Streaming and SSE

Overview

Introduction to Streaming

Streaming allows your server to send data to clients piece-by-piece (in chunks) rather than waiting to generate the entire response all at once. This is especially useful for large payloads, real-time logs, AI text generation, or any scenario where you want to reduce Time to First Byte (TTFB) and show progress immediately.

Hono provides built-in streaming helpers that leverage web-standard ReadableStream and WritableStream objects, making them compatible with modern server runtimes like Node.js, Bun, Deno, and Cloudflare Workers.

Streaming Text and Custom Content

The stream helper gives you low-level control over a chunked text response. You can write strings or binary data to the stream progressively and close it when you are finished.

Copyimport { Hono } from 'hono'
import { stream } from 'hono/streaming'
 
const app = new Hono()
 
app.get('/stream', (c) => {
  return stream(c, async (stream) => {
    await stream.write('Hello ')
    await stream.sleep(1000)
    await stream.write('World!')
    await stream.writeln(' This is a new line.')
  })
})

If you specifically want to stream plain text with the correct text/plain headers and safety settings, use streamText:

Copyimport { Hono } from 'hono'
import { streamText } from 'hono/streaming'
 
const app = new Hono()
 
app.get('/text', (c) => {
  return streamText(c, async (stream) => {
    await stream.write('Chunk 1\n')
    await stream.sleep(500)
    await stream.write('Chunk 2\n')
  })
})

Server-Sent Events (SSE)

Server-Sent Events allow a server to push real-time updates to a client over an HTTP connection using event streams (text/event-stream).

Hono provides streamSSE and the SSEStreamingApi class to simplify sending structured messages that include event names, unique IDs, retry intervals, and data payloads.

Copyimport { Hono } from 'hono'
import { streamSSE } from 'hono/streaming'
 
const app = new Hono()
 
app.get('/sse', (c) => {
  let id = 0
  return streamSSE(c, async (stream) => {
    while (true) {
      await stream.writeSSE({
        data: `Server time is ${new Date().toISOString()}`,
        event: 'time-update',
        id: String(id++),
        retry: 3000,
      })
      await stream.sleep(1000)
    }
  })
})

Key Concepts

• Chunked Transfer Encoding: An HTTP mechanism where data is sent in a series of chunks, allowing the browser to render content before the server finishes generating the whole response.
• ReadableStream / WritableStream: Standard web APIs used underneath Hono's streaming helpers to pipe data from the server handler to the client connection.
• Server-Sent Events (SSE): A lightweight protocol for pushing text-based messages from server to client over a single long-lived HTTP connection.

Streaming API Reference

The streaming helpers pass a stream instance into your callback function. Here are the available methods on the stream objects:

Related

• Context API Methods
• Handling WebSockets