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+---
+title: Simple Server-Sent Events based PubSub Server
+url: simple-server-sent-events-based-pubsub-server.html
+date: 2020-03-22T12:00:00+02:00
+type: post
+draft: false
+---
+
+## 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](https://kafka.apache.org/) or [RabbitMQ](https://www.rabbitmq.com/) 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
+
+- provide a simple server that relays messages to all the connected clients,
+- messages can be posted on specific topics,
+- messages get sent via [Server-Sent
+  Events](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events)
+  to all the subscribers.
+
+## 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](/assets/simple-pubsub-server/pubsub-overview.png)
+
+**These are some naive characteristics we want to achieve:**
+
+- producer is publishing messages to subscribe topic,
+- consumer is receiving messages from subscribed topic,
+- servers is also known as Broker,
+- broker does not store messages or tracks success,
+- broker uses
+  [FIFO](https://en.wikipedia.org/wiki/FIFO_(computing_and_electronics)) method
+  for delivering messages,
+- if consumer wants to receive messages from a topic, producer and consumer
+  topics must match,
+- consumer can subscribe to multiple topics,
+- producer can publish to multiple topics,
+- each message has a messageId.
+
+**Known drawbacks:**
+
+- messages will not be stored in a persistent queue or unreceived messages like
+  [DeadLetterQueue](https://en.wikipedia.org/wiki/Dead_letter_queue) so old
+  messages could be lost on server restart,
+- [Server-Sent
+  Events](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events)
+  opens a long-running connection between the client and the server so make sure
+  if your setup is load balanced that the load balancer in this case can have
+  long opened connection,
+- no system moderation due to the dynamic nature of creating queues.
+
+## Server-Sent Events
+
+Read more about it on [official specification
+page](https://html.spec.whatwg.org/multipage/server-sent-events.html).
+
+### Current browser support
+
+![Browser support](/assets/simple-pubsub-server/caniuse.png)
+
+Check
+[https://caniuse.com/#feat=eventsource](https://caniuse.com/#feat=eventsource)
+for latest information about browser support.
+
+### Known issues
+
+- Firefox 52 and below do not support EventSource in web/shared workers
+- In Firefox prior to version 36 server-sent events do not reconnect
+  automatically in case of a connection interrupt (bug)
+- Reportedly, CORS in EventSource is currently supported in Firefox 10+, Opera
+  12+, Chrome 26+, Safari 7.0+.
+- Antivirus software may block the event streaming data chunks.
+
+Source: [https://caniuse.com/#feat=eventsource](https://caniuse.com/#feat=eventsource)
+
+### Message format
+
+The simplest message that can be sent is only with data attribute:
+
+```bash
+data: this is a simple message
+<blank line>
+```
+
+You can send message IDs to be used if the connection is dropped:
+
+```bash
+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):
+
+```bash
+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:
+
+```bash
+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](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_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](/assets/simple-pubsub-server/chrome-debugging.png)
+
+## Server implementation
+
+For the sake of this example we will use [Node.js](https://nodejs.org/en/) with
+[Express](https://expressjs.com) as our router since this is the easiest way to
+get started and we will use already written SSE library for node
+[sse-pubsub](https://www.npmjs.com/package/sse-pubsub) so we don't reinvent the
+wheel.
+
+```bash
+npm init --yes
+
+npm install express
+npm install body-parser
+npm install sse-pubsub
+```
+
+Basic implementation of a server (`server.js`):
+
+```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.
+
+```json
+{
+  "topic": "sample-topic",
+  "event": "sample-event",
+  "message": { "name": "John" }
+}
+```
+
+## Publisher and subscriber clients
+
+### Publisher and subscriber in action
+
+<video src="/assets/simple-pubsub-server/clients.m4v" controls></video>
+
+You can download [the code](../simple-pubsub-server/sse-pubsub-server.zip) 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.
+
+```html
+<!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.
+
+```html
+<!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
+
+- [Using server-sent events](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events)
+- [Using SSE Instead Of WebSockets For Unidirectional Data Flow Over HTTP/2](https://www.smashingmagazine.com/2018/02/sse-websockets-data-flow-http2/)
+- [What is Server-Sent Events?](https://apifriends.com/api-streaming/server-sent-events/)
+- [An HTTP/2 extension for bidirectional messaging communication](https://tools.ietf.org/id/draft-xie-bidirectional-messaging-01.html)
+- [Introduction to HTTP/2](https://developers.google.com/web/fundamentals/performance/http2)
+- [The WebSocket API (WebSockets)](https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API)
+