Transcript

Understanding the HTTP Request / Response Cycle

Oracle APEX Architecture and Listener Options

 

Session 3 – The HTTP Request Response Cycle

 

>> John:  To begin with, the HTTP request-response cycle. HTTP is a stateless protocol. You issue an HTTP request using typically a post or get method. 

 

I’ll go to the most basic get method here. This shouldn’t be the URL. The URL doesn’t have to come from a browser of course, it can come from pretty much anything, but the URL just doesn’t get request. It contacts the web listener sending it a message requesting a page. The web listener does whatever is necessary and returns the page usually in the form of HTML but not necessarily. 

 

To break down what a URL actually is. You specify a protocol then delimiter colon, slash, slash (://), username delimiter colon (:) and the password and an at symbol (@), host.domain delimiter colon (:), port number delimiter a slash (/), then a path delimited with slash (/), filename with a dot (.) has a delimiter and then extension followed by a question mark (?) and then a set of parameters, parameter=value pass. Your entire URL may have all of these elements. It usually won’t. 

 

For example, a typical URL – I have on the slide just that. FTP – if I were to launch my browser

 

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and give it an FTP, type URL FTP://. I’ll log on. What might my log on be? Perhaps give it a username, Oracle. Password, Oracle. Not working high security here, @ and then my server .168.192.168.56.101 and see what we’ve got. 

 

I didn’t specify the entire URL at that point, just part of it. If we look down here, check my status line in the bottom. If I hover over hello world there, you’re seeing the status line at the bottom of the browser FTP://oracle:oracle is the username and password, @, the host name and then specifying a file helloworld.txt. Clicking on that, we get “message is sent” and back comes the content of the file. 

 

That’s all a URL is. You issue a URL, the client then hangs until the server side returns the page of HTML or whatever it may be. That will be a fairly basic URL. 

 

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In the APEX environment your URL is more likely going to be of this form. 

 

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You’ll be at the form http://machine, then a port typically 8080, then the path probably APEX or it could be something else, and then the / and then f and the question mark (?). What’s that – the f and the question mark? We’re breaking it down again. 

 

An example, I’ll go to http:// and the destination I’m going to go to, just at this point, one of Oracle’s demonstration sites. We could go to 

 

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APEX.oracle.com. I happen to know this listener is listening on port 80, the virtual path is being configured is pls/otn. This is part of the Oracle Technology Network. And then f as a filename. Remember after the filename, we can have a question mark (?) and the parameter string. The parameter string I will give it is p=31517. Machine name, port, path, filename f, parameter string p=31517. Send that URL off and we get back a page for a demonstration APEX application. 

 

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What actually happened when we ran that? The web listener on the other side received the HTTP request and path it. It looks at that path pls/otn and the web listener, whatever it is, maps that onto something else. In the case of my FTP URL, it maps it onto a filename and copied it back. In the case of an APEX Listener, that gets mapped onto a call to run a PL/SQL procedure. Which procedure? 

 

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The procedure called f, which is perhaps not the most helpful procedure but the procedure called f. What is f? 

 

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On any database where APEX being stored – if I select owner and object 5 from DBA objects where object name = f, this does need to be in quotes (“), we see there’s a public synonym and if we investigate further we see that’s mapped onto that procedure. Now we know that f in this state is not a file name, it’s a PL/SQL procedure. And what is it? That’s the procedure. 

 

This is the procedure, the f procedure, this is the heart of APEX. You invoke the procedure with optionally one or more parameters. I pass through one parameter which is p equals (=) and I gave it p=31517 and that is the page identifier. You request a page by name or by number and there are few other extras you’re going to add on to the p arguments which should be subsession identifier, a few other parameters you can drive it with, and in some circumstances you’ll set extra ones. 

 

That’s what an APEX application is. It’s a URL that invokes the f procedure, the f procedure then generates a page of HTML which is sent back to you. 

 

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How is that page generated? That becomes the next major point of discussion.

 

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