[ Revised ] 15 elementary truths about XML

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

Thank you John, Bjoern, Peter, Michael, Andrew, Michael, and Toby for your excellent feedback.

Based on your feedback, I revised the statements.  Do you agree with the current formulation of each statement?  /Roger

1. An XML document is a sequence of zeros and ones called bits.

2. A byte consists of 8 bits. 

3. Thus, the content an XML document is a sequence of bytes.

4. Here is an example of a byte: 00110001

5. That byte may be interpreted in various ways by software applications. For example, it may be interpreted as:

    - corresponding to an integer in base two. 
      In base 10 it represents the integer 49.

    - corresponding to a character.
      In the ASCII character encoding scheme it 
     represents the character 1.  

6. XML processors always interpret the bytes in XML documents as characters.

7. Thus, XML processors interpret the content of XML documents as a sequence of characters.

8. There are various character encoding schemes, such as ASCII and UTF-8. Some character encoding schemes require more than one byte to encode a character.

9. An XML processor may identify the character encoding scheme used by an XML document either by its encoding attribute in the XML declaration or by some out-of-band means.

10. An XML processor is software that reads the bytes in an XML document and makes them available to XML applications. 

11. An XML application is software that processes the output of an XML processor. Metaphorically, an XML application is a layer of software on top of an XML processor.

12. An XML Schema validator is an XML application.

13. XML applications may interpret the bytes in XML documents differently than how an XML processor interprets the bytes.

14. For example, consider the XML Schema that declares an element A with a Boolean data type:

    <element name="A" type="boolean" />

    Suppose the value of <A> is the byte 00110001. 
    The element declaration informs the XML Schema validator 
    and the XML Schema validator interprets the byte as the 
    Boolean value "true."

15. Thus, an XML processor interprets the byte 00110001 as representing the character 1 whereas an XML Schema validator interprets the same byte  as representing the Boolean value "true."