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Centralized Databases

Centralized database is a system with a single logical

database located at one site under the control of single

DBMS

It is perceived that in next decade most organizations

will be move towards distributed database systems

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Distributed Databases (1/4)

Alogically interrelated collection of shared data (anda description of this data), physically distributed over

a computer network.

Distributed DBMS is software system that permits themanagement of the distributed database and makesthe distribution transparent to users.

DDBMS consists of a single database that is split intoa number of fragments, each of which is stored on oneor more computers under the control of separateDBMS; computers are connected by a communicationnetwork

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Each site is capable of independently processing userrequests that require access to local data, and also

capable of processing data stored on other computersin the network

User access distributed databases via two differentapplications:

Local Applications (Those that require data from local site)

Global Applications (Those that require data from other

sites, A DDBS must have at least one Global Application)

Note, it is not necessary for every site to have its ownlocal database

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ADDBMS has the following characteristics:

Collection of logically-related shared data.

Data split into fragments.

Fragments may be replicated.

Fragments/replicas allocated to sites.

Sites linked by a communications network.

Data at each site is under control of a DBMS.

DBMSs handle local applications autonomously.

Each DBMS participates in at least one global application.

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Disadvantages of DDBMS

Complexity

Cost

Security

Integrity control more difficult

Lack of standards

Lack of experience

Database design more complex

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Types of DDBMS (1/2)

Homogeneous DDBMS All sites use same DBMS product.

Much easier to design and manage.

Approach provides incremental growth and allows increasedperformance.

Heterogeneous DDBMS Sites may run different DBMS products, with possibly different

underlying data models.

Occurs when sites have implemented their own databases andintegration is considered later.

Translations required to allow for: Different hardware.

Different DBMS products.

Different hardware and different DBMS products.

Typical solution is to use gateways.

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Types of DDBMS (2/2)

Multidatabase System (MDBS) DDBMS in which each site maintains complete autonomy.

DBMS that resides transparently on top of existing database

and file systems and presents a single database to its users.

A

llows users to access and share data without requiringphysical database integration.

Unfederated MDBS (where there are no local users) and

federated MDBS.

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Reference Architecture for DDBMS (1/2)

Due to diversity, no accepted architecture equivalentto ANSI/SPARC 3-level architecture.

A reference architecture consists of:

Set of global external schemas.

Global conceptual schema (GCS).

Fragmentation schema and allocation schema.

Set of schemas for each local DBMS conforming to 3-level

ANSI/SPARC.

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Reference Architecture for DDBMS (2/2)

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Components of a DDBMS (1/3)

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Local DBMS Component (LDBMS) A standard DBMS, responsible for controlling the local

data at each site that has a database

It has its own local data dictionary

It is homogeneous system (i.e. the local DBMS componentis the same product)

Data Communication Component (DC)

Software that enables all sites to communicate with eachother

Contains information about the sites and the links

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Global Data Dictionary (GD) Holds information specific to the distributed nature of the

system

Such as fragmentation and allocation schemes

Distributed DBMS

The controlling unit of the entire system

Communication services to provide access to remote sites

Perform distributed query processing, query optimizationand remote data access

Concurrency control in distributed environment

Recovery services in distributed environment

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