Simple Server-Sent Events based PubSub Server

by Mitja Felicijan

Before we continue …

Publisher Subscriber model is nothing new and there are many amazing solutions out there, so writing a new one would be a waste of time if other solutions wouldn't have quite complex install procedures and weren't so hard to maintain. But to be fair, comparing this simple server with something like Kafka or RabbitMQ is laughable at the least. Those solutions are enterprise grade and have many mechanisms there to ensure messages aren't lost and much more. Regardless of these drawbacks, this method has been tested on a large website and worked until now without any problems. So now, that we got that cleared up, let's continue.

Wiki definition: Publish/subscribe messaging, or pub/sub messaging, is a form of asynchronous service-to-service communication used in serverless and microservices architectures. In a pub/sub model, any message published to a topic is immediately received by all the subscribers to the topic.

General goals

How exactly does the pub/sub model work?

The easiest way to explain this is with diagram bellow. Basic function is simple. We have subscribers that receive messages, and we have publishers that create and post messages. Similar model is also well know pattern that works on a premise of consumers and producers, and they take similar roles.

How PubSub works

These are some naive characteristics we want to achieve:

Known drawbacks:

Server-Sent Events

Read more about it on official specification page.

Current browser support

Browser support

Check https://caniuse.com/#feat=eventsource for latest information about browser support.

Known issues

Source: https://caniuse.com/#feat=eventsource

Message format

The simplest message that can be sent is only with data attribute:

data: this is a simple message
<blank line>

You can send message IDs to be used if the connection is dropped:

id: 33
data: this is line one
data: this is line two
<blank line>

And you can specify your own event types (the above messages will all trigger the message event):

id: 36
event: price
data: 103.34
<blank line>

Server requirements

The important thing is how you send headers and which headers are sent by the server that triggers browser to threat response as a EventStream.

Headers responsible for this are:

Content-Type: text/event-stream
Cache-Control: no-cache
Connection: keep-alive

Debugging with Google Chrome

Google Chrome provides build-in debugging and exploration tool for Server-Sent Events which is quite nice and available from Developer Tools under Network tab.

You can debug only client side events that get received and not the server ones. For debugging server events add console.log to server.js code and print out events.

Google Chrome Developer Tools EventStream

Server implementation

For the sake of this example we will use Node.js with Express as our router since this is the easiest way to get started and we will use already written SSE library for node sse-pubsub so we don't reinvent the wheel.

npm init --yes

npm install express
npm install body-parser
npm install sse-pubsub

Basic implementation of a server (server.js):

const express = require('express');
const bodyParser = require('body-parser');
const SSETopic = require('sse-pubsub');

const app = express();
const port = process.env.PORT || 4000;

// topics container
const sseTopics = {};

app.use(bodyParser.json());

// open for all cors
app.all('*', (req, res, next) => {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Headers', 'X-Requested-With, Content-Type');
  next();
});

// preflight request error fix
app.options('*', async (req, res) => {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Headers', 'X-Requested-With, Content-Type');
  res.send('OK');
});

// serve the event streams
app.get('/stream/:topic', async (req, res, next) => {
  const topic = req.params.topic;

  if (!(topic in sseTopics)) {
    sseTopics[topic] = new SSETopic({
      pingInterval: 0,
      maxStreamDuration: 15000,
    });
  }

  // subscribing client to topic
  sseTopics[topic].subscribe(req, res);
});

// accepts new messages into topic
app.post('/publish', async (req, res) => {
  let body = req.body;
  let status = 200;

  console.log('Incoming message:', req.body);

  if (
    body.hasOwnProperty('topic') &&
    body.hasOwnProperty('event') &&
    body.hasOwnProperty('message')
  ) {
    const topic = req.body.topic;
    const event = req.body.event;
    const message = req.body.message;

    if (topic in sseTopics) {
      // sends message to all the subscribers
      sseTopics[topic].publish(message, event);
    }
  } else {
    status = 400;
  }

  res.status(status).send({
    status,
  });
});

// returns JSON object of all opened topics
app.get('/status', async (req, res) => {
  res.send(sseTopics);
});

// health-check endpoint
app.get('/', async (req, res) => {
  res.send('OK');
});

// return a 404 if no routes match
app.use((req, res, next) => {
  res.set('Cache-Control', 'private, no-store');
  res.status(404).end('Not found');
});

// starts the server
app.listen(port, () => {
  console.log(`PubSub server running on http://localhost:${port}`);
});

Our custom message format

Each message posted on a server must be in a specific format that out server accepts. Having structure like this allows us to have multiple separated type of events on each topic.

With this we can separate streams and only receive events that belong to the topic.

One example would be, that we have index page and we want to receive messages about new upvotes or new subscribers but we don't want to follow events for other pages. This reduces clutter and overall network. And structure is much nicer and maintanable.

{
  "topic": "sample-topic",
  "event": "sample-event",
  "message": { "name": "John" }
}

Publisher and subscriber clients

Publisher and subscriber in action

You can download the code and follow along.

Publisher

As talked about above publisher is the one that send messages to the broker/server. Message inside the payload can be whatever you want (string, object, array). I would however personally avoid send large chunks of data like blobs and such.

<!DOCTYPE html>
<html lang="en">

  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Publisher</title>
  </head>

  <body>

    <h1>Publisher</h1>

    <fieldset>
      <p>
        <label>Server:</label>
        <input type="text" id="server" value="http://localhost:4000">
      </p>
      <p>
        <label>Topic:</label>
        <input type="text" id="topic" value="sample-topic">
      </p>
      <p>
        <label>Event:</label>
        <input type="text" id="event" value="sample-event">
      </p>
      <p>
        <label>Message:</label>
        <input type="text" id="message" value='{"name": "John"}'>
      </p>
      <p>
        <button type="button" id="button">Publish message to topic</button>
      </p>
    </fieldset>

    <script>

      const button = document.querySelector('#button');
      const server = document.querySelector('#server');
      const topic = document.querySelector('#topic');
      const event = document.querySelector('#event');
      const message = document.querySelector('#message');

      button.addEventListener('click', async (evt) => {
        const req = await fetch(`${server.value}/publish`, {
          method: 'post',
          headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json',
          },
          body: JSON.stringify({
            topic: topic.value,
            event: event.value,
            message: JSON.parse(message.value),
          }),
        });

        const res = await req.json();
        console.log(res);
      });

    </script>

  </body>

</html>

Subscriber

Subscriber is responsible for receiving new messages that come from server via publisher. The code bellow is very rudimentary but works and follows the implementation guidelines for EventSource.

You can use either Developer Tools Console to see incoming messages or you can defer to Debugging with Google Chrome section above to see all EventStream messages.

Don't be alarmed if the subscriber gets disconnected from the server every so often. The code we have here resets connection every 15s but it automatically get reconnected and fetches all messages up to last received message id. This setting can be adjusted in server.js file; search for the maxStreamDuration variable.

<!DOCTYPE html>
<html lang="en">

  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Subscriber</title>
    <link rel="stylesheet" href="style.css">
  </head>

  <body>

    <h1>Subscriber</h1>

    <fieldset>
      <p>
        <label>Server:</label>
        <input type="text" id="server" value="http://localhost:4000">
      </p>
      <p>
        <label>Topic:</label>
        <input type="text" id="topic" value="sample-topic">
      </p>
      <p>
        <label>Event:</label>
        <input type="text" id="event" value="sample-event">
      </p>
      <p>
        <button type="button" id="button">Subscribe to topic</button>
      </p>
    </fieldset>

    <script>

      const button = document.querySelector('#button');
      const server = document.querySelector('#server');
      const topic = document.querySelector('#topic');
      const event = document.querySelector('#event');

      button.addEventListener('click', async (evt) => {

        let es = new EventSource(`${server.value}/stream/${topic.value}`);

        es.addEventListener(event.value, function (evt) {
          console.log(`incoming message`, JSON.parse(evt.data));
        });

        es.addEventListener('open', function (evt) {
          console.log('connected', evt);
        });

        es.addEventListener('error', function (evt) {
          console.log('error', evt);
        });

      });

    </script>

  </body>

</html>

Reading further