1 00:00:01,150 --> 00:00:02,580 ‫Welcome back. 2 00:00:02,580 --> 00:00:05,000 ‫Let's now learn all about the event loop, 3 00:00:05,000 --> 00:00:08,150 ‫which is the heart of the Node.js architecture. 4 00:00:08,150 --> 00:00:10,580 ‫And this is probably the most important lecture 5 00:00:10,580 --> 00:00:13,500 ‫in this section, so make sure you really understand 6 00:00:13,500 --> 00:00:16,510 ‫everything that I show you during this video. 7 00:00:16,510 --> 00:00:17,813 ‫So, let's get started. 8 00:00:18,800 --> 00:00:21,880 ‫So, here is a diagram similar to the last lecture 9 00:00:21,880 --> 00:00:25,170 ‫so that we know exactly what we're talking about here. 10 00:00:25,170 --> 00:00:28,240 ‫So, we're still in a Node process in the single thread 11 00:00:28,240 --> 00:00:30,810 ‫where the event loop runs, all right? 12 00:00:30,810 --> 00:00:32,530 ‫Now, the first thing that you need 13 00:00:32,530 --> 00:00:34,750 ‫to know is that the event loop is 14 00:00:34,750 --> 00:00:36,350 ‫where all the application code 15 00:00:36,350 --> 00:00:39,550 ‫that is inside callback functions is executed. 16 00:00:39,550 --> 00:00:43,640 ‫So, basically, all code that is not top level code will run 17 00:00:43,640 --> 00:00:45,250 ‫in the event loop. 18 00:00:45,250 --> 00:00:48,450 ‫Some parts might get offloaded to the thread pool 19 00:00:48,450 --> 00:00:50,140 ‫as we saw in the last lecture, 20 00:00:50,140 --> 00:00:53,430 ‫but it's the event loop that takes care of all this. 21 00:00:53,430 --> 00:00:56,370 ‫As I said before, it really is the heart 22 00:00:56,370 --> 00:00:58,360 ‫of the Node architecture. 23 00:00:58,360 --> 00:01:01,660 ‫Okay, now, as I mentioned many times in the first part 24 00:01:01,660 --> 00:01:04,300 ‫of the course, Node.js is all built 25 00:01:04,300 --> 00:01:05,860 ‫around callback functions. 26 00:01:05,860 --> 00:01:08,160 ‫So, functions that are called as soon 27 00:01:08,160 --> 00:01:11,700 ‫as some work is finished some time in the future. 28 00:01:11,700 --> 00:01:12,960 ‫Remember that? 29 00:01:12,960 --> 00:01:15,310 ‫And it works this way because Node 30 00:01:15,310 --> 00:01:17,750 ‫uses an event-triggered architecture, 31 00:01:17,750 --> 00:01:20,480 ‫which is something that we're gonna talk about in one 32 00:01:20,480 --> 00:01:22,100 ‫of the next videos. 33 00:01:22,100 --> 00:01:24,180 ‫But what you need to know for now is 34 00:01:24,180 --> 00:01:26,020 ‫that things like our application 35 00:01:26,020 --> 00:01:31,020 ‫receiving an HTTP request on our server or a timer expiring 36 00:01:31,130 --> 00:01:34,860 ‫or a file finishing to read, all these will emit events 37 00:01:34,860 --> 00:01:37,350 ‫as soon as they are done with their work, 38 00:01:37,350 --> 00:01:40,150 ‫and our event loop will then pick up these events 39 00:01:40,150 --> 00:01:41,880 ‫and call the callback functions 40 00:01:41,880 --> 00:01:44,750 ‫that are associated with each event. 41 00:01:44,750 --> 00:01:46,520 ‫Okay, make sense? 42 00:01:46,520 --> 00:01:49,680 ‫So, again, the event loop receives events 43 00:01:49,680 --> 00:01:51,800 ‫each time something important happens, 44 00:01:51,800 --> 00:01:54,440 ‫and will then call the necessary callbacks 45 00:01:54,440 --> 00:01:56,443 ‫such as we define in our code. 46 00:01:57,300 --> 00:01:59,690 ‫So, in summary, it's usually said 47 00:01:59,690 --> 00:02:02,600 ‫that the event loop does the orchestration, 48 00:02:02,600 --> 00:02:05,310 ‫which simply means that it receives events, 49 00:02:05,310 --> 00:02:07,330 ‫calls their callback functions, 50 00:02:07,330 --> 00:02:11,290 ‫and offloads the more expensive tasks to the thread pool. 51 00:02:11,290 --> 00:02:14,670 ‫Now, how does all this actually work behind the scenes? 52 00:02:14,670 --> 00:02:17,960 ‫In what order are these callbacks executed? 53 00:02:17,960 --> 00:02:20,543 ‫Well, that is what we're gonna find out next. 54 00:02:21,460 --> 00:02:24,630 ‫So, remember, when we start our Node application, 55 00:02:24,630 --> 00:02:27,430 ‫the event loop starts running right away. 56 00:02:27,430 --> 00:02:29,720 ‫Now, the event loop has multiple phases, 57 00:02:29,720 --> 00:02:32,330 ‫and each phase has a callback queue, 58 00:02:32,330 --> 00:02:35,060 ‫which are the callbacks coming from the events 59 00:02:35,060 --> 00:02:36,690 ‫that the event loop receives, 60 00:02:36,690 --> 00:02:39,830 ‫just as we talked about in the last slide. 61 00:02:39,830 --> 00:02:41,830 ‫Now, in some places you will read 62 00:02:41,830 --> 00:02:45,520 ‫that there is only one callback queue or one event queue, 63 00:02:45,520 --> 00:02:49,290 ‫but in fact, as I said, the event loop has many phases 64 00:02:49,290 --> 00:02:52,290 ‫where each phase has its own callback queue. 65 00:02:52,290 --> 00:02:56,090 ‫So, let's now take a look at the four most important phases. 66 00:02:56,090 --> 00:02:58,090 ‫There are one or two other phases 67 00:02:58,090 --> 00:02:59,890 ‫that are used internally by Node, 68 00:02:59,890 --> 00:03:01,360 ‫but these are not that important 69 00:03:01,360 --> 00:03:03,530 ‫and I'm not gonna talk about them. 70 00:03:03,530 --> 00:03:05,230 ‫So, the first phase takes care 71 00:03:05,230 --> 00:03:07,860 ‫of callbacks of expired timers, 72 00:03:07,860 --> 00:03:10,880 ‫for example, from the setTimeout() function. 73 00:03:10,880 --> 00:03:13,210 ‫So, if there are callback functions from timers 74 00:03:13,210 --> 00:03:15,750 ‫that just expired, these are the first ones 75 00:03:15,750 --> 00:03:18,010 ‫to be processed by the event loop. 76 00:03:18,010 --> 00:03:21,040 ‫If a timer expires later during the time when one 77 00:03:21,040 --> 00:03:23,370 ‫of the other phases are being processed, 78 00:03:23,370 --> 00:03:26,860 ‫well, then the callback of that timer will only be called 79 00:03:26,860 --> 00:03:30,450 ‫as soon as the event loop comes back to this first phase. 80 00:03:30,450 --> 00:03:31,580 ‫Make sense? 81 00:03:31,580 --> 00:03:34,520 ‫And it works like this in all four phases. 82 00:03:34,520 --> 00:03:37,250 ‫So, callbacks in each queue are processed one 83 00:03:37,250 --> 00:03:40,810 ‫by one until there are no ones left in the queue, 84 00:03:40,810 --> 00:03:44,630 ‫and only then, the event loop will enter the next phase. 85 00:03:44,630 --> 00:03:49,180 ‫Next up, we have I/O polling and execution of I/O callbacks. 86 00:03:49,180 --> 00:03:52,840 ‫And again, remember that I/O stands for input/output. 87 00:03:52,840 --> 00:03:56,040 ‫So, polling basically means looking for new I/O events 88 00:03:56,040 --> 00:03:58,060 ‫that are ready to be processed 89 00:03:58,060 --> 00:04:00,890 ‫and putting them into the callback queue. 90 00:04:00,890 --> 00:04:04,110 ‫And remember that in the context of a Node application, 91 00:04:04,110 --> 00:04:08,710 ‫I/O means mainly stuff like networking and file access, 92 00:04:08,710 --> 00:04:12,150 ‫and so, it's in this phase where probably 99% 93 00:04:12,150 --> 00:04:14,270 ‫of our code gets executed, 94 00:04:14,270 --> 00:04:16,640 ‫simply because in a typical Node app, 95 00:04:16,640 --> 00:04:20,500 ‫the bulk of what we need to do is related to networking 96 00:04:20,500 --> 00:04:23,170 ‫and also, file accessing. 97 00:04:23,170 --> 00:04:26,490 ‫The next phase is for setImmediate callbacks, 98 00:04:26,490 --> 00:04:29,250 ‫and setImmediate is a special kind of timer 99 00:04:29,250 --> 00:04:32,810 ‫that we can use if we want to process callbacks immediately 100 00:04:32,810 --> 00:04:36,130 ‫after the I/O polling and execution phase, 101 00:04:36,130 --> 00:04:39,173 ‫which can be important in some more advanced use cases. 102 00:04:40,110 --> 00:04:40,943 ‫All right. 103 00:04:40,943 --> 00:04:44,940 ‫And finally, the fourth phase is for close callbacks, 104 00:04:44,940 --> 00:04:47,530 ‫which are, again, not that important for us, 105 00:04:47,530 --> 00:04:50,820 ‫but I put this here anyway for the sake of completeness. 106 00:04:50,820 --> 00:04:54,450 ‫Basically, in this phase, all close events are processed, 107 00:04:54,450 --> 00:04:58,920 ‫for example, for when a web server or a WebSocket shut down. 108 00:04:58,920 --> 00:05:01,920 ‫So, these are the four phases in the event loop, 109 00:05:01,920 --> 00:05:05,400 ‫but besides these four callback queues that we just saw, 110 00:05:05,400 --> 00:05:08,330 ‫there are actually also two other queues, 111 00:05:08,330 --> 00:05:11,600 ‫the nextTick() queue and the other microtasks queue, 112 00:05:11,600 --> 00:05:14,780 ‫which is mainly for resolved promises. 113 00:05:14,780 --> 00:05:16,890 ‫If you're not familiar with promises, 114 00:05:16,890 --> 00:05:20,240 ‫we will talk about them a bit in a later section. 115 00:05:20,240 --> 00:05:22,620 ‫Anyway, if there are any callbacks in one 116 00:05:22,620 --> 00:05:24,750 ‫of these two queues to be processed, 117 00:05:24,750 --> 00:05:27,840 ‫they will be executed right after the current phase 118 00:05:27,840 --> 00:05:30,250 ‫of the event loop finishes instead of waiting 119 00:05:30,250 --> 00:05:32,380 ‫for the entire loop to finish. 120 00:05:32,380 --> 00:05:33,370 ‫Okay? 121 00:05:33,370 --> 00:05:36,680 ‫So, in other words, after each of these four phases, 122 00:05:36,680 --> 00:05:40,340 ‫if there are any callbacks in these two special queues, 123 00:05:40,340 --> 00:05:42,880 ‫they will be executed right away. 124 00:05:42,880 --> 00:05:46,030 ‫Now, for example, imagine that a promise resolves 125 00:05:46,030 --> 00:05:49,730 ‫and returns some data from an API call while the callback 126 00:05:49,730 --> 00:05:52,690 ‫of an expired timer is running. 127 00:05:52,690 --> 00:05:56,050 ‫So, in this case, the promise callback will be executed 128 00:05:56,050 --> 00:05:59,230 ‫right after the one from the timer finishes. 129 00:05:59,230 --> 00:06:00,490 ‫Okay? 130 00:06:00,490 --> 00:06:03,480 ‫And the same logic also applies to the nextTick() queue, 131 00:06:03,480 --> 00:06:05,290 ‫which we didn't talk about yet. 132 00:06:05,290 --> 00:06:09,060 ‫So, basically, process the nextTick() is a function 133 00:06:09,060 --> 00:06:11,610 ‫that we can use when we really, really need 134 00:06:11,610 --> 00:06:13,740 ‫to execute a certain callback 135 00:06:13,740 --> 00:06:16,290 ‫right after the current event loop phase. 136 00:06:16,290 --> 00:06:18,810 ‫It's a bit similar to setImmediate, 137 00:06:18,810 --> 00:06:21,540 ‫with the difference that setImmediate only runs 138 00:06:21,540 --> 00:06:23,400 ‫after the I/O callback phase. 139 00:06:23,400 --> 00:06:24,600 ‫What is similar, though, 140 00:06:24,600 --> 00:06:27,930 ‫is that both are for really advanced use cases, 141 00:06:27,930 --> 00:06:30,080 ‫and we're probably not even gonna need them 142 00:06:30,080 --> 00:06:31,580 ‫throughout this course. 143 00:06:31,580 --> 00:06:34,660 ‫But anyway, I wanted to include this more complex stuff here 144 00:06:34,660 --> 00:06:36,700 ‫as well so that you have the tools 145 00:06:36,700 --> 00:06:38,940 ‫that you need if you really need 146 00:06:38,940 --> 00:06:42,980 ‫to dig deep into Node.js if you want to. 147 00:06:42,980 --> 00:06:46,210 ‫All right, and with that, we actually finished one tick 148 00:06:46,210 --> 00:06:50,360 ‫of the event loop, and a tick is basically just one cycle 149 00:06:50,360 --> 00:06:51,580 ‫in this loop. 150 00:06:51,580 --> 00:06:54,840 ‫So, now it's time to decide whether the loop should continue 151 00:06:54,840 --> 00:06:58,520 ‫to the next tick or if the program should exit. 152 00:06:58,520 --> 00:07:00,720 ‫And how does Node do that? 153 00:07:00,720 --> 00:07:02,310 ‫Well, it's very simple. 154 00:07:02,310 --> 00:07:05,100 ‫Node simply checks whether there are any timers 155 00:07:05,100 --> 00:07:08,440 ‫or I/O tasks that are still running in the background, 156 00:07:08,440 --> 00:07:12,180 ‫and if there aren't any, then it will exit the application. 157 00:07:12,180 --> 00:07:15,430 ‫But if there are any pending timers or I/O tasks, 158 00:07:15,430 --> 00:07:17,870 ‫well, then it will continue running the event loop 159 00:07:17,870 --> 00:07:20,500 ‫and go straight to the next tick. 160 00:07:20,500 --> 00:07:22,030 ‫So, for example, when we're listening 161 00:07:22,030 --> 00:07:24,740 ‫for incoming HTTP requests like we did 162 00:07:24,740 --> 00:07:27,770 ‫in our Node farm project in a previous section, 163 00:07:27,770 --> 00:07:30,600 ‫we were basically running an I/O task, 164 00:07:30,600 --> 00:07:32,320 ‫and that is why the event loop, 165 00:07:32,320 --> 00:07:34,640 ‫and therefore, Node.js, keep running 166 00:07:34,640 --> 00:07:38,600 ‫and keep listening for new HTTP requests coming in 167 00:07:38,600 --> 00:07:41,920 ‫instead of just exiting the application. 168 00:07:41,920 --> 00:07:44,450 ‫Also, when we're writing or reading a file 169 00:07:44,450 --> 00:07:47,480 ‫in the background, that's also an I/O task, 170 00:07:47,480 --> 00:07:50,210 ‫and so, it makes sense that the app doesn't exit 171 00:07:50,210 --> 00:07:53,260 ‫while it's working with that file, right? 172 00:07:53,260 --> 00:07:55,780 ‫Okay, and this is basically what you should know 173 00:07:55,780 --> 00:07:57,930 ‫about the Node.js event loop. 174 00:07:57,930 --> 00:08:00,260 ‫If you need even more detail than this, 175 00:08:00,260 --> 00:08:01,760 ‫well, you can always try to read 176 00:08:01,760 --> 00:08:04,860 ‫the official Node documentation, which should be quite easy 177 00:08:04,860 --> 00:08:07,060 ‫for you to understand at this point now 178 00:08:07,060 --> 00:08:10,570 ‫that you already understand most of the event loop anyway. 179 00:08:10,570 --> 00:08:12,830 ‫And I just wanna emphasize that it's really, 180 00:08:12,830 --> 00:08:15,940 ‫really important for you to correctly understand 181 00:08:15,940 --> 00:08:18,280 ‫the event loop so that you can write your own 182 00:08:18,280 --> 00:08:21,390 ‫performing code and also debug your own code 183 00:08:21,390 --> 00:08:24,173 ‫when something goes wrong in an unexpected way. 184 00:08:25,720 --> 00:08:27,770 ‫And now, just to finish, let's review some 185 00:08:27,770 --> 00:08:29,810 ‫of the stuff we talked about here. 186 00:08:29,810 --> 00:08:32,490 ‫So, in a nutshell, the most important thing 187 00:08:32,490 --> 00:08:34,620 ‫that I want you to understand from this lecture, 188 00:08:34,620 --> 00:08:36,740 ‫and maybe from this entire course, 189 00:08:36,740 --> 00:08:38,560 ‫is that event loop is what makes 190 00:08:38,560 --> 00:08:41,630 ‫asynchronous programming possible in Node.js, 191 00:08:41,630 --> 00:08:45,190 ‫making it the most important feature in Node's design 192 00:08:45,190 --> 00:08:47,580 ‫and making Node.js completely different 193 00:08:47,580 --> 00:08:49,050 ‫from other platforms. 194 00:08:49,050 --> 00:08:51,600 ‫It takes care of all incoming events 195 00:08:51,600 --> 00:08:55,120 ‫and performs orchestration by offloading heavier tasks 196 00:08:55,120 --> 00:08:58,840 ‫into the thread pool, and doing the most simple work itself. 197 00:08:58,840 --> 00:09:01,520 ‫Also, remember that we need the event loop 198 00:09:01,520 --> 00:09:05,260 ‫because in Node.js everything works in one single thread, 199 00:09:05,260 --> 00:09:07,260 ‫and so, you can have thousands or millions 200 00:09:07,260 --> 00:09:11,230 ‫of users accessing the same thread at the same time. 201 00:09:11,230 --> 00:09:13,690 ‫This makes Node so lightweight and scalable, 202 00:09:13,690 --> 00:09:16,290 ‫but at the same time, it comes with the danger 203 00:09:16,290 --> 00:09:17,930 ‫of blocking our single thread, 204 00:09:17,930 --> 00:09:20,140 ‫which would make the entire app slow 205 00:09:20,140 --> 00:09:25,140 ‫or even stop for all your users accessing the app, right? 206 00:09:25,340 --> 00:09:28,110 ‫Now, in other languages like PHP running on 207 00:09:28,110 --> 00:09:31,300 ‫an Apache server, basically, a new thread is created 208 00:09:31,300 --> 00:09:35,200 ‫for each new user, which is way more resource-intensive. 209 00:09:35,200 --> 00:09:37,180 ‫But on the other hand, there is no danger 210 00:09:37,180 --> 00:09:39,160 ‫of blocking, right? 211 00:09:39,160 --> 00:09:41,430 ‫So, that whole model makes it a bit easier 212 00:09:41,430 --> 00:09:44,340 ‫to use PHP for beginners, but of course, 213 00:09:44,340 --> 00:09:46,540 ‫it comes with its own disadvantages, 214 00:09:46,540 --> 00:09:48,890 ‫which I'm not gonna go into at this point. 215 00:09:48,890 --> 00:09:50,690 ‫Anyway, let me again remind you 216 00:09:50,690 --> 00:09:54,860 ‫that it's your responsibility to not block the event loop, 217 00:09:54,860 --> 00:09:58,150 ‫and so here's a couple of guidelines for that. 218 00:09:58,150 --> 00:10:00,490 ‫First off, don't use the sync versions 219 00:10:00,490 --> 00:10:02,850 ‫of functions in the fs, crypto, 220 00:10:02,850 --> 00:10:06,450 ‫or zlib modules in your callback functions, okay? 221 00:10:06,450 --> 00:10:09,210 ‫So, in our first project, we actually did use 222 00:10:09,210 --> 00:10:12,610 ‫the synchronous version, but it was in the top level code, 223 00:10:12,610 --> 00:10:15,000 ‫so outside of any callback. 224 00:10:15,000 --> 00:10:18,520 ‫And since that code runs before the event loop even starts, 225 00:10:18,520 --> 00:10:22,200 ‫well, it's not problem to use the synchronous version there. 226 00:10:22,200 --> 00:10:24,910 ‫Also, and this is probably pretty obviously, 227 00:10:24,910 --> 00:10:28,140 ‫don't perform very complex calculations in the event loop. 228 00:10:28,140 --> 00:10:29,730 ‫So, stuff like crunching millions 229 00:10:29,730 --> 00:10:33,890 ‫of numbers in loops inside of loops, or something like that. 230 00:10:33,890 --> 00:10:37,550 ‫Next, be careful with JSON in very large objects 231 00:10:37,550 --> 00:10:40,480 ‫because at some point, it can start to take a long time 232 00:10:40,480 --> 00:10:43,440 ‫to parse, or to stringify, JSON. 233 00:10:43,440 --> 00:10:47,170 ‫And finally, don't use all too complex regular expressions, 234 00:10:47,170 --> 00:10:49,840 ‫for example, with multiple nested quantifiers 235 00:10:49,840 --> 00:10:52,130 ‫or back references, because again, 236 00:10:52,130 --> 00:10:54,810 ‫they can take longer than expected. 237 00:10:54,810 --> 00:10:56,680 ‫These are, of course, just a couple 238 00:10:56,680 --> 00:10:59,700 ‫of high level guidelines, but they will get you started 239 00:10:59,700 --> 00:11:00,803 ‫on the right path. 240 00:11:01,650 --> 00:11:03,490 ‫Now, there are some potential solutions 241 00:11:03,490 --> 00:11:06,390 ‫to these blocking problems, like manually offloading 242 00:11:06,390 --> 00:11:09,750 ‫to the thread pool or using child processes, 243 00:11:09,750 --> 00:11:12,070 ‫and we might talk about this by the end of the course 244 00:11:12,070 --> 00:11:14,370 ‫or some time in the future, but for now, 245 00:11:14,370 --> 00:11:17,460 ‫it's important that you understand and follow this advice 246 00:11:17,460 --> 00:11:20,110 ‫of really not blocking the event loop. 247 00:11:20,110 --> 00:11:21,600 ‫All right. 248 00:11:21,600 --> 00:11:24,520 ‫Next up, I'm gonna give you a small example 249 00:11:24,520 --> 00:11:25,990 ‫to show you some of the stuff 250 00:11:25,990 --> 00:11:27,640 ‫that we talked about in practice.