# SseStream

**Kind:** Class

**Source:** [`packages/core/router/sse-stream.ts`](https://github.com/nestjs/nest/blob/master/packages/core/router/sse-stream.ts#L75)

**Part of:** [Core](subsystem-packages-core)

Adapted from https://raw.githubusercontent.com/EventSource/node-ssestream
Transforms "messages" to W3C event stream content.
See https://html.spec.whatwg.org/multipage/server-sent-events.html
A message is an object with one or more of the following properties:
- data (String or object, which gets turned into JSON)
- type
- id
- retry
- comment

If constructed with a HTTP Request, it will optimise the socket for streaming.
If this stream is piped to an HTTP Response, it will set appropriate headers.

`SseStream` is a transform stream that converts message objects into W3C Server-Sent Events (SSE) wire-format payloads. It can optimize an incoming HTTP request for streaming and configures appropriate response headers when piped to an HTTP response.

**Extends:** `Transform`

## Methods

| Method | Signature | Returns |
|---|---|---|
| `pipe` | `pipe(destination: T, options: { additionalHeaders?: AdditionalHeaders; statusCode?: number; end?: boolean; })` | `T` |
| `commitHeaders` | `commitHeaders()` | `void` |
| `_transform` | `_transform(message: MessageEvent, encoding: string, callback: (error?: Error | null, data?: any) => void)` | `void` |
| `writeMessage` | `writeMessage(message: MessageEvent, cb: (error: Error | null | undefined) => void)` | `void` |

## Where it refuses work

- `SseStream` stops the work with an early return when `this._headersCommitted || !this._destination`.
- `SseStream` stops the work with an early return when `this._destination.writableEnded`.

## Diagram

```mermaid
graph LR
  A[HTTP Request] --> B[SseStream]
  C[Message objects<br/>data, type, id, retry, comment] --> B
  B -->|Formats as SSE frames| D[HTTP Response]
  D --> E[EventSource client]
```

## AI Coding Instructions

- Write message objects to the stream using standard stream APIs such as `stream.write({ data, type, id })`.
- Use `data` for strings or JSON-serializable objects; objects are serialized to JSON before being sent to clients.
- Pipe the stream directly to the HTTP response so `SseStream` can apply the required SSE response headers.
- Construct the stream with the request when available to enable request socket optimizations for long-lived connections.
- Handle request or response closure and call `end()` to avoid retaining SSE streams after clients disconnect.