1 00:00:01,120 --> 00:00:03,120 ‫In this video and the next one 2 00:00:03,120 --> 00:00:06,030 ‫we're gonna dive into yet another important piece 3 00:00:06,030 --> 00:00:08,870 ‫of the Node JS architecture with JS events 4 00:00:08,870 --> 00:00:11,093 ‫and event-driven architecture. 5 00:00:12,370 --> 00:00:15,210 ‫So most of Node's core modules, like the ones 6 00:00:15,210 --> 00:00:19,780 ‫we already used, like HTTP, File System, and Timers 7 00:00:19,780 --> 00:00:23,200 ‫are built around an event-driven architecture, 8 00:00:23,200 --> 00:00:25,830 ‫and we can of course also use this architecture 9 00:00:25,830 --> 00:00:28,720 ‫to our advantage in our own code. 10 00:00:28,720 --> 00:00:31,910 ‫And the concept is actually quite simple. 11 00:00:31,910 --> 00:00:34,780 ‫So, in Node, there are certain objects called 12 00:00:34,780 --> 00:00:37,843 ‫event emitters that emit named events 13 00:00:37,843 --> 00:00:41,520 ‫as soon as something important happens in the app, 14 00:00:41,520 --> 00:00:45,360 ‫like a request hitting server, or a timer expiring, 15 00:00:45,360 --> 00:00:47,660 ‫or a file finishing to read. 16 00:00:47,660 --> 00:00:51,030 ‫These events can then be picked up by event listeners 17 00:00:51,030 --> 00:00:54,380 ‫that we developers set up, which will fire off 18 00:00:54,380 --> 00:00:58,870 ‫callback functions that are attached to each listener, okay. 19 00:00:58,870 --> 00:01:02,150 ‫So again, on one hand, we have event emitters, 20 00:01:02,150 --> 00:01:05,470 ‫and on the other hand event listeners that will react 21 00:01:05,470 --> 00:01:08,940 ‫to emitted events by calling callback function. 22 00:01:08,940 --> 00:01:10,540 ‫Simple, right? 23 00:01:10,540 --> 00:01:12,900 ‫And probably the best way to understand this 24 00:01:12,900 --> 00:01:14,630 ‫is to look at an example. 25 00:01:14,630 --> 00:01:17,760 ‫And so let's briefly understand how Node use 26 00:01:17,760 --> 00:01:21,330 ‫the event-driven architecture to handle server requests 27 00:01:21,330 --> 00:01:25,950 ‫in the HTTP module that we already used in another section. 28 00:01:25,950 --> 00:01:28,600 ‫So when we want to create a server, 29 00:01:28,600 --> 00:01:30,384 ‫we use the Create Server method 30 00:01:30,384 --> 00:01:33,900 ‫and save it to a Server Variable. 31 00:01:33,900 --> 00:01:36,260 ‫This implementation here is a bit different 32 00:01:36,260 --> 00:01:40,270 ‫from what we did before, but it works the exact same way. 33 00:01:40,270 --> 00:01:44,040 ‫Anyway, this server.on method is how we actually 34 00:01:44,040 --> 00:01:49,040 ‫create a listener, and in this case for the "request" event. 35 00:01:49,070 --> 00:01:51,390 ‫So let's say we have our server running, 36 00:01:51,390 --> 00:01:53,770 ‫and a new request is made. 37 00:01:53,770 --> 00:01:57,410 ‫The server acts as an emitter, and will automatically 38 00:01:57,410 --> 00:02:01,220 ‫emit an event called "request" each time that a request 39 00:02:01,220 --> 00:02:02,810 ‫hits the server. 40 00:02:02,810 --> 00:02:04,350 ‫Simple, right? 41 00:02:04,350 --> 00:02:07,100 ‫Then, since we already have a listener set up 42 00:02:07,100 --> 00:02:10,830 ‫for this exact event, the callback function that we attached 43 00:02:10,830 --> 00:02:14,000 ‫to this listener will automatically be called. 44 00:02:14,000 --> 00:02:16,780 ‫And this kind of function we already know from before, 45 00:02:16,780 --> 00:02:20,040 ‫it will simply send some data back to the client. 46 00:02:20,040 --> 00:02:22,970 ‫Now, it works this way because behind the scenes 47 00:02:22,970 --> 00:02:26,074 ‫the server is actually an instance of the Node JS 48 00:02:26,074 --> 00:02:28,980 ‫EventEmitter class, so it inherits all this 49 00:02:28,980 --> 00:02:32,500 ‫event emitting and listening logic from that EventEmitter 50 00:02:32,500 --> 00:02:35,410 ‫class, and don't worry, we're going to use it all 51 00:02:35,410 --> 00:02:38,410 ‫right in the next video, so that you become familiar 52 00:02:38,410 --> 00:02:41,210 ‫with all these logic. 53 00:02:41,210 --> 00:02:43,340 ‫Now, just to finish this lecture, I believe it's 54 00:02:43,340 --> 00:02:46,800 ‫important to mention that this EventEmitter logic 55 00:02:46,800 --> 00:02:49,960 ‫is called the Observer Pattern in Javascript programming 56 00:02:49,960 --> 00:02:53,160 ‫in general, and it's quite a popular pattern with 57 00:02:53,160 --> 00:02:54,740 ‫many used cases. 58 00:02:54,740 --> 00:02:57,090 ‫So the idea is I set there an observer, 59 00:02:57,090 --> 00:03:00,370 ‫in this case the event listener, which keeps waiting, 60 00:03:00,370 --> 00:03:03,450 ‫keeps observing the subject that will eventually 61 00:03:03,450 --> 00:03:06,486 ‫emit the event that the listener is waiting for. 62 00:03:06,486 --> 00:03:09,680 ‫And the opposite of this pattern is simply functions 63 00:03:09,680 --> 00:03:12,370 ‫calling other functions, which is something that 64 00:03:12,370 --> 00:03:14,530 ‫we're more used to actually, right? 65 00:03:14,530 --> 00:03:16,490 ‫But the observer pattern has been designed 66 00:03:16,490 --> 00:03:19,020 ‫to react rather than to call. 67 00:03:19,020 --> 00:03:22,330 ‫And that is because there is a huge benefit of using 68 00:03:22,330 --> 00:03:25,248 ‫this architecture, which is the fact that everything 69 00:03:25,248 --> 00:03:27,420 ‫is more de-coupled. 70 00:03:27,420 --> 00:03:30,340 ‫We don't have, for example, functions from the File System 71 00:03:30,340 --> 00:03:33,610 ‫module calling functions from the HTTP module 72 00:03:33,610 --> 00:03:35,770 ‫because it would be a huge mess. 73 00:03:35,770 --> 00:03:38,690 ‫Instead, these modules are nicely de-coupled 74 00:03:38,690 --> 00:03:41,470 ‫and self-contained, each emitting events that 75 00:03:41,470 --> 00:03:44,380 ‫other functions, even if they come from other modules 76 00:03:44,380 --> 00:03:46,120 ‫can respond to. 77 00:03:46,120 --> 00:03:49,170 ‫Also, using an event-driven architecture makes it 78 00:03:49,170 --> 00:03:52,390 ‫way more straight forward to react multiple times 79 00:03:52,390 --> 00:03:53,760 ‫to the same event. 80 00:03:53,760 --> 00:03:58,210 ‫All we have to do is to set up multiple listeners, right? 81 00:03:58,210 --> 00:04:01,760 ‫Okay and that is Node's event-driven architecture 82 00:04:01,760 --> 00:04:03,020 ‫in a nutshell. 83 00:04:03,020 --> 00:04:06,050 ‫And don't worry if this seems a bit too theoretical, 84 00:04:06,050 --> 00:04:08,730 ‫you will see this logic being used in many situations 85 00:04:08,730 --> 00:04:12,183 ‫throughout the course, starting right in the next video.