Sse Event Stream Format

// Server sending different SSE fields
const http = require('http');

http.createServer((req, res) => {
  res.writeHead(200, {
    'Content-Type': 'text/event-stream',
    'Cache-Control': 'no-cache',
    'Connection': 'keep-alive',
  });

  // Comments start with ':'
  res.write(': This is a comment line\n\n');

  // Send event with all fields
  res.write(
    'id: event-001\n' +
    'event: user-update\n' +
    'data: {"name": "Alice", "status": "online"}\n' +
    '\n'
  );

  // Send another event
  res.write(
    'id: event-002\n' +
    'data: {"message": "Hello World"}\n' +
    '\n'
  );

  // Multi-line data field
  res.write(
    'id: event-003\n' +
    'data: {\n' +
    'data: "line1",\n' +
    'data: "line2"\n' +
    'data: }\n' +
    '\n'
  );

  // Set reconnection time
  res.write('retry: 5000\n\n');

  // Send without event type
  res.write(
    'id: event-004\n' +
    'data: unnamed event\n' +
    '\n'
  );

}).listen(3000);

const source = new EventSource('/events');

// Handle named events
source.addEventListener('user-update', (event) => {
  console.log('Event type:', event.type);
  console.log('Event ID:', event.lastEventId);
  console.log('Data:', event.data);
});

// Handle unnamed events (onmessage)
source.onmessage = (event) => {
  console.log('Unnamed event');
  console.log('ID:', event.lastEventId);
  console.log('Data:', event.data);
};

// Track last event ID for reconnection
let lastEventId = null;
source.onmessage = (event) => {
  lastEventId = event.lastEventId || lastEventId;
  processEvent(event.data);
};

// The client sends Last-Event-ID header on reconnection
// Server can read it from the request headers

function sendSSEEvent(res, options = {}) {
  const {
    data,
    event,
    id,
    retry,
  } = options;

  let message = '';

  // Event type (optional)
  if (event) {
    message += `event: ${event}\n`;
  }

  // Data (required)
  if (data) {
    // Handle multi-line data
    const lines = String(data).split('\n');
    lines.forEach(line => {
      message += `data: ${line}\n`;
    });
  }

  // Event ID (optional)
  if (id) {
    message += `id: ${id}\n`;
  }

  // Retry interval (optional)
  if (retry) {
    message += `retry: ${retry}\n`;
  }

  // Empty line terminates the event
  message += '\n';

  res.write(message);
}

// Usage
sendSSEEvent(res, {
  event: 'stock-price',
  data: JSON.stringify({
    symbol: 'AAPL',
    price: 175.50,
    change: 2.30,
  }),
  id: `stock-${Date.now()}`,
});

sendSSEEvent(res, {
  data: 'Simple message without type or ID',
});