Distributed Databases

1
Distributed Databases

• David Nelson
• CAT
• May 2006

2
Contents

• Internet Databases
• Client/Server Architectures, advantages and
  disadvantages
• Web Database Approaches
• XML
• Semi-structured data
• Distributed Database Systems
• Definitions
• Homogeneous and Heterogeneous Systems
• Federated DBMS Systems
• Interoperability
• The Grid

3
Traditional Architecture

• Traditional Database Systems are based on a
  two-tier client-server architecture

• User interface
• Main business and data processing logic

Client
Database Server

• Server-side validation
• Database access

4
Web Architecture

• Need for enterprise scalability causes problems
  which can be solved by a three-tier architecture
• Generalised to n-tier

• User interface

Client
Application Server

• Business logic
• Data processing logic

• Server-side validation
• Database access

Database Server
5
Web as a Database Platform

• Advantages
• DBMS advantages
• Simplicity
• Platform independence
• Graphical User Interface
• Standardization
• Cross-platform support
• Transparent network access
• Scalable deployment
• Innovation

6
Web as a Database Platform

• Disadvantages
• Reliability
• Security
• Cost
• Scalability
• Limited HTML Functionality
• Statelessness
• Bandwidth
• Performance
• Immaturity of development tools

7
Approaches

• CGI
• Server Side Includes
• HTTP Cookies
• API (non-CGI gateways)
• ODBC
• Java (JDBC, JSQL, JRB)
• JavaScript, JScript
• Microsoft Active Platform (ASP, ADO, ActiveX)
• PHP (Hypertext Preprocessor)
• XML

8
Extensible Markup Language (XML)

• A simplified version of SGML, designed
  specifically for Web documents
• a meta-language to create customised tags which
  provide functionality not available in HTML
• links can point to multiple documents
• links can be bi-directional
• links to relative objects
• broken into
• document data
• document type definition (DTD) for well-formed
  documents
• stylesheet (XSL standard)

9
Semi-structured Data

• Typical data models (e.g. relational) are
  structured
• i.e. has a separate schema
• Semi-structured data is self describing
• aka schemaless
• no separate description of the type/structure of
  data
• e.g. XML

10
Sample XML Database
unicode

• lt?xml version 1.0 encoding UTF-8
  standaloneyesgt
• lt?xmlstylesheet type text/xsl
  hrefstaff_list.xslgt
• lt!DOCTYPE STAFFLIST SYSTEM staff_list.dtdgt
• ltSTAFFLISTgt
• ltSTAFF branchNo B005gt
• ltSTAFFNOgtSL21lt/STAFFNOgt
• ltNAMEgt
• ltFNAMEgtJohnlt/FNAMEgtltLNAMEgtWhitelt/LNAMEgt
• lt/NAMEgt
• ltPOSITIONgtManagergt
• lt/STAFFgt
• ltSTAFF branchNoB003gt
• lt/STAFFLISTgt

root element only 1 per document
attribute
elements ordered attributes unordered
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Sample DTD

• lt!ELEMENT STAFFLIST (STAFF)gt
• lt!ELEMENT STAFF (NAME, POSITION, DOB?, SALARY)gt
• lt!ELEMENT NAME (FNAME, LNAME)gt
• lt!ELEMENT FNAME (PCDATA)gt
• lt!ELEMENT LNAME (PCDATA)gt
• lt!ELEMENT POSITION (PCDATA)gt
• lt!ATTLIST STAFF branchNo CDATA IMPLIED)gt

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Sample StyleSheet

• lt?xml version 1.0?gt
• ltxslstylesheet xmlnsxsl http//www.w3.org/TR/
  WD-xslgt
• ltxsltemplate match /gt
• lthtmlgtltbodygt
• ltcentergtlth2gtDreamHome Estate agentslt/h2gtlt/center
  gt
• lttable border 1 bgcolor ffffffgt
• lttrgt
• ltthgtstaffNolt/thgt
• --- repeat for other column headings
• ltxslfor-each selectSTAFFLIST/STAFFgt
• lttrgtltxslvalue-of-selectSTAFFNO/gtlt/tdgt
• lttrgtltxslvalue-of-selectNAME/FNAME/gtlt/tdgtlt/t
  rgt
• lt/xslfor-eachgtlt/tablegtlt/bodygtlt/htmlgt
• lt/xsl-stylesheetgt

13
Benefits of XML

• Simplicity
• Open standard and platform/vendor-independent
• Extensibility
• Reuse
• Separation of content and presentation
• Improved load balancing

14
Benefits of XML

• Support for integration of data from multiple
  sources
• Ability to describe data from a wide variety of
  applications
• More advanced search engines
• XQuery

15
XML Schema

• ltxsdgroup-name STAFFTYPE
• ltxsdelementnameSTAFFgt
• ltxsdcomplexTypegt
• ltxsdsequencegt
• ltxsdelement name STAFFNO
  typeSTAFFNOTYPE/gt
• ltxsdelement name NAMEgt
• ltxsdcomplexTypegt
• ltxsdsequencegt
• ltxsdelement name FNAMEgt
  type xsdstring/gt
• ltxsdelement name LNAMEgt
  type xsdstring/gt
• lt/xsdsequencegt
• lt/xsdcomplexTypegt
• ...

16
Querying

• W3C working group has produced
• XML Query Requirements
• XML Query Data Model
• XML Query Algebra
• projection, iteration, selection, join, sorting,
  aggregation
• XQuery - a query language for XML

17
XQuery Queries

• List the staff at branch B005 with a salary
  greater than 15000
• FOR S IN document(staff_list.xml)//STAFF
• WHERE S/SALARY gt 15000 AND
• S/_at_branchNo B005
• RETURN S/STAFFNO

18
XML Databases

• Native
• XML is the primary data store of the DBMS
• Semi-structured databases
• e.g. Lore
• XML Enabled
• Traditional RDBMS provides mappings between XML
  and data store
• Can be stored
• Course grained
• Medium grained
• Fine grained
• E.g. Oracle, SQL Server, SQL2003

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Fine Grained Approach

• Good for queries which need to inspect/manipulate
  specific elements in the XML document
• Not good for queries which manipulate (e.g.
  retrieve/store) the entire document

Child
Element ( parent)
Document
CharData
Attribute
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Course Grained Approach

• One table
• Best for queries which manipulate whole document
• e.g. retrieve/store a document
• Worst for queries which manipulate elements
• e.g. retrieve children of a tag

21
Medium Grained Approach

• A compromise between fine and course grained
• Slice document tree up into sections
• Store sub-sections using a course grained
  approach
• Good for both types of queries

22
Distributed Db Definitions

• Distributed Database System
• the ability of the DDBS users to run applications
  at each node
• Federated Database System
• a DDBS is usually a single application
  distributed over various sites
• a FDBS is a cooperating multiple system
• a simple solution for interoperability (as
  discussed later)

23
Distributed Database Systems

• System needs facilities to be able to
• perform distributed query optimization
• manage distributed transactions
• manage data replication
• Homogeneous DDBS simplest case
• several sites, each running their own
  applications on same DBMS with same schema and
  transactions
• location transparency
• can communicate over large distances, and are
  autonomous

24
Heterogeneous DDBMS

• Several existing databases (using different
  DBMSs) linked into a single system
• Problems
• variation in costs of operation between sites
• some operations may not be available at some
  sites
• some DBMSs cannot read records of others
• varying base types
• Requesting site must
• have detailed knowledge of operation of remote
  system
• assume remote system has only rudimentary
  functionality
• make programmer do query composition by hand

25
Federated DBMS

• A collection of independently managed,
  heterogeneous database systems
• allow partial and controlled sharing of data
  without affecting existing applications

Federated schema
Federated to local schema mapping
Local schema
Federated schema
Federated to local schema mapping
Local schema
26
Interoperability

• The web is the ultimate interoperable database
  platform
• Need to be able to query using various sources on
  the web
• Without duplication of data as in a data warehouse

27
Interoperability

• IEEE (1990) Definition
• the ability of two or more systems or components
  to exchange information and to use the
  information that has been exchanged
• IEEE Standard Computer Dictionary A Compilation
  of IEEE Standard Computer Glossaries
• Current simple solutions
• transformation
• mediation

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Interoperability Definition 2

• The ability to request and receive services
  between various systems and use their
  functionality
• More than data exchange
• Implies a close integration

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Interoperability Definitions 3

• Semantic Interoperability
• agreements about content description standards
• Ontologies
• Structural Interoperability
• Specifying semantic schemas such that they can be
  shared, e.g. RDF
• Syntactic Interoperability
• How to tag and mark data to facilitate exchange
• E.g. XML

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Features

• Exchange of messages and requests
• Use of each others functionality
• Client-server abilities
• Distribution
• Operate multiple systems as single unit
• Communication despite incompatibilities
• Extensibility and evolution

31
The Problems and Difficulties

• Different data models
• There can be major semantic differences even
  within the same data model
• Properties may be called by different names
• Different data types may be used
• What about recreating local defined functions?
• All this implies we know where they are and we
  have a physical means of getting to them

32
The Problems and Difficulties

• Databases are by their nature protectors of
  data, they do not share easily
• Many (particularly legacy systems) do not have
  any form of web interface
• Most databases are security protected
• Databases do not advertise their services to the
  web
• Even client/Server databases operate within a
  cocoon of silence

33
EBCDIC

• EBCDIC /eb's-dik/, /eb'seedik/, or
  /eb'k-dik/ n.
• abbreviation, Extended Binary Coded Decimal
  Interchange Code
• A character set used on early IBM computers. It
  exists in at least six mutually incompatible
  versions, all featuring such delights as
  non-contiguous letter sequences and the absence
  of several punctuation characters fairly
  important for modern computer languages (exactly
  which characters are absent varies according to
  which version of EBCDIC you're looking at). IBM
  adapted EBCDIC from punched card code in the
  early 1960s and promulgated it as a
  customer-control tactic, spurning the already
  established ASCII standard. Today, IBM claims to
  be an open-systems company, but IBM's own
  description of the EBCDIC variants and how to
  convert between them is still an internally
  classified top-secret.
• EBCDIC is the most common alternate character
  code but there are others.
• http//www.cheverus.org/advanced/data/EBCDIC.html

34
Some Simple Integration Problems 1

• Differing schema
• author char(50) author_surname char(50)
• author_inits char(10)
• title varchar(300) title varchar(200)
• keyword set(char(30)) keywd array(8) (char(30))
• - both are valid schema in SQL-3
• also A.N.Other, A N Other, Other N A, ...

35
Some Simple Problems 2

• Homogeneous Models
• the same information may be held as attribute
  name, relation name or a value in different
  databases
• e.g. library fines
• as a dedicated relation Fine(amount, borrowed_id)
• as an attribute Loan(id, isbn, date_out, fine)
• or as a value Charge(1.25, fine)

36
Complex Problems

• Heterogeneous models
• Need to relate model constructions to one
  another, for example
• relate classes in object-oriented to user-defined
  types in object-relational
• All problems are magnified at this level!

37
Data Models

• We are only touching the surface in repositories
  and data warehouses

38
XML RDF

• Resource Description Framework
• XML Schema defines a grammar
• therefore we have all the problems shown
  previously (e.g. names)
• RDF provides a way to encode domain models
• an infrastructure that enables the encoding,
  exchange and reuse of structured meta-data (W3C)
• this is what we need for interoperable systems

39
RDF Data Model

• RDF Data Model consists of three objects
• Resource
• anything that can have a URL
• Property
• a specific attribute which is used to describe a
  resource
• Statement
• a combination of a resource, a property and a
  value
• known as the subject, predicate and object
• e.g. The author of http//www.myhome.net/staff_li
  st.xml is Fred Smith

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RDF Example

• The statement would be defined in RDF
  (simplified) as
• lt?xml version"1.0"?gt
• ltRDFgt
• ltDescription about" http//www.myhome.net/staff_
  list.xml "gt
• ltauthorgtFred Smithlt/authorgt
• ltcreatedgt25 May 2006lt/createdgt
• lt/Descriptiongt
• lt/RDFgt

41
The Grid

• Original Motivation
• the need for a distributed computing
  infrastructure for advanced science and
  engineering (Walker)
• Used originally in science for large number
  crunching applications
• but now finding larger appeal
• Compare to the national power grid
• Interoperability is a key issue

42
Examples of Computational and Information Grids

• NASA Information Power Grid
• access to large-scale computing resources, large
  databases, and high-end instruments
• dynamically co-allocated resources (e.g.
  supercomputers)
• AstroGrid
• a virtual observatory
• European Data Grid
• high energy physics, biology and Earth
  observation
• distributed, large-scale data intensive computing

43
Summary

• Distributed and web-databases are increasingly
  important areas
• XML is being increasingly used in data models,
  data transmission and data integration
• Interoperability is the key issue and the major
  research area in database systems
• XML and RDF have the potential as a stepping
  stone to achieving this
• The Grid is an example of a system which could
  require interoperability to integrate database
  systems

44
Further Reading

• Connolly and Begg, Database Systems, chapters
  22,23,29,and 30.
• Ozsu and Valduriez, Principles of Distributed
  Database Systems, 2nd edition
• everything you ever wanted to know about
  distributed database systems
• Chaudri and Zicari, Succeeding with Object
  Databases, 2001
• D Walker, Emerging Distributed Computing
  Technologies, Cardiff University
• http//www.cs.cf.ac.uk/User/David.W.Walker/IGDS/Gr
  idCourse.htm
• an introduction to the Grid
• XML and RDF
• www.w3schools.com