:
     - tutorial/sql-databases.md
     - tutorial/bigger-applications.md
     - tutorial/stream-json-lines.md
+    - tutorial/server-sent-events.md
     - tutorial/background-tasks.md
     - tutorial/metadata.md
     - tutorial/static-files.md