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1. XML and Its Applications
Ben Y. Zhao, CS294-7
Spring 1999

2. Overview: The XML Language
 What is XML
 Document Type Definitions
 XML and DTD Example
 XML APIs: DOM and SAX
 Pros and Cons

3. What is XML?
 Extensible Markup Language is a simplified
subset of Standard Generalized Markup Language.
 Tags can be arbitrarily named, and can be used to
encode semantic information about enclosed data.
 Documents can be well-formed, or optionally
validated against Document Type Definitions.
 XML documents can be componentized, and be
distributed across networks.
 Creates a self-describing, text-based framework
around text and binary data

4. Document Type Definitions
 DTD: Concise structural definition of an XML
document type.
 DTDs are optional, well-formed+DTD=>valid
 DTDs provide a way to enforce XML documents’
compliance to constraints on XML documents.
 Optional elements and attributes add additional
flexibility to support evolving schemas.
 DTDs can be embedded in the XML document,
providing a self-sufficient validating document.

5. XML and DTD Example (open editor)
<?xml version =“1.0” encoding="US-ASCII"?>
<!ELEMENT PERSON (NAME, (EMAIL)+)>
<!ELEMENT NAME (FIRSTNAME, LASTNAME)>
<!ELEMENT FIRSTNAME (#PCDATA)>
<!ELEMENT LASTNAME (#PCDATA)>
<!ELEMENT EMAIL (#PCDATA)>
<?xml version =“1.0”?>
<PERSON>
<NAME>
<FIRSTNAME>Ben</FIRSTNAME>
<LASTNAME>Zhao</LASTNAME>
</NAME>
<EMAIL>ravenben@cs.berkeley.edu</EMAIL>
<EMAIL>ravenben@joker.cs.berkeley.edu</EMAIL>
</PERSON>

6. XML APIs: DOM and SAX
 DOM (Document Object Model)
– Provides a definitive API for accessing hierarchical
description languages like XML, HTML
– Specifies interfaces for accessing all part of a document
– Includes inheritance, typing, and constants
 SAX (Simple API for XML)
– An event-driven parser API
– API reports parsing events to application via callbacks
– Optimized for parsing large documents by eliminating
need to keep tree structure in memory

7. Pros and Cons
Pros
 Simple (human readable)
 Standard (easy to
integrate, widely adopted)
 Portable (cross-platform
data exchange)
 Flexible
(handles complex data)
 Extensible
(dynamic data model)
Cons
 Text-based means space
consuming
 Standardization is still a
problem to be solved
 Evolutionary model means
ill-defined functionality
core

8. Overview: Current XML Efforts
 XML Tools
 Evolving Recommendations
 Industry Databases
 XML Query Languages
 Research Query Engines
 Relevance to Systems Research

9. XML Tools
 Parsers
– Existing parsers support DOM or SAX
– Varying XML compliance and performance
 Editors for XSL, XML and DTDs
 Browsers
 Converters
– Applications that convert from and to XML
 Document Management
– Lightweight searching and indexing tools
– Difference engines

10. Related Evolving Recommendations
 Namespaces: qualifying names with URI references
 XML-data, defines XML vocabulary for schemas
(definitions of characteristics of classes of objects)
 XLink
– XML Linking language, sophisticated link styles
 XPointers
– XML Pointers to all parts of XML documents
 RDF (Resource Description Framework)
– model for using XML to describe metadata on the web
 DCD(Document Content Description)
– XML-data + RDF

11. Industry XML Databases
 ObjectStore eXcelon
– Middle tier server that imports from different DB stores
– XQL queries applied to integrated data
– Provides “cache server” for XML imported from
heterogeneous DB backends
– Focuses on web applications as access methods to DBs
 Poet XML Repository
– Object oriented database with standard DB
functionality, with OQL
– Focuses on use of XML to faciliate EDI

12. XML Query Languages
 XML-QL (AT&T, Inria, U.Wash.)
– Very similar to SQL
– Optimizations and other DB techniques applicable
– Data integration and conversion from hetero. sources
 XQL (Microsoft)
– Based on the XSL transformation language
– Context based and XML-specific query matching
– Departure from the database-centric SQL format
 LOREL (Stanford)
– See notes from Last Week’s LOREL presentation

13. Research Query Engines
 LORE (Stanford)
– Based on the LOREL query language
– A feature-rich DB approach to XML storage and query,
with context-free indexing, path indexing through
dataguides, query optimizations, and views
 XSet (UCB, Ninja)
– Streamlined XML search engine implemented in Java
– Focus on high performance rather than feature set
– Small size favors integration into low-level applications
– Research issues on next slide

14. Relevance to Systems Work
 FSML: XML meta-index for fast access to files
 Distributed Service Discovery (Ninja SDS)
– Service descriptions encoded in XML
 Semantically Enhanced Web searching
 Data exchange across heterogeneous platforms
 Low overhead scripting language for thin clients
 User preferences
– Embedded logic and scripting inside XML

15. Discussion
 XML: flexible description language with optional
DTD validation
 Provides flexible framework for marking data with
inferred semantics
 Provides additional push towards standardization,
but not as a result of the language itself
 Are the benefits of the XML movement due to
something intrinsic in the language?
 Description language of choice? Pervasive among
future applications?