NOTE This file was generated by Descript 1 00:00:00,090 --> 00:00:02,910 Before we start creating websites of our own, it pays to have a 2 00:00:02,910 --> 00:00:07,110 rough idea of what goes on in the background when we type a URL in 3 00:00:07,110 --> 00:00:08,640 the address bar of our web browsers. 4 00:00:08,760 --> 00:00:10,920 It takes a surprising amount of work! 5 00:00:11,460 --> 00:00:14,700 For our browsers, the first step is to find the IP address of 6 00:00:14,700 --> 00:00:15,960 the domain we want to access. 7 00:00:16,660 --> 00:00:20,640 Our browers don't use addresses like google.com or facebook.com directly. 8 00:00:21,000 --> 00:00:24,180 Instead they use IP addresses, which are used to identify 9 00:00:24,390 --> 00:00:26,040 internet connected devices. 10 00:00:26,190 --> 00:00:30,090 When we type the URL for a given site, our browser has to figure out which 11 00:00:30,090 --> 00:00:32,310 IP address corresponds to that site. 12 00:00:32,640 --> 00:00:35,310 And this whole process is called name resolution. 13 00:00:35,935 --> 00:00:40,795 To do this, the browser uses a domain name system server, also called a DNS 14 00:00:40,795 --> 00:00:45,565 server, which checks an index of URLs and their corresponding IP addresses. 15 00:00:45,934 --> 00:00:48,805 This index is a little like a phone book for computers. 16 00:00:49,795 --> 00:00:52,680 For sites you've visited before, the IP address can often be 17 00:00:52,680 --> 00:00:55,280 found in one or a local caches. 18 00:00:55,550 --> 00:00:59,209 Your browser maintains one such cache, your operating system and your 19 00:00:59,209 --> 00:01:01,519 router also maintain caches like this. 20 00:01:01,760 --> 00:01:04,250 These helps speed up the process of name resolution. 21 00:01:04,759 --> 00:01:07,580 But if a matching record isn't found in one of these caches in 22 00:01:07,580 --> 00:01:09,080 your computer or in your router. 23 00:01:09,320 --> 00:01:12,140 Then it's up to your internet service provider to tell your 24 00:01:12,140 --> 00:01:15,199 browser the IP address of the site you're hoping to access. 25 00:01:15,560 --> 00:01:17,449 If a matching IP address is found. 26 00:01:17,670 --> 00:01:20,410 Then your browser will let them to connect to it or rather 27 00:01:20,699 --> 00:01:22,440 connect to the server behind it. 28 00:01:22,980 --> 00:01:25,860 A lot of things happen during this process but we don't have 29 00:01:25,860 --> 00:01:27,350 to worry too much about it. 30 00:01:27,370 --> 00:01:30,360 It's purpose is to ensure that it reliable connection can be 31 00:01:30,360 --> 00:01:32,400 established between the two devices. 32 00:01:32,580 --> 00:01:35,130 Your browser and the server you're connecting to. 33 00:01:35,400 --> 00:01:38,310 If a reliable connection is established, the browser can then 34 00:01:38,310 --> 00:01:40,920 send an HTTP request to the server. 35 00:01:41,100 --> 00:01:43,890 This is where it tells the server what it actually wants from it. 36 00:01:44,340 --> 00:01:47,940 When accessing a website, the browser generally makes GET requests. 37 00:01:48,090 --> 00:01:52,140 These are used to ask for content from the server, such as plain text documents, 38 00:01:52,290 --> 00:01:54,540 images, videos, or anything else. 39 00:01:55,110 --> 00:01:58,950 The request the browser makes also includes a lot of meta information, 40 00:01:59,160 --> 00:02:00,840 such as your language preferences. 41 00:02:01,605 --> 00:02:04,995 In this request, your browser will also send any relevant 42 00:02:05,225 --> 00:02:07,625 HTTP cookies to the server. 43 00:02:08,135 --> 00:02:11,385 Cookies are small pieces of data stored by your browser and they 44 00:02:11,385 --> 00:02:15,215 are frequently used to preserve information between page visits. 45 00:02:15,725 --> 00:02:19,095 Common use cases are things like keeping track of items in a user's 46 00:02:19,115 --> 00:02:23,525 shopping cart or keeping users logged in across several page loads. 47 00:02:23,815 --> 00:02:27,150 A GET request isn't the only type of request we can make. 48 00:02:27,490 --> 00:02:30,160 We'll be covering some other requests types, such as POST 49 00:02:30,360 --> 00:02:31,710 throughout the course as well. 50 00:02:32,130 --> 00:02:35,970 After our browser sends the request for us, it's up to the site server to handle 51 00:02:35,970 --> 00:02:40,860 the request and give us back what we asked for in the form of an HTTP response. 52 00:02:41,310 --> 00:02:43,440 As well as the actual content, the browser requested. 53 00:02:43,740 --> 00:02:47,860 HTTP responses contain lots of other stuff such as the time the response 54 00:02:47,860 --> 00:02:52,020 was sent, information about the content being returned and a lot more. 55 00:02:52,765 --> 00:02:56,424 The server may not always be able to fulfill our request and the response 56 00:02:56,424 --> 00:02:59,875 contains a line to let the browser know the status of the request. 57 00:03:00,084 --> 00:03:03,144 This line includes the HTTP version being used. 58 00:03:03,295 --> 00:03:07,644 A status code for the response and some explanatory phase for the status code. 59 00:03:08,125 --> 00:03:11,635 You're probably familiar with the 404 status code, which indicates 60 00:03:11,635 --> 00:03:13,135 that a resource could not be found. 61 00:03:13,674 --> 00:03:16,435 It usually means that we're trying to access a page which doesn't exist. 62 00:03:16,800 --> 00:03:20,640 Another important status code is 200 which means that the request was 63 00:03:20,640 --> 00:03:22,620 successful and that there were no issues. 64 00:03:23,100 --> 00:03:26,190 A lot of people think 404 is the most common status code but 65 00:03:26,190 --> 00:03:28,560 naturally 200 is much more common. 66 00:03:28,950 --> 00:03:31,950 Assuming the server was able to fulfill the response to view the desire 67 00:03:31,950 --> 00:03:35,579 page the browser will be served a document as part of their response. 68 00:03:35,940 --> 00:03:39,210 Documents served in response to website access requests are 69 00:03:39,210 --> 00:03:42,210 generally written using HTML 5. 70 00:03:42,450 --> 00:03:46,470 This document describes the structure and meaning of the pages' content and 71 00:03:46,470 --> 00:03:49,440 would have contained links to other resources, which will also get fetched 72 00:03:49,440 --> 00:03:51,630 by the browser with separate requests. 73 00:03:51,930 --> 00:03:55,230 That can include style sheets, which describe the appearance of the pages' 74 00:03:55,230 --> 00:03:59,820 content, images, video files, fonts, and potentially JavaScript files. 75 00:04:00,660 --> 00:04:05,010 For each of these, the browser has to send additional requests to the server, just 76 00:04:05,010 --> 00:04:07,260 like it did for the HTML document itself. 77 00:04:07,710 --> 00:04:11,820 And while these additional resources download, the browser begins constructing 78 00:04:11,820 --> 00:04:16,140 a model of the site structure using the HTML code and starts painting 79 00:04:16,140 --> 00:04:17,909 content to the browser window. 80 00:04:18,420 --> 00:04:21,600 Once the painting is complete and all of the sites resources 81 00:04:21,659 --> 00:04:24,900 have finished downloading, we're left with a complete webpage. 82 00:04:25,290 --> 00:04:27,990 In this course, we'll focus on steps three and four. 83 00:04:28,110 --> 00:04:31,230 We'll write client-side code to interact with servers through a web 84 00:04:31,230 --> 00:04:35,340 browser, and we'll also write server side code to handle those requests. 85 00:04:35,670 --> 00:04:38,340 Throughout the next couple of sections we'll take a closer look 86 00:04:38,490 --> 00:04:41,280 at the technologies involved in displaying a webpage to the user. 87 00:04:41,670 --> 00:04:44,460 Then we'll look into writing server side code to handle user 88 00:04:44,460 --> 00:04:46,860 requests with Python and Flask. 89 00:04:47,070 --> 00:04:48,090 Thank you guys for watching. 90 00:04:48,300 --> 00:04:49,680 I'll see you in the next one.