Event Driven Applications

Overview

Today, we will leverage the ability of Javascript to raise events, monitor events, and perform operations in response to events occurring.

Class Outline

• Code Review
• Warmup question
• Lecture

Learning Objectives

Students will be able to

Describe and Define

• Events
• … specifically, the Node event system
• Event driven architecture

Execute

• Implement the Observer pattern using Publish/Subscribe
• Create a modular, event based system.A

Notes

Event Driven Programming

Nearly everything in the world is “Event Driven”

Humans respond to events billions of times every day. Your eyes react to light. You hit the brakes when the car in front of you slows down. Your skin forms a blister when burned.

Machines can be event driven as well. Self driving cars can stay in their lane by “reading” the road lines in real time. Thermostats constantly turn the heat/air on or off in response to the temperature.

How can we leverage this in a software application?

• Everything in JS is an object
• Most actions in JS are event driven
  • UI Events
  • Express Routes
  • (soon) Model Lifecycle Hooks
  • (later) React … everything
• Now, we harness that power

Emitting Events

I just did something important and I want the whole world to know about it

express-server.js

let SQL = "DELETE FROM sometable WHERE id = $1"
let values = [request.query.id];
client.query(SQL, values)
  .then( results => {
    emit('delete', request.query.id);
    res.send('Record Deleted')
  });

Something happened that I need to care about and do something with

some-other-module.js

// Whenever the "delete" event has been emitted anywhere in my code base
// Run this function
events.on('delete', (data) => {
    sendEmail({
        to: 'admin@here.com',
        subject: 'Someone deleted part of the database',
        body: `Record id: ${data} was removed`
    });
});