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SWU, Computer systems and technologies

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The Objective of This Lecture To give you a very high-level overview of some of the tools for Web Programming Use several typical tools to show the basics of Web Programming Not meant for advanced Web developers The next in-class tutorial will be on DB Programming We will assume minimal interaction with DBMS in this lecture to focus on the Web programming part

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Dynamic Content We are all used to fetching pages from a Web server. Most are prepared by a human and put on the Web site for others to read. Each reader sees the same page. The Web browser (client) can perform client-side computations on the page to present it differently. And the content can be dynamic via Javascript code in the page that operates as the page is being rendered (e.g. layout) and as it is being viewed (e.g. mouse operations) We, however, are interested in server-side dynamic content.

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Server Side Coding CGI (standard protocol for server-client communication) CGI PHP (open source) PHP ASP.NET (Microsoft proprietary) ASP.NET JSP (JavaServer Pages) JSP Python, e.g. Django (web framework) (open source) PythonDjango (web framework) Ruby, e.g. Ruby on Rails (open source) RubyRuby on Rails

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Server-side Dynamic Pages A request to the Web server returns a new, machinegenerated page, e.g. google search result page, travelocitys flight purchase page,... The server receives the request and executes some code to create a new HTML document (or other type) and returns that. What makes this interesting is that the request can specify inputs that govern how the page is created. This is very similar to calling an R function with different inputs and generating output in the form of an HTML page

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Client Side Coding HTML stands for HyperText Markup Language, and is the predominant markup language for web pages. Javascript Formally called EMCAScript, Javascript is a ubiquitous client side programming tool, often implemented as part of a web browser in order to provide enhanced user interfaces and dynamic websites. Its typically used to enable programmatic access to computational objects within a host environment. Applet and ActiveX. AppletActiveX Applet (e.g. Java Applet): small application that performs one specific task, sometimes running within a larger program, perhaps as a plug-in. AJAX (Asynchronous Javascript And XML) AJAX Group of technologies that provides new methods of using Javascript, PHP and other languages to improve the user experience

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Client-Server components One application acts as a client and connects to the server and sends a request; the server sends a reply. Our client is a browser and specifically an HTML form that provides ways to specify inputs to the server. Our server is an R script on the Web server that gets the inputs and generates an HTML page based these inputs. To provide dynamic access to R functionality via the Web, you need to create both pieces.

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Transactions The web is (as we already know) based on HTTP, which is a simple request/response protocol. A browser requests a web page, and the server supplies it. That's it. These events together constitute a single web transaction. Unfortunately, a transaction-based system is not, on its own, sufficiently powerful to create the kinds of web- based systems that are needed for modern applications.

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The "welcome.php" file can now use the $_POST variable to catch the form data (notice that the names of the form fields will automatically be the ID keys in the $_POST array): Welcome. You are years old! The $_POST Variable

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Why Use $_POST Variables sent with HTTP POST are not shown in the URL Variables have no length limit However, because the variables are not displayed in the URL, it is not possible to bookmark the page. The PHP $_REQUEST variable contains the contents of both $_GET, $_POST, and $_COOKIE. The PHP $_REQUEST variable can be used to get the result from form data sent with both the GET and POST methods. Welcome. You are years old!

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What is a Cookie? A cookie is often used to identify a user. A cookie is a small file that the server embeds on the user's computer. Each time the same computer requests a page with a browser, it will send the cookie too. With PHP, it can both creating and retrieving cookie values. The setcookie() function is used to set a cookie. The setcookie() function must appear BEFORE the tag. Setcookie (name, value, expire, path, domain) ;

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How to Create a Cookie? - Example In the example below, we will create a cookie named "user" and assign the value "Alex Ivanov" to it. We also specify that the cookie should expire after one hour:

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How to Retrieve a Cookie Value? The PHP $_COOKIE variable is used to retrieve a cookie value. In the example below, we retrieve the value of the cookie named "user" and display it on a page:

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How to Retrieve a Cookie Value? In the following example we use the isset() function to find out if a cookie has been set:

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How to Delete a Cookie? When deleting a cookie you should assure that the expiration date is in the past. Delete example:

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PHP Session Variables A PHP session is used to store user information on the server for later use (i.e. username, shopping items, etc). However, session information is temporary and will be deleted after the user has left the website. If you need a permanent storage you may want to store the data in a database. Sessions work by creating a unique id (UID) for each visitor and store variables based on this UID. The UID is either stored in a cookie or is propagated in the URL.

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Starting a PHP Session Before you can store user information in your PHP session, you must first start up the session. Note: The session_start() function must appear BEFORE the tag: The code above will register the user's session with the server, allow you to start saving user information, and assign a UID for that user's session.

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Storing a Session Variable The correct way to store and retrieve session variables is to use the PHP $_SESSION variable: Output: Pageviews = 1

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Storing a Session Variable In the example below, we create a simple page-views counter. The isset() function checks if the "views" variable has already been set. If "views" has been set, we can increment our counter. If "views" doesn't exist, we create a "views variable, and set it to 1:

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Destroying a Session If you wish to delete some session data, you can use the unset() or the session_destroy() function. The unset() function is used to free the specified session variable: You can also completely destroy the session by calling the session_destroy() function:

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