@fastify/sse

Server-Sent Events plugin for Fastify. Provides first-class SSE support with clean API integration, session management, and streaming capabilities.

Features

• 🚀 Clean API: Route-level SSE support with { sse: true }
• 📡 Streaming: Native support for Node.js streams and async iterators
• 🔄 Reconnection: Built-in message replay with Last-Event-ID
• 💓 Heartbeat: Configurable keep-alive mechanism
• 🎯 Lifecycle: Full integration with Fastify hooks and error handling
• 📝 TypeScript: Complete type definitions included
• ⚡ Performance: Efficient backpressure handling

Install

npm i @fastify/sse

Quick Start

const fastify = require('fastify')({ logger: true })

// Register the plugin
await fastify.register(require('@fastify/sse'))

// Create an SSE endpoint
fastify.get('/events', { sse: true }, async (request, reply) => {
  // Send a message
  await reply.sse.send({ data: 'Hello SSE!' })
  
  // Send with full options
  await reply.sse.send({
    id: '123',
    event: 'update', 
    data: { message: 'Hello World' },
    retry: 1000
  })
})

await fastify.listen({ port: 3000 })

API

Plugin Registration

await fastify.register(require('@fastify/sse'), {
  // Optional: heartbeat interval in milliseconds (default: 30000)
  heartbeatInterval: 30000,
  
  // Optional: default serializer (default: JSON.stringify)
  serializer: (data) => JSON.stringify(data)
})

Route Configuration

// Enable SSE for a route
fastify.get('/events', { sse: true }, handler)

// With options
fastify.get('/events', { 
  sse: {
    heartbeat: false,           // Disable heartbeat for this route
    serializer: customSerializer // Custom serializer for this route
  }
}, handler)

reply.sse.send(source)

Send SSE messages. Accepts various source types:

Single Message

// Simple data
await reply.sse.send({ data: 'hello' })

// Full SSE message
await reply.sse.send({
  id: '123',
  event: 'update',
  data: { message: 'Hello' },
  retry: 1000
})

// Plain string
await reply.sse.send('plain text message')

Streaming Sources

// Async generator
async function* generateEvents() {
  for (let i = 0; i < 10; i++) {
    yield { id: i, data: `Event ${i}` }
    await sleep(1000)
  }
}
await reply.sse.send(generateEvents())

// Node.js Readable stream
const stream = fs.createReadStream('data.jsonl')
await reply.sse.send(stream)

// Transform existing stream
const transformStream = new Transform({
  transform(chunk, encoding, callback) {
    callback(null, { data: chunk.toString() })
  }
})
someSource.pipe(transformStream)
await reply.sse.send(transformStream)

reply.sse.stream()

Create a transform stream for use in pipeline operations:

// Use with pipeline for efficient streaming
const { pipeline } = require('stream/promises')
const fs = require('fs')

fastify.get('/file-stream', { sse: true }, async (request, reply) => {
  const fileStream = fs.createReadStream('data.jsonl')
  
  // Parse each line as JSON and convert to SSE format
  const parseTransform = new Transform({
    transform(chunk, encoding, callback) {
      const lines = chunk.toString().split('\n').filter(Boolean)
      for (const line of lines) {
        try {
          const data = JSON.parse(line)
          this.push({ id: data.id, data })
        } catch (err) {
          // Skip invalid JSON lines
        }
      }
      callback()
    }
  })
  
  // Stream file data through SSE
  await pipeline(
    fileStream,
    parseTransform,
    reply.sse.stream(),
    reply.raw,
    { end: false }
  )
})

Connection Management

fastify.get('/live', { sse: true }, async (request, reply) => {
  // Keep connection alive (prevents automatic close)
  reply.sse.keepAlive()

  // Send initial message
  await reply.sse.send({ data: 'Connected' })

  // Check if keepAlive was called
  console.log('Keep alive status:', reply.sse.shouldKeepAlive) // true

  // Set up periodic updates
  const interval = setInterval(async () => {
    if (reply.sse.isConnected) {
      await reply.sse.send({ data: 'ping' })
    } else {
      clearInterval(interval)
    }
  }, 1000)

  // Clean up when connection closes
  reply.sse.onClose(() => {
    clearInterval(interval)
    console.log('Connection closed')
  })
})

Message Replay

Handle client reconnections with Last-Event-ID:

const messageHistory = []

fastify.get('/events', { sse: true }, async (request, reply) => {
  // Handle replay on reconnection
  await reply.sse.replay(async (lastEventId) => {
    // Find messages after the last received ID
    const startIndex = messageHistory.findIndex(msg => msg.id === lastEventId)
    const messagesToReplay = startIndex !== -1 
      ? messageHistory.slice(startIndex + 1)
      : messageHistory
    
    // Send missed messages
    for (const message of messagesToReplay) {
      await reply.sse.send(message)
    }
  })
  
  // Send new message
  const newMessage = { id: Date.now(), data: 'New event' }
  messageHistory.push(newMessage)
  await reply.sse.send(newMessage)
})

Properties and Methods

Properties

• reply.sse.lastEventId: Client's last received event ID (string | null)
• reply.sse.isConnected: Connection status (boolean)
• reply.sse.shouldKeepAlive: Whether connection should be kept alive after handler completion (boolean)

Methods

• reply.sse.send(source): Send SSE messages from various source types
• reply.sse.stream(): Create a transform stream for pipeline operations
• reply.sse.keepAlive(): Prevent connection from auto-closing after handler returns
• reply.sse.close(): Manually close the connection
• reply.sse.replay(callback): Handle message replay using Last-Event-ID
• reply.sse.onClose(callback): Register callback for when connection closes
• reply.sse.sendHeaders(): Manually send headers (called automatically on first write)

Advanced Usage

Fallback to Regular Responses

Routes with { sse: true } automatically fall back to regular handlers when the client doesn't request SSE:

fastify.get('/data', { sse: true }, async (request, reply) => {
  const data = await getData()
  
  // Check if this is an SSE request
  if (request.headers.accept?.includes('text/event-stream')) {
    // SSE client - stream the data
    await reply.sse.send({ data })
  } else {
    // Regular client - return JSON
    return { data }
  }
})

Error Handling

fastify.get('/stream', { sse: true }, async (request, reply) => {
  try {
    async function* riskyGenerator() {
      yield { data: 'before error' }
      throw new Error('Something went wrong')
    }
    
    await reply.sse.send(riskyGenerator())
  } catch (error) {
    // Handle errors gracefully
    await reply.sse.send({ 
      event: 'error',
      data: { message: 'Stream error occurred' }
    })
  }
})

Custom Serialization

// Plugin-level serializer
await fastify.register(require('@fastify/sse'), {
  serializer: (data) => {
    // Custom serialization logic
    return typeof data === 'string' ? data : JSON.stringify(data)
  }
})

// Route-level serializer
fastify.get('/custom', {
  sse: {
    serializer: (data) => `CUSTOM:${JSON.stringify(data)}`
  }
}, async (request, reply) => {
  await reply.sse.send({ data: 'test' }) // Outputs: "CUSTOM:\"test\""
})

Testing

Testing SSE endpoints is simplified with standard Fastify injection:

const response = await fastify.inject({
  method: 'GET',
  url: '/events',
  headers: {
    accept: 'text/event-stream'
  }
})

assert.strictEqual(response.statusCode, 200)
assert.strictEqual(response.headers['content-type'], 'text/event-stream')
assert.ok(response.body.includes('data: "Hello SSE!"'))

Client-Side Usage

<!DOCTYPE html>
<html>
<head>
  <title>SSE Client</title>
</head>
<body>
  <div id="messages"></div>
  
  <script>
    const eventSource = new EventSource('/events')
    const messagesDiv = document.getElementById('messages')
    
    eventSource.onmessage = function(event) {
      const data = JSON.parse(event.data)
      messagesDiv.innerHTML += '<div>' + JSON.stringify(data) + '</div>'
    }
    
    eventSource.addEventListener('update', function(event) {
      console.log('Update event:', JSON.parse(event.data))
    })
    
    eventSource.onerror = function(event) {
      console.error('SSE error:', event)
    }
  </script>
</body>
</html>

TypeScript

Full TypeScript support included:

import fastify from 'fastify'
import fastifySSE, { SSEMessage } from '@fastify/sse'

const app = fastify()
await app.register(fastifySSE)

app.get('/events', { sse: true }, async (request, reply) => {
  const message: SSEMessage = {
    id: '123',
    event: 'test',
    data: { hello: 'world' }
  }

  await reply.sse.send(message)

  // TypeScript knows about SSE properties
  console.log(reply.sse.isConnected) // boolean
  console.log(reply.sse.lastEventId) // string | null
  console.log(reply.sse.shouldKeepAlive) // boolean
})

Examples

See the examples directory for complete working examples:

• Basic Usage - Simple SSE endpoints
• More examples coming soon...

Comparison with fastify-sse-v2

| Feature | fastify-sse-v2 | @fastify/sse | |---------|---------------|--------------| | Basic SSE | ✅ | ✅ | | Async Iterators | ✅ | ✅ | | Stream Support | ✅ | ✅ Enhanced | | Session Management | ❌ | ✅ | | Last-Event-ID | ❌ | ✅ | | Connection Health | ❌ | ✅ | | Fastify Integration | ⚠️ Limited | ✅ Full | | Testing Support | ❌ | ✅ |

License

MIT