You've already seen some HTTP responses, but 0:00 let me pull one up just to refresh, so telnet httpbin.org 80. 0:04 And I'll GET the resource /xml using HTTP 0:09 version 1.1 at the HOST httpbin.org. 0:13 Upon making this request, what comes up soon thereafter is the response. 0:18 This HTTP response is formatted in mostly the same way, so let's check it out. 0:23 The first line is the status line, which starts out exactly the way the request 0:29 line ended, with http/ then the version number, currently, 1.1. 0:33 Then comes the response code, then a space, then the status message. 0:37 We could spend an entire course talking about response codes and 0:43 messages but this is not that course. 0:47 Some that you should be familiar with are 200 level codes which, in general, 0:49 are success messages. 0:54 Particularly the response code 200 which is a message meaning okay. 0:55 Another you've seen is 404, which has the message not found. 1:02 Other 400 level codes, in general, 1:07 refer to something that went wrong in the client's request. 1:10 You also may see a response code in the 300s, 1:15 which indicates that the resource requested may have moved. 1:18 But in any case, the rest of the response contains instructions about where to 1:21 go next, that is, where to redirect. 1:26 Finally, there are 500 level error codes, which is used for 1:29 indicating that something went wrong on the server while processing the request. 1:33 Remember, a 500 level code means that something went wrong on the server. 1:38 Moving on, following the status line is a section of headers. 1:43 This section is formatted in the same way as request headers, with a header name, 1:48 then a colon and a space, followed by the value for that header. 1:52 These headers communicate information about the response, metadata, so to speak. 1:56 In our example, the headers include the type of server handling the request, 2:01 nginx in this case. 2:05 The exact date and time the response was went and 2:07 the type of content that's included in the body below, in this case, XML. 2:10 When the response includes HTML you'll see a content type of text/HTML. 2:15 Following the list of headers is a blank line, and then comes the response body, 2:21 which is also called the payload. 2:25 In this case, we made a request to the XML resource, so 2:27 the response body or payload is the actual XML that comes back. 2:31 If we had requested HTML, the payload would contain the HTML that comes back. 2:36 Now that you've seen the general format of HTTP, there's one point I'd like 2:42 to drive home in this request response cycle of an HTTP conversation. 2:46 And that is that HTTP itself is what's called a stateless protocol. 2:50 Stateless means that there's no record of previous interactions, and 2:56 each interaction is processed with only with 3:01 the information that comes with that particular interaction. 3:03 Put it in different way, that means that there is nothing adherent at HTTP that 3:07 allows it to remember a previous request and use it to inform a later one. 3:13 When a web application appears as if it remembers a previous request, 3:18 like remembering your user name when you return to Facebook, 3:22 this is a feature that's part of an application that using HTTP. 3:25 But not part of HTTP itself. 3:29 HTTP messages are like self-destructing notes that disappear after they're read. 3:33 Okay, you get it, HTTP is stateless. 3:38