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You've already seen some HTTP responses,
but

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let me pull one up just to refresh,
so telnet httpbin.org 80.

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And I'll GET the resource /xml using HTTP

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version 1.1 at the HOST httpbin.org.

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Upon making this request, what comes
up soon thereafter is the response.

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This HTTP response is formatted in mostly
the same way, so let's check it out.

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The first line is the status line, which
starts out exactly the way the request

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line ended, with http/ then
the version number, currently, 1.1.

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Then comes the response code,
then a space, then the status message.

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We could spend an entire course
talking about response codes and

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messages but this is not that course.

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Some that you should be familiar with
are 200 level codes which, in general,

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are success messages.

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Particularly the response code 200
which is a message meaning okay.

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Another you've seen is 404,
which has the message not found.

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Other 400 level codes, in general,

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refer to something that went
wrong in the client's request.

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You also may see a response
code in the 300s,

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which indicates that the resource
requested may have moved.

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But in any case, the rest of the response
contains instructions about where to

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go next, that is, where to redirect.

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Finally, there are 500 level error codes,
which is used for

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indicating that something went wrong on
the server while processing the request.

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Remember, a 500 level code means that
something went wrong on the server.

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Moving on, following the status
line is a section of headers.

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This section is formatted in the same way
as request headers, with a header name,

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then a colon and a space,
followed by the value for that header.

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These headers communicate information
about the response, metadata, so to speak.

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In our example, the headers include
the type of server handling the request,

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nginx in this case.

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The exact date and
time the response was went and

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the type of content that's included
in the body below, in this case, XML.

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When the response includes HTML you'll
see a content type of text/HTML.

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Following the list of headers is a blank
line, and then comes the response body,

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which is also called the payload.

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In this case,
we made a request to the XML resource, so

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the response body or
payload is the actual XML that comes back.

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If we had requested HTML, the payload
would contain the HTML that comes back.

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Now that you've seen the general format
of HTTP, there's one point I'd like

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to drive home in this request response
cycle of an HTTP conversation.

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And that is that HTTP itself is
what's called a stateless protocol.

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Stateless means that there's no
record of previous interactions, and

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each interaction is
processed with only with

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the information that comes with
that particular interaction.

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Put it in different way, that means that
there is nothing inherent to HTTP that

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allows it to remember a previous request
and use it to inform a later one.

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When a web application appears as
if it remembers a previous request,

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like remembering your user name
when you return to Facebook,

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this is a feature that's part of
an application that using HTTP.

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But not part of HTTP itself.

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HTTP messages are like self-destructing
notes that disappear after they're read.

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Okay, you get it, HTTP is stateless.