Distributed Database

Management Systems
In this chapter, you will learn:
• What a distributed database management system (DDBMS)
is and what its components are
• How database implementation is affected by different levels
of data and process distribution
• How transactions are managed in a distributed database
environment
• How database design is affected by the distributed database
environment
The Evolution of Distributed
Database Management Systems
• Distributed database management system
(DDBMS)
• Governs storage and processing of logically related
data over interconnected computer systems in
which both data and processing functions are
distributed among several sites
The Evolution of Distributed Database
Management Systems (continued)
• Centralized database required that corporate data be
stored in a single central site
• Dynamic business environment and centralized
database’s shortcomings spawned a demand for
applications based on data access from different
sources at multiple locations
DDBMS Advantages and
Disadvantages
•Advantages include:
• Data are located near “greatest demand”
site
• Faster data access
• Faster data processing
• Growth facilitation
• Improved communications
DDBMS Advantages and
Disadvantages (continued)
•Advantages include (continued):
•Reduced operating costs
•User-friendly interface
•Less danger of a single-point failure
•Processor independence
DDBMS Advantages and
Disadvantages (continued)
•Disadvantages include:
—Complexity of management and control
—Security
—Lack of standards
—Increased storage requirements
—Increased training cost
Characteristics of Distributed
Management Systems
●Application interface
●Validation
●Transformation
●Query optimization
●Mapping
●I/O interface
Characteristics of Distributed
Management Systems (continued)
• Formatting
• Security
• Backup and recovery
• DB administration
• Concurrency control
• Transaction management
Characteristics of Distributed
Management Systems (continued)
• Must perform all the functions of
centralized DBMS
• Must handle all necessary functions
imposed by distribution of data and
processing
• Must perform these additional functions
transparently to the end user
DDBMS Components
• Must include (at least) the following
components:
• Computer workstations
• Network hardware and software
• Communications media
• Transaction processor (application processor,
transaction manager)
• Software component found in each computer
that requests data
DDBMS Components (continued)
• Must include (at least) the following
components (continued):
• Data processor or data manager
• Software component residing on each
computer that stores and retrieves data
located at the site
• May be a centralized DBMS
Single-Site Processing, Single-
Site Data (SPSD)
• All processing is done on single CPU or host
computer (mainframe, midrange, or PC)
• All data are stored on host computer’s local disk
• Processing cannot be done on end user’s side of
system
Single-Site Processing,
Single-Site Data (SPSD)
(continued)
• Typical of most mainframe and midrange
computer DBMSs
• DBMS is located on host computer, which is
accessed by dumb terminals connected to it
• Also typical of first generation of single-user
microcomputer databases
Multiple-Site Processing, Single-
Site Data (MPSD)
• Multiple processes run on different computers sharing
single data repository
• MPSD scenario requires network file server running
conventional applications that are accessed through
LAN
• Many multiuser accounting applications, running
under personal computer network, fit such a
description
Multiple-Site Processing,
Multiple-Site Data (MPMD)
• Fully distributed database management system with
support for multiple data processors and transaction
processors at multiple sites
• Classified as either homogeneous or heterogeneous
• Homogeneous DDBMSs
– Integrate only one type of centralized DBMS over a
network
Multiple-Site Processing, Multiple-
Site Data (MPMD) (continued)
• Heterogeneous DDBMSs
• Integrate different types of centralized DBMSs over a
network
• Fully heterogeneous DDBMS
• Support different DBMSs that may even support different
data models (relational, hierarchical, or network) running
under different computer systems, such as mainframes
and microcomputers
Distributed Database
Transparency Features
• Allow end user to feel like database’s only user
• Features include:
–Distribution transparency
–Transaction transparency
–Failure transparency
–Performance transparency
–Heterogeneity transparency
Distribution Transparency
• Allows management of physically dispersed
database as though it were a centralized
database
• Following three levels of distribution
transparency are recognized:
• Fragmentation transparency
• Location transparency
• Local mapping transparency
Transaction Transparency
• Ensures database transactions will maintain
distributed database’s integrity and consistency
Distributed Requests and
Distributed Transactions
• Distributed transaction
• Can update or request data from several different remote
sites on network
• Remote request
• Lets single SQL statement access data to be processed by
single remote database processor
• Remote transaction
• Accesses data at single remote site
Distributed Requests and Distributed
Transactions (continued)
• Distributed transaction
• Allows transaction to reference several
different (local or remote) DP sites
• Distributed request
• Lets single SQL statement reference data
located at several different local or remote DP
sites
Distributed Concurrency Control
• Multisite, multiple-process operations are much more
likely to create data inconsistencies and deadlocked
transactions than are single-site systems
Two-Phase Commit Protocol

• Distributed databases make it possible for transaction

to access data at several sites
• Final COMMIT must not be issued until all sites have
committed their parts of transaction
• Two-phase commit protocol requires each individual
DP’s transaction log entry be written before database
fragment is actually updated
Performance Transparency and
Query Optimization
• Objective of query optimization routine is to minimize
total cost associated with execution of request
• Costs associated with request are function of:
• Access time (I/O) cost
• Communication cost
• CPU time cost
• Must provide distribution transparency as well as
replica transparency
Performance Transparency and
Query Optimization (continued)
• Replica transparency
• DDBMS’s ability to hide existence of multiple copies
of data from user
• Query optimization techniques include:
• Manual or automatic
• Static or dynamic
• Statistically based or rule-based algorithms
Distributed Database Design

• Data fragmentation
• How to partition database into fragments
• Data replication
• Which fragments to replicate
• Data allocation
• Where to locate those fragments and replicas
Data Fragmentation
• Breaks single object into two or more segments or
fragments
• Each fragment can be stored at any site over
computer network
• Information about data fragmentation is stored in
distributed data catalog (DDC), from which it is
accessed by TP to process user requests
Data Fragmentation Strategies
• Horizontal fragmentation
• Division of a relation into subsets (fragments) of
tuples (rows)
• Vertical fragmentation
• Division of a relation into attribute (column) subsets
• Mixed fragmentation
• Combination of horizontal and vertical strategies
Data Replication
• Storage of data copies at multiple sites served by
computer network
• Fragment copies can be stored at several sites to
serve specific information requirements
• Can enhance data availability and response time
• Can help to reduce communication and total query
costs
Data Replication scenarios
• Fully replicated database
• Stores multiple copies of each database fragment at
multiple sites
• Can be impractical due to amount of overhead
• Partially replicated database
• Stores multiple copies of some database fragments
at multiple sites
• Most DDBMSs are able to handle the partially
replicated database well
Replication scenarios (continued)
•Un-replicated database
•Stores each database fragment at
single site
•No duplicate database fragments
Data Allocation
• Deciding where to locate data
• Allocation strategies
• Centralized data allocation
• Entire database is stored at one site
• Partitioned data allocation
• Database is divided into several disjointed parts
(fragments) and stored at several sites
Data Allocation (continued)
•Replicated data allocation
•Copies of one or more database
fragments are stored at several sites
•Data distribution over computer
network is achieved through data
partition, data replication, or
combination of both
Client/Server vs. DDBMS
•Way in which computers interact to form
system
•Features user of resources, or client, and
provider of resources, or server
•Can be used to implement a DBMS in
which client is the TP and server is the
DP
Client/Server vs. DDBMS
(continued)
•Client/server advantages
• Less expensive than alternate
minicomputer or mainframe solutions
• Allow end user to use microcomputer’s
GUI, thereby improving functionality and
simplicity
• More people in job market have PC skills
than mainframe skills
• PC is well established in workplace
Client/Server vs. DDBMS
(continued)
•Client/server advantages
(continued)
•Numerous data analysis and query
tools exist to facilitate interaction with
DBMSs available in PC market
•Considerable cost advantage to
offloading applications development
from mainframe to powerful PCs
Client/Server vs. DDBMS
(continued)
• Client/server disadvantages
• Creates more complex environment
• Different platforms (LANs, operating
systems, and so on) are often difficult to
manage
• An increase in number of users and processing
sites often paves the way for security
problems
Client/Server vs. DDBMS
(continued)
•Possible to spread data access to much
wider circle of users
•Increases demand for people with
broad knowledge of computers and
software
•Increases burden of training and cost
of maintaining the environment
C. J. Date’s Twelve Commandments for DDBMS
• Local site independence
• Site independent, autonomous & centralized DBMS
• Site responsible for security, concurrency control, backup &
recovery
• Central site independence
• No site relies on a central or any other site
• All sites have same capabilities
• Failure independence
• System not affected by node failures
• System must be in a continuous operation even in case of node
failure or network expansion
C. J. Date’s Twelve
Commandments for DDBMS
• Location transparency
• User does not need to know location of data in order to
retrieve that data
• Fragmentation transparency
• User only sees one logical database
• User is not aware of data fragmentation
• Replication transparency
• User only sees one logical database
• User is not aware that data is replicated
C. J. Date’s Twelve Commandments for DDBMS
• Distributed query processing
• A query may be processed at several sites
• Query optimization is performed transparently by the
DDBMS
• Distributed transaction processing
• A transaction may update data at several sites
• A transaction is transparently executed at several sites
• Hardware independence
• System must run on any hardware platform
• IBM/DEC/HP/PC’s, etc.
C. J. Date’s Twelve
Commandments for DDBMS
• Operating system independence
• The system must run on any operating system
• Some sites may run UNIX; some PC/DOS
• Network independence
• System must run on any network platform
• Different hardware, different operating systems, different
communication networks
• Database independence
• System must support any vendor’s database product
• One site may run ORACLE while other sites may run INGRES
Summary
• Distributed database stores logically related data in
two or more physically independent sites connected
via computer network
• Distributed processing is division of logical database
processing among two or more network nodes
• Distributed databases require distributed processing
• Main components of DDBMS are transaction
processor and data processor
Summary (continued)
• Current database systems can be classified by extent
to which they support processing and data
distribution
• Homogeneous distributed database system integrates
only one particular type of DBMS over computer
network
• Heterogeneous distributed database system
integrates several different types of DBMSs over
computer network
Summary (continued)
• DDBMS characteristics are best described as set of
transparencies
• Transaction is formed by one or more database
requests
• Distributed concurrency control is required in network
of distributed databases
• Distributed DBMS evaluates every data request to find
optimum access path in distributed database
Summary (continued)
• The design of distributed database must consider
fragmentation and replication of data
• Database can be replicated over several different sites
on computer network
• Client/server architecture refers to way in which two
computers interact over computer network to form a
system