1 00:00:01,400 --> 00:00:03,624 ‫So, remember that in the last lecture, 2 00:00:03,624 --> 00:00:06,000 ‫we talked about the Node architecture, 3 00:00:06,000 --> 00:00:07,670 ‫where two fundamental parts 4 00:00:07,670 --> 00:00:10,810 ‫are the thread pool and the event loop. 5 00:00:10,810 --> 00:00:12,630 ‫So in this lecture, you're gonna learn 6 00:00:12,630 --> 00:00:14,619 ‫all about threads and a thread pool. 7 00:00:14,619 --> 00:00:15,771 ‫And in the next one, 8 00:00:15,771 --> 00:00:19,510 ‫we will then dive deep into the event loop. 9 00:00:19,510 --> 00:00:20,883 ‫So, let's get started. 10 00:00:22,230 --> 00:00:25,660 ‫So, first off, when we use Node on a computer, 11 00:00:25,660 --> 00:00:28,210 ‫it means that there is a Node process 12 00:00:28,210 --> 00:00:30,110 ‫running on that computer. 13 00:00:30,110 --> 00:00:33,530 ‫And the process is just a program in execution. 14 00:00:33,530 --> 00:00:35,900 ‫And we already learned that Node.js 15 00:00:35,900 --> 00:00:38,490 ‫is basically a C++ program, 16 00:00:38,490 --> 00:00:42,040 ‫which will therefore start a process when it's running. 17 00:00:42,040 --> 00:00:44,100 ‫This is important because in Node, 18 00:00:44,100 --> 00:00:46,980 ‫we actually have access to a process variable, 19 00:00:46,980 --> 00:00:49,670 ‫which we're gonna use later in this course. 20 00:00:49,670 --> 00:00:52,510 ‫Now in that process, Node.js runs 21 00:00:52,510 --> 00:00:54,860 ‫in a so called, single thread. 22 00:00:54,860 --> 00:00:58,641 ‫And a thread is basically just a sequence of instructions. 23 00:00:58,641 --> 00:01:01,062 ‫But it's not important to deeply understand 24 00:01:01,062 --> 00:01:03,690 ‫what a thread or a process is. 25 00:01:03,690 --> 00:01:05,740 ‫That is more about computer science. 26 00:01:05,740 --> 00:01:08,130 ‫Just imagine a thread as being a box 27 00:01:08,130 --> 00:01:11,483 ‫where our code is executed in a computer's processor. 28 00:01:12,750 --> 00:01:15,273 ‫Now, what is important to understand here, 29 00:01:15,273 --> 00:01:19,100 ‫is the fact that Node runs in just one thread, 30 00:01:19,100 --> 00:01:22,346 ‫which makes it easy to block Node applications. 31 00:01:22,346 --> 00:01:25,273 ‫And this is something that we talked about before, actually. 32 00:01:25,273 --> 00:01:28,650 ‫But it's something really, really important to remember 33 00:01:28,650 --> 00:01:30,586 ‫because this is one of the unique features 34 00:01:30,586 --> 00:01:33,029 ‫that Node.js brings to the table. 35 00:01:33,029 --> 00:01:36,370 ‫So, again, if you run your Node application, 36 00:01:36,370 --> 00:01:38,710 ‫it'll run in just a single thread. 37 00:01:38,710 --> 00:01:41,834 ‫No matter if you have 10 users or 10 million users 38 00:01:41,834 --> 00:01:45,030 ‫accessing your application at the same. 39 00:01:45,030 --> 00:01:47,210 ‫And so you need to be very careful 40 00:01:47,210 --> 00:01:49,610 ‫about not blocking that thread. 41 00:01:49,610 --> 00:01:51,042 ‫And we will of course, take care of that 42 00:01:51,042 --> 00:01:53,723 ‫throughout the project in this course. 43 00:01:54,890 --> 00:01:57,010 ‫Moving on, let's now quickly understand 44 00:01:57,010 --> 00:01:59,700 ‫exactly what happens in a single thread 45 00:01:59,700 --> 00:02:02,040 ‫when you start your Node application. 46 00:02:02,040 --> 00:02:04,730 ‫So when the program is initialized, 47 00:02:04,730 --> 00:02:07,440 ‫all the top level code is executed, 48 00:02:07,440 --> 00:02:09,060 ‫which means all the code that is 49 00:02:09,060 --> 00:02:10,933 ‫not inside any callback function. 50 00:02:12,020 --> 00:02:16,200 ‫Also, all the modules that your app needs are required 51 00:02:16,200 --> 00:02:18,560 ‫and all the callbacks are registered, 52 00:02:18,560 --> 00:02:20,300 ‫just like the ones that we used 53 00:02:20,300 --> 00:02:23,390 ‫for our HTP server in the Node Farm App. 54 00:02:23,390 --> 00:02:24,589 ‫Remember that? 55 00:02:24,589 --> 00:02:29,589 ‫Then after all that, the event loop finally starts running. 56 00:02:29,890 --> 00:02:33,000 ‫And again, more about the event in loop in the next video. 57 00:02:33,000 --> 00:02:35,500 ‫What you need to know for now is that the event loop 58 00:02:35,500 --> 00:02:38,410 ‫is where most of the work is done in your app. 59 00:02:38,410 --> 00:02:42,600 ‫So, it's really the heart of the entire Node architecture. 60 00:02:42,600 --> 00:02:46,640 ‫But here is the catch, some tasks are actually too heavy. 61 00:02:46,640 --> 00:02:50,570 ‫They are too expensive to be executed in the event loop 62 00:02:50,570 --> 00:02:53,510 ‫because they would then block the single thread. 63 00:02:53,510 --> 00:02:56,770 ‫And so, that's where the thread pool comes in, 64 00:02:56,770 --> 00:02:58,610 ‫which just like the event loop, 65 00:02:58,610 --> 00:03:01,670 ‫is provided to Node.js by the libuv library 66 00:03:01,670 --> 00:03:03,890 ‫that we talked about before. 67 00:03:03,890 --> 00:03:07,140 ‫So, the thread pool gives us four additional threads 68 00:03:07,140 --> 00:03:10,560 ‫that are completely separate from the main single thread. 69 00:03:10,560 --> 00:03:14,170 ‫And we can actually configure it up to 128 threads. 70 00:03:14,170 --> 00:03:16,670 ‫But usually, these four are enough. 71 00:03:16,670 --> 00:03:19,630 ‫So these threads together formed a thread pool. 72 00:03:19,630 --> 00:03:21,840 ‫And the event loop can then automatically 73 00:03:21,840 --> 00:03:25,490 ‫offload heavy tasks to the thread pool. 74 00:03:25,490 --> 00:03:28,490 ‫And all this happens automatically behind the scenes. 75 00:03:28,490 --> 00:03:30,680 ‫It's not us developers who decide 76 00:03:30,680 --> 00:03:33,253 ‫what goes to thread pool and what doesn't. 77 00:03:34,780 --> 00:03:37,849 ‫Now, the expensive tasks that do get offloaded 78 00:03:37,849 --> 00:03:41,390 ‫are all operations dealing with files, 79 00:03:41,390 --> 00:03:44,898 ‫everything related to cryptography, like caching passwords, 80 00:03:44,898 --> 00:03:48,860 ‫then all compression stuff, and also DNS lookups, 81 00:03:48,860 --> 00:03:50,850 ‫which basically matches web domains 82 00:03:50,850 --> 00:03:54,020 ‫to their corresponding real IP addresses. 83 00:03:54,020 --> 00:03:55,500 ‫So this is the stuff that would 84 00:03:55,500 --> 00:03:57,870 ‫most easily block the main thread. 85 00:03:57,870 --> 00:04:00,460 ‫And so, Node takes care of automatically 86 00:04:00,460 --> 00:04:02,890 ‫offloading them into the thread pool, 87 00:04:02,890 --> 00:04:05,830 ‫where they don't block our event loop. 88 00:04:05,830 --> 00:04:07,640 ‫And that is the most important thing 89 00:04:07,640 --> 00:04:09,923 ‫that I want you to retain from this video. 90 00:04:11,370 --> 00:04:15,693 ‫So, let's now move on and talk about the event loop itself.