HTML Form Processing

Weasel pp 247-270 (reference)Kookaburra pp 199-222 (read)

Web Client-Server Model

http response

http requestSpecify the URL of the “file” you want.

Send back the “file” to the client.

What is in an http request?

• Obviously, the URL of the “file” you want.• Anything else?

• Note: A URL not only helps you find the server, but it also tells the server what file you want.

What is in an http request?

• Information about the client browser– Firefox, IE, Safari, etc.– Browser Version– Platform (Windows, Unix, MacOS, etc.)

• Also the URL itself can include variables– http://www.site.com/makepage.php?pageid=48&display=wide

URL Variables

http://www.site.com/makepage.php?pageid=48&display=wide

• ? Defines the URLs query string• pageid and display are query variables• & is a delimiter, i.e., put between query

variables

Can an http request send anything else?

Yes,• It can send form variables to the server.• HTML includes elements for creating forms– Text boxes <input type=“text” />– Buttons <input type=“submit” />– Check boxes– Etc.

• The data entered into these form elements can be sent to the server using an http request.

HTML Forms

• Consider the following HTML code<form action=“script.php”> Enter Email: <input type=“text” name=“email”/> <br /><input type=“submit” /></form>

• When you click the submit button, the browser– requests the file “script.php”– and sends the server the data that user enters

email = “myname@place.org”

Summary: Data sent by client

1. URL itself2. URL variables 3. Information about the client browser4. Any form data

What does the server do with the data?

The Server 1. receives an http request

2. parses URL and message body of the http request

3. puts the data into variables (associative arrays) that can be used by any scripting language

Variables provided by the server.

• $_GET – stores all the URL variables

• $_POST – stores all the Form variables

• $_SERVER – stores server information including info about the client browsers that just made the request.

Example

• http://www.site.com/add.php?xval=5&yval=7

• add.php<?php

$xValue = $_GET['xval'];$yValue = $_GET['yval'];

$sum = $xValue + $yValue;

echo $xValue,' + ', $yValue, ' = ', $sum;

?>

5+ 7 = 12

Example

<?php

$xValue = $_GET['xval'];$yValue = $_GET['yval'];

$sum = $xValue + $yValue;

echo $xValue,' + ', $yValue, ' = ', $sum;

?>

<form action="add.php" method="get"><input name="xval" /> +<input name="yval“ /><input type="submit" value="equals" />

</form>

4 5

4 + 5 = 9

Get vs. Post

<?php

$sum = $_POST['xval'] + $_POST['yval']

echo $_POST['xval'],' + ', $_POST['yval'], ' = ', $sum;

?>

<form action="add.php" method=“post"><input name="xval" /> +<input name="yval“ /><input type="submit" value="equals" />

</form>

4 5

4 + 5 = 9

GET vs. POST

GET – First way• Browser adds the form

variables to the URL

• Then sends the http request• Form variables are visible in

the URL• Browsers limit the length a

URL• Limit for IE is 2,048

characters

POST – Improved way• Browser adds the form

variables to the message body of the http request

• Then sends the http request• Must be used when sending

non-text data, binary files, images, etc.

• Must be used when sending long data (> 2KB)

Details

Client-side: The web browser • extracts the form data, i.e.,

the values the user types, and the variables, i.e., the names of the form elements.

• places variables and values in either – the URL (if method=“get”) or – the body of the http request

(if method=“post”).

Server-side: The web server• receives the http request.• Parses the header of the

http request which indicates if there are form variables.

• extracts the variables• places them in a data

structure, and• passes the structure to the

requested script.

One more look

<?php

$sum = $_POST['xval'] + $_POST['yval']

echo $_POST['xval'],' + ', $_POST['yval'], ' = ', $sum;

?>

<form action="add.php" method=“post"><input name="xval" /> +<input name="yval“ /><input type="submit" value="equals" />

</form>

4 5

4 + 5 = 9

Quirks of Web-page Based Interfaces

• Ever notice the delay in submitting a form?• Ever notice that forms tend to always send

back a confirmation message, even when you don’t need one?

• Ever notice the page entirely refreshes whenever a button is clicked?

Mechanics of Web-based InterfacesUser requests a FORM (add.html)

Sends the FORM (add.html)

User fills out the FORM and clicks submit

Send FORM variables and requests action “add.php”Time

Sends back output of “add.php” could be a new FORM

Server receives the FORM variables

Server executes “add.php”

User fills out the new FORM and clicks submit

Send FORM variables and requests new action

Synchronous Communication

• A Phone conversation is synchronous– Generally, people wait for an answer before asking the next

question– You talk, I talk, you talk, I talk, …

• Web analogy– When submitting a form, you have to wait for a response before

you can move to the next action.• This is why web-interfaces are sometimes quirky:

– We are used to asynchronous interaction.– Web client-server interaction is actually synchronous.– Note many “experts” mistakenly consider web client-server

interaction to be asynchronous. They are wrong!

Asynchronous Communication

• Email is a classic example of asynchronous communication– You do NOT have to wait for an email reply to send

out other emails. • Web client-server analogy– You do NOT have to wait for an http response to

send out another request, even to the same server• When you look at it this way, web client-server

interaction seems asynchronous, but it is not.

Why it appears Asynchronous

• Consider opening multiple browser windows and connecting to different servers

• Consider slow servers

• The response order is NOT always the same as the request order.

• This appears to be asynchronous communication like email, but….

But…

• You can get an email out of the blue.• A software application can send a message out

of the blue.• Surprisingly, a web browser cannot get an http

response without first making a request.• Pop-ups are actually initiated by spyware or

plug-ins that make requests without the user’s explicit action.

http is Synchronous

• A server won’t do anything unless the client makes a request.

• Then, the only thing the server can do is send back exactly one response.

• Truly asynchronous communication would mean– The server could initiate communication when

appropriate.– A single request from a client could trigger more

than one response from the server at any time

Confused? Here is a Summary

• The general communication between web client and web server has an asynchronous feel– because clients and servers support multiple

simultaneous requests/responses• However, http is a synchronous protocol– Requests from clients drive all the responses from

servers• Later in the semester, we’ll learn about AJAX, which

is a way to support true asynchronous communication between client and server.

Still Confused?

• A phone conversation is synchronous– You talk, I talk, you talk, I talk.

• Surfing the web is a synchronous conversation– Click a link (request), get a page (response), click a link (request),

get a page (response)• Web clients and servers support multiple simultaneous

conversations.– Like having multiple mouths and sets of ears– The conversations feel asynchronous because you can switch from

one to the other but they are really simultaneous synchronous conversations.

– Take that! high paid web consultants who say http is asynchronous.

Why is this important?

• Synchronous form interaction is annoying.– You have to refresh an entire form just to update

one part of it.– Then, you end up sending the entire form back

again even if only part of it changed.• Without using asynchronous frameworks like

AJAX, synchronous form interaction is the only way to build web applications.

Example

• If there is time, we’ll look at a neat example

• Registration Form