[Big Data Application] HTML and HTTP

Uniform Resources Locators

• In the Web, functionality of pointers is provided by Uniform Resource Locators (URLs).
• URL example:  http://www.myweb.com/dbstore/application/dbprogram 
• The first part indicates how the document is to be accessed

      - “http” indicates that the document is to be accessed using the Hyper Text Transfer Protocol.

      - The second part gives the unique name of a machine on the Internet.

• The rest of the URL identifies the document within the machine.

       - The local identification can be: The path name of a file on the machine, or An identifier (path name) of a program(search), plus arguments(?) to be passed to the program. E.g. http://www.google.com/search?q=database

 

HTML and HTTP

• HTML provides formatting, hypertext link, and image display features.
• HTML also provides input features

        - Select from a set of options : Pop-up menus, radio buttons, check lists

        - Enter values : Text boxes

        - Filled in input sent back to the server, to be acted upon by an executable at the server

• HyperText Transfer Protocol (HTTP) used for communication with the Web server

Client Side Scripting and Applets

• Browsers can fetch certain scripts (client-side scripts) or programs along with documents, and execute them in “safe mode” at the client site

       - Javascript, Macromedia Flash and Shockwave for animation/games, VRML, Applets

• Client-side scripts/programs allow documents to be active

       - Permit flexible interaction with the user. : Executing programs at the client site speeds up interaction by avoiding many round trips to server

       - Animation by executing programs at the local site

       - Ensure that values entered by users satisfy some correctness checks

• Security mechanisms needed to ensure that malicious scripts do not cause damage to the client machine

       - Easy for limited capability scripting languages, harder for general purpose programming languages like Java

• E.g. Java’s security system ensures that the Java applet code does not make any system calls directly

       - Disallows dangerous actions such as file writes

       - Notifies the user about potentially dangerous actions, and allows the option to abort the program or to continue execution.

 

Web Servers

• A Web server can easily serve as a front end to a variety of information services.
• The document name in a URL may identify an executable program, that, when run, generates a HTML document.

       - When a HTTP server receives a request for such a document, it executes the program, and sends back the HTML document that is generated.

       - The Web client can pass extra arguments with the name of the document.

• To install a new service on the Web, one simply needs to create and install an executable that provides that service.