BA372 - XML & Web Services

BA372 - XML & 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 a financial spreadsheet and I would like to know the value of my portfolio.
- 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, specially formatted and laid-out HTML pages.
- Every time their HTML front-end changes, I (might) have to change my program parsing it.
- 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 -- facing my customer.
- 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.

Adopted as W3C standard in 1998.

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.'

An XML document or fragment can be represented as a tree (similar to a directory/folder tree).

Problem: Can you draw the <collection> tree?

Trees are 'nice' data structures to represent and parse.

e.g., look at how your browser shows an arbitrary XML document. (vs. the document rendered as HTML).

Document Object Model (DOM) is a W3C-governed standard for representing XML documents as 'trees.'

An entire XML document can be read into a tree and stored in memory; e.g., as a DOM object.

When the tree is properly constructed; i.e., as a tree, the document is said to be well formed.

Only well-formed documents can be reliably parsed ==> XML makes things very rigid & predictable ==> easy on programs to parse and 'take apart.'

Problem: what are some examples of a not well-formed XML tree?

Problem: make an XML document, introduce some well formedness errors and try pick up the 'broken' document with your browser; what do you observe?

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 name="Business Systems Design & Development">

  <abbreviation>BA 372</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>

Note how an element or 'node' such as <section> can have attributes, similar to attributes in HTML tags.

Problem: When to make something an attribute rather than an element?

But how does the receiving computer know which XML elements I made up? How, therefore, can it 'parse' my XML?

Answer 1: If the document is well formed; a document tree can always be formed.

However, what about more complex constraints?

<student> must have both a <first_name> and a <last_name>.

<first_name>s and <last_name>s are atomic; i.e., they must contain nothing but characters.

A <section> has an attribute number.

Answer 2: you have to provide a declaration of your syntax (grammar + vocabulary): Document Type Definition (DTD) or XML Schema (XSD).

XML (Harold & Means (2001), p. 30)	DTD
<person> <name> <first>Allen</first> <last>Turing</last> </name> <profession >computer scientist</profession> <profession>mathematician</profession> <profession>cryptographer</profession> </person>	<!ELEMENT first (#PCDATA)> <!ELEMENT last (#PCDATA)> <!ELEMENT profession (#PCDATA)> <!ELEMENT name (first, last)> <!ELEMENT person (name, profession*)>

Notice that DTDs and XSDs themselves are XML documents. However!!! their syntax is fixed and predetermined!

Look how Altova's XMLSpy visualizes an XML Schema (XSD) as a tree (not a public site).

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:

<!DOCTYPE person [
<!ELEMENT first (#PCDATA)>
<!ELEMENT last (#PCDATA)>
<!ELEMENT profession (#PCDATA)>

<!ELEMENT name (first, last)>
<!ELEMENT person (name, profession*)>
]>

<person>

<name>
<first>Allen</first>
<last>Turing</last>
</name>

<profession >computer scientist</profession>
<profession>mathematician</profession>
<profession>cryptographer</profession>

</person>

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

<!DOCTYPE person SYSTEM "http://www.mywebsite.com/person.dtd">
<person>

<name>
<first>Allen</first>
<last>Turing</last>
</name>

<profession >computer scientist</profession>
<profession>mathematician</profession>
<profession>cryptographer</profession>

</person>

Examples of XML-based formats:

MathML

XBRL: Extensible Business Reporting Language

HydroML

Music(X)ML

And a zillion others.

Web service: HTTP/XML-based data provider: two types:

REpresentational State Transfer (REST): HTTP response contains unadorned, unencapsulated, native XML.

Digital Library for Earth System Education (DLESE) search for the term "ocean" and return a maximum of 10 search results, starting at position 0: http://www.dlese.org/dds/services/ddsws1-1?verb=Search&q=ocean&s=0&n=10

NorthWest Airlines (NWA) RSS feed on fare promotions: http://www.nwa.com/rss/fare_promo_rss.xml

Yahoo REST search service: http://search.yahooapis.com/WebSearchService/V1/webSearch?appid=foo&query=eggplant

Service-Oriented (previously: Simple Object) Access Protocol (SOAP):

Organizational framework for Web services.

Provides a structure for Web-based services; both request & response:

Example.

Can easily be wrapped inside HTTP.

Often used in conjunction with Web Services Description Language (WSDL).

XML-based format for describing Web services.

Example from W3C's pages.

e.g., Amazon's E-Commerce Service's WSDL.

Problem: What is a so-called Service-Oriented Architecture (SOA):

Problem: Isn't this what OOP was supposed to give us?

Problem: So, what's the difference? Why would XML-based Web services be able to accomplish what OOP did not?