Server-Sent Events don't work in Next API routes

[trezy]

Bug report

Describe the bug

When using Next's API routes, chunks that are written with res.write aren't sent until after res.end() is called.

To Reproduce

Steps to reproduce the behavior, please provide code snippets or a repository:

1. Create the following API route in a Next app:

export default async (req, res) => {
  let intervalID = null

  res.setHeader('Content-Type', 'text/event-stream')
  res.write('data: CONNECTION ESTABLISHED\n')

  const end = () => {
    if (intervalID) {
      clearTimeout(intervalID)
    }
  }

  req.on('aborted', end)
  req.on('close', end)

  const sendData = () => {
    const timestamp = (new Date).toISOString()
    res.write(`data: ${timestamp}\n`)
  }

  intervalID = setInterval(sendData, 1000)
}

2. Connect to the route with a tool that supports Server-Sent Events (i.e. Postwoman).

Expected behavior

The route sends a new event to the connection every second.

Actual behavior

The route doesn't send any data to the connection unless a call to res.end() is added to the route.

System information

• OS: macOS
• Version of Next.js: 9.1.5

Additional context

When using other HTTP frameworks (Express, Koa, http, etc) this method works as expected. It's explicitly supported by Node's http.incomingMessage and http.ServerResponse classes which, from what I understand, Next uses as a base for the req and res that are passed into Next API routes.

I'd hazard a guess that #5855 was caused by the same issue, but considered unrelated because the issue was obscured by the express-sse library.

There are also two Spectrum topics about this (here and here) that haven't garnered much attention yet.

Supporting Websockets and SSE in Next API routes may be related, but fixing support for SSE should be a lower barrier than adding support Websockets. All of the inner workings are there, we just need to get the plumbing repaired.

[trezy]

I forgot to mention that this works in Micro routes, as well. I'm trying to eliminate the need for my Micro API by moving everything into Next, but this is a blocker for me.

[msand]

You can use a custom server.js to workaround this for now:

require('dotenv').config();
const app = require('express')();
const server = require('http').Server(app);
const next = require('next');

const DSN = process.env.DSN || 'postgres://postgres:postgres@localhost/db';
const dev = process.env.NODE_ENV !== 'production';

const nextApp = next({ dev });
const nextHandler = nextApp.getRequestHandler();

nextApp.prepare().then(() => {
  app.get('*', (req, res) => {
    if (req.url === '/stream') {
      res.writeHead(200, {
        Connection: 'keep-alive',
        'Cache-Control': 'no-cache',
        'Content-Type': 'text/event-stream',
      });
      res.write('data: Processing...\n\n');
      setTimeout(() => {
        res.write('data: Processing2...\n\n');
      }, 10000);
    } else {
      return nextHandler(req, res);
    }
  });

  require('../websocket/initWebSocketServer')(server, DSN);

  const port = 8080;
  server.listen(port, err => {
    if (err) throw err;
    console.log('> Ready on http://localhost:' + port);
  });
});

componentDidMount() {
    this.source = new EventSource('/stream')
    this.source.onmessage = function(e) {
      console.log(e)
    }
}

[msand]

I would still recommend to keep any server sent event and websocket handlers in separate processes in production. It's very likely that the frequency of updates to those parts of the business logic are quite different. Your front-end most likely changes more often than the types of events you handle / need to push to the clients from the servers. If you only make changes to one, you probably don't want to restart the processes responsible for the other(s). Better to keep the connections alive rather than cause a flood of reconnections / server restarts for changes which have no effect.

[trezy]

@msand The main reason I'm trying to avoid using a custom server is that I'm deploying to Now. Using a custom server would break all of the wonderful serverless functionality I get there.

Your second point is fair. What I'm trying to do is create an SSE stream for data that would otherwise be handled with basic polling. The server is already dealing with constant reconnections in that case, so an SSE stream actually results in fewer reconnections.

I suppose I could set up a small webserver in the same repo that just uses a separate Now builder. That would allow the processes to remain separate, though it'd still cause all of the SSE connections to abort and reconnect when there are any changes to the project.

Even with those points, I can see plenty of scenarios in which it makes sense to be able to run an SSE endpoint from one of Next's API routes. Additionally, in the docs it's specifically stated that...

• req: An instance of http.IncomingMessage, plus some pre-built middlewares you can see here
• res: An instance of http.ServerResponse, plus some helper functions you can see here

Since it's specifically stated that res is an instance of http.ServerResponse, I'd expect it to behave exactly the way http.ServerResponse behaves in any other circumstance. Either the documentation should change to reflect the quirks of the implementation or, preferably, res.write should be fixed to behave the way it does in any other circumstance.

[msand]

@trezy It seems the issue is that the middleware adds a gzip encoding which the browser has negotiated using the header:

Accept-Encoding: gzip, deflate, br

If you add

Content-Encoding: none

then it seems to work:

  res.writeHead(200, {
    Connection: 'keep-alive',
    'Content-Encoding': 'none',
    'Cache-Control': 'no-cache',
    'Content-Type': 'text/event-stream',
  });

[msand]

Alternatively, gzip your content

[trezy]

Oh, that's super interesting! I'll give that a shot and report back. In the meantime, it'd still be nice for this quirk (and any similar ones) to be noted somewhere in the docs.

[msand]

Yeah, it's more a consequence of having some helpers, would be nice with a mode which can turn all of it off, and only makes it a plain req res pair

[msand]

Actually, this seems to be documented here: https://github.com/expressjs/compression#server-sent-events

Have to call res.flush() when you think there's enough data for the compression to work efficiently

export default (req, res) => {
  res.writeHead(200, {
    'Cache-Control': 'no-cache',
    'Content-Type': 'text/event-stream',
  });
  res.write('data: Processing...');
  /* https://github.com/expressjs/compression#server-sent-events
    Because of the nature of compression this module does not work out of the box with
    server-sent events. To compress content, a window of the output needs to be
    buffered up in order to get good compression. Typically when using server-sent
    events, there are certain block of data that need to reach the client.

    You can achieve this by calling res.flush() when you need the data written to
    actually make it to the client.
*/
  res.flush();
  setTimeout(() => {
    res.write('data: Processing2...');
    res.flush();
  }, 1000);
};

[msand]

It then applies gzip compression for you

[uxFeranmi]

I have switched to using a custom express server. That's the only way I could get it to work. I guess that's cool since I can do more with express. Before deciding to integrate express, I had tried the things mentioned above, none worked. 1. Turned off gzip compression by setting the option in next.config.js. The behavior remained the same. I inspected the headers on the client (using postman) and confirmed the gzip encoding was removed, but that didn't seem to fix the problem. 2. Calling res.flush had no effect either. Instead I get a warning in the console that flush is deprecated and to use flushHeaders instead. But that's not what I want. This is a rather strange bug.. 😔

…

On Thursday, 9 January 2020, Mikael Sand ***@***.***> wrote: It then applies gzip compression for you — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#9965>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AHJC5ZDO2YUJFN5JLQPLYGLQ46QBFANCNFSM4KDHWFMA> .

[kavuri]

I have been trying to get SSE work in nextjs, but could not get working. With custom server and native node httpServer req,res it works, but with Nextjs 'res', no messages are sent to the client.
I started using Nextjs to get the advantages of server-side rendering, SLS and having the server and client code together. Using express beats the purpose
Any pointers on how this could work? This is a blocking problem for me

[uxFeranmi]

Hey @kavuri. It is possible to integrate a custom Node.js server (e.g using express) with your next.js app. That way, you can still get Server-Side Rendering without these Next.js limitations.

See this page of the official documentation for details: https://nextjs.org/docs/advanced-features/custom-server

Also, check out how I implemented this in my own app which I mentioned in the comment above yours: https://github.com/uxFeranmi/react-woocommerce/blob/master/server.js

[kavuri]

@uxFeranmi I could use the custom server method as mentioned here https://nextjs.org/docs/advanced-features/custom-server to write messages as res.write(...). But in the Next app, I do not see any messages in my page
I have created a sample page index.js and a react component App.js in pages dir as under

import EventSource from 'eventsource'

class App extends React.Component {
   constructor(props) {
      super(props) 
      this.events = new EventSource('http://localhost:3000/test')
      this.events.onopen = function() {
        console.log('connection is opened');
      }
      this.events.onerror = function() {
        console.log('error in opening conn.');
      }
   }
   componentDidMount() {
      this.events.onmessage = (event) => {
         console.log('got message..',event)
         this.data = JSON.parse(event.data)
      }
   }

   componentWillUnmount() {
      // cleanup
   }

   render() {
      return (
         <div>
           <h1>{this.data}</h1>
         </div>
      );
   }
}

index.js

import App from './App.js'

function HomePage() {
  return <div><App /></div>
}
export default HomePage

My custom server.js

const { createServer } = require('http')
const { parse } = require('url')
const next = require('next')
const fs = require('fs')

const port = parseInt(process.env.PORT, 10) || 3000
const dev = process.env.NODE_ENV !== 'production'
const app = next({ dev })
const handle = app.getRequestHandler()

console.log('starting server...')
function listen(req, res) {
    console.log('listening for incoming orders...');
    // Create a change stream. The 'change' event gets emitted when there's a
    // change in the database
    fs.watch('./', (eventType, filename) => {
      if (filename) {
        var obj = {"text": filename}
        console.log('sending:',obj);
        res.write('data:' + JSON.stringify(obj));
      }
    });
  
    res.on('close', () => {
       console.log('closing connection');
    });
}


app.prepare().then(() => {
  createServer((req, res) => {
    const parsedUrl = parse(req.url, true)
    const { pathname, query } = parsedUrl

    if (pathname === '/test') {
         const headers = {
        'Content-Type': 'text/event-stream',
        'Connection': 'keep-alive',
        'Cache-Control': 'no-cache'
      };
      res.writeHead(200, headers);
      res.write('\n')
      listen(req, res)
    } else {
      handle(req, res, parsedUrl)
    }
  }).listen(port, err => {
    if (err) throw err
    console.log(`> Ready on http://localhost:${port}`)
  })
})

I am not getting any message in the index page. But if I open the url http://localhost:3000/test, I get the messages, which means that the EventSource itself is working, but the Next server side rendering for the eventsource is not. Or maybe I am doing something wrong! Any pointers?