The Web: HTTP, HTML, XML and Web Services

HyperText Transfer Protocol (HTTP)

• HTTP is a protocol: Stamper, D.A. (1999) Business Data Communications: "convention used for establishing (data) transmission rules." (p. 599).
• Spainhour & Eckstein (p.379). "Hypertext Transfer Protocol is the language web clients and servers use to communicate with each other."
• Protocol for letting web browsers (client requests) and web servers (server responses) communicate.

• Client (e.g., web browser) - server (e.g., web server) model.
  

• Basic understanding is a must(!) for a basic understanding of how the web works.
• Some HTTP is "filtered back" to users through error message in browsers; e.g., 404 Error: File or Directory Not Found.

• Created by Tim Berners-Lee.
  

• Three parts to each HTTP client request and each HTTP server response:
1. Request or response line: specifies the contents of the request or the status of the response, respectively.
2. Headers (optional): specifies configurations, acceptable formats, and lots of other things the browser and the server want to tell each other; e.g., the date and time the file was last modified; whether the file should be cached or not, etc.
3. Body (optional): additional data; e.g., the actual data passed back from the HTTP server or any data the server must know in order to execute a request.
   

• Example of an HTTP transaction:
1. HTTP client (browser) initializes transaction:

1. Request line: <method> <document address> <HTTP version number>

   GET classes.bus.oregonstate.edu/ba471/index.htm HTTP/1.1

   • GET: request method (Get me the data represented by the URL).
     
   • classes.bus.oregonstate.edu: domain name of the OSU COB web server (IP address: )
   • ba471/index.htm: off of the server's root folder, get me the index.htm file which is in the ba471 subfolder.
   • HTTP/1.1: I, browser, speak HTTP 1.1. Please do not sent me anything back more recent than HTTP 1.1.

2. Headers:

User-Agent: Mozilla/4.7(WinNT; I)
Accept: image/gif, image/jpeg, image/x-bitmap, text/html

3. Body: nothing

1. HTTP server responds:
1. Response line: <HTTP version> <status code> <status description>

   HTTP/1.1 200 OK

    

   Response Code	Response Meaning
   100 - 199	General information
   200 - 299	Client request successful
   300 - 399	Client request redirected, action necessary
   400 - 499	Error; e.g., client request incomplete or file not found
   500 - 599	Server errors

    
2. Headers:

Date: Thu. 25 May 2006 09:21:56 UTC
Server: Apache/1.3.12
Last-Modified: Wed. 24 May 2006 08:18:54 UTC
Content-Type: text/html
Content-Length: 6517

3. Body: HTML or text.
 
• HTTP is a so-called stateless protocol: each transaction is independent of the previous ones; i.e., the process has no memory.
• Simplicity: browsers and servers do not need to keep information around.
• Would get complex with multiple users.
• Don't know how long info must be kept around.
• Problem: are conversations and interactions really stateless? E.g., booking a flight, buying a book on the Internet? If not, what to do?

Example: buy a book on the Internet:

1. Client issues request for information about book X.
2. Server replies with price and request for credit card #.
3. Client issues request containing credit card #.

But!!! HTTP is stateless --> step 3 is independent of steps 1 and 2 --> In step 3, the server has no memory about steps 1 and 2, including which book is being bought! --> What to do?
• 'memory to go:' The server passes all the information back to the client and forgets about the client. But at the next request, the client passes everything it got back in the previous step to the server again, plus any new information it must submit.

• 'write it down:' The server (or actually, one of its helper programs) sticks everything it receives elsewhere on the system (files or database) and retrieves it at the next client request.

Problem: How does the server know who's who?

• Cookies: Store base information needed in subsequent requests on the client:
• Client request is received by server.
• Server includes the cookie information as a response header:
Set-Cookie: mysite.com_id=1234567890
• If client is cookie enabled, cookie is stored in client's cookies file (persistent).
• At next client request to mysite.com, cookie information is included as request header(s):
Cookie: mysite.com_id=1234567890

eXtensible Markup Language (XML) and Web services

• The Web for humans: HyperText Markup Language (HTML):
  • is a language for formatting documents.
  • sections of text are marked-up according to how they should be displayed:

    this should be in italics and bolded

    <i>this should be in italics <b>and bolded</b></i>

• But what about programs?
  • I am an on-line bookstore and I want to sell other stores' books (and get a commission).
  • I am an on-line newspaper and I want to show the NYSE and NASDAQ stock quotes on my web site.
  • I am an on-line weather site and I want to simultaneously display the current temperatures in 10 large cities.
  • I am a mortgage broker and I need to know the current rates of my top 50 lenders.
  • I am the OSU library and I would like to show my patrons which books are available in the UofO's library.
  • I am www.teachengineering.com and I would like to show my users how many NSDL documents have something to do with 'water'.
  • etc.
    

• HTML is no good:
  • Each site that I would need to visit provides its own HTML pages.
  • I'd like to first collect all the information from multiple sites and then organize it on a single web page.
  • So I need some HTML, but only at the very end.
  • How do I get the first bit (collecting all the information in the first place) done?

• eXtensible Markup Language (XML) to the rescue:
  • Like HTML, XML is a markup language.
  • However, instead of formatting instructions, it communicates contents.

  example: from Harold, E.R. & Means, W.S. (2001) XML in a Nutshell; O'Reilly & Associates

  <collection>
  
    <painting>
      <title>Memory of the Garden at Etten</title>
      <artist>Vincent van Gogh</artist>
      <date>Nov. 1888</date>
      <description>Two women look to the left. A third works in her garden</description>
    </painting>
  
    <painting>
      <title>The Swing</title>
      <artist>Pierre Auguste Renoir</artist>
      <date>1886</date>
      <description>Girl on a swing. Two men and a toddler watch</description>
    </painting>
  
  </collection>
  

• Notice how the markup tags convey meaning, yet say nothing about how this information must be displayed.
• Also notice how the concepts are hierarchical; i.e., 'nested:'
  • a collection can contain more than one painting.
  • titles and artists appear only within paintings.

• This strict hierarchy is easy for programs to 'parse.'
• Since XML (like HTML) is text-based, it is compatible across computing platforms.

• Problem: Who determines which 'tags' or 'elements' are available? What if I would like to create some XML to represent course student enrollment? Which XML elements are there for me to use?
• Answer: you make them up yourself!

  For example:

  <course>
  
    <abbreviation>BA 471</abbreviation>
  
    <section number="001">
      <instructor>
         <first_name>Rene</first_name>
         <last_name>Reitsma<last_name>
      </instructor> 
  
      <student>
         <first_name>John</first_name>
         <last_name>Smith<last_name>
      </student> 
  
      <student>
         <first_name>Mary</first_name>
         <last_name>Jones<last_name>
      </student> 
    </section >
  
    <section number="002">
       ...
    </section >
  
  </course>
  

• Problem:  OK, but how does the receiving computer know which XML elements I made up? How, therefore, can it 'parse' my XML?

• Answer: you have to provide a declaration of your syntax (grammar + vocabulary): Document Type Definition (DTD) or XML Schema.

• Problem: So how do the XML document and the DTD get associated with each other? How does the receiving computer know which DTD goes with which XML document?

• Internal DTD: DTD and XML stored in the same document:

• External DTD: XML document contains a reference to a DTD elsewhere on the Web.

• Examples of XML vocabularies:
• MathML: Mathematics Markup Language.
• XBRL: Extensible Business Reporting Language
• HydroML
• Music(X)ML
• And a zillion others.
  
• Digital library meta data (Open Archive Initiative): TeachEngineering metadata.

• Web service: HTTP/XML-based data provider.
  

• Putting it all together: www.teachengineering.com.

• Problem: Can you think of other Web service applications?