1---DistributedDBDesign-Basic Concepts

8/7/2019 1---DistributedDBDesign-Basic Concepts

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Distributed Database Design

BASICS


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Introduction Distributed Database defn:

A logicallly interrelated collection of shareddata, physically distributed over a computernetwork

Distributed DBMS defn:

the software system that permits themanagement of the distributed database andmakes the distribution transparent to users


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Distributed Systems Data spread over multiple machines

(also referred to as sites or nodes).

Network interconnects the machines

Data shared by users on multiplemachines


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Advantages Organisational structure - many

organisations cover several sites Shareability -users at different sites can share Local autonomy each site is able to retain a

degree of control over data stored locally

Improved availability- node failure will notmake system inoperable

Improved Reliability- replicated data allowsdata accessability Improved Performance - data located near

site Modular Growth - easier expansion


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Disadvantages Complexity - more complex than centralised

Cost - added network and maintenance costs

Security - network must be made secure Integrity control - more difficult to ensure

proper coordination among sites.

Lack of standards experience - no tools ormethodologies

Complex Design- Database design morecomplex


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Same software/schema on all sites, data may be

partitioned among sites Goal: provide a view of a single database, hiding

details of distribution

Characteristics All sites use same DBMS product.

Much easier to design and manage.

Approach provides incremental growth and allows

increased performance.

Homogeneous DDBMSClassification of DDBMS


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Different software/schema on different sites

Goal: integrate existing databases to provide

useful functionalityCharacteristics Sites may run different DBMS products, with possibly different

underlying data models.

Occurs when sites have implemented their own databases and

integration is considered later.

Translations required to allow for:

Differenthardware.

DifferentDBMS products.

DifferentHW and differentDBMS products.

Typical solution is to use gateways.

HeterogeneousDDBMS


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Classification Contd

Type of DBMS LAN network WAN network

Homogenous

Heterogeneous

Data managementand financialapplications

Inter-divisionalinformationsystems

Travel managementand finanacialapplications

Integrated bankingand inter-bankingsystems

Examples of typical applications:


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Design Issues with DDBMS In designing a distributed database, the same issues are faced

as for a centralized database plus, in addition:

Fragmentation

Relation may be divided into a number of sub-relations,which are then distributed.

Allocation:

Each fragment is stored at site with "optimal" distribution.

Replication

Copy of fragment may be maintained at several sites.


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Functions of DDBMS

extended communication to allow the transfer of

queries and data among sites extended system catalog to store data distribution

details

distributed query processing , including query

optimisation extended concurrency control to maintain consistency

of replicated data

extended recovery services to take account of failuresof individual sites and comms links

Functions of a Centralised DBMS plus:


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Component Architecture LocalDBMS (LDBMS) - responsible for local data

Transaction, Buffer and Recovery Managers and Scheduler

DataCommunications (DC) component- allows all sites to

communicate with each other lobalsystemcatalog (GSC) - catalog information re:

fragmentation and allocation schema

DistributedDBMS (DDBMS) - controlling unit of the entiresystem

ComponentsofaDDBMS


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Due to diversity, no accepted architecture

equivalent to ANSI/SPARC 3-level

architecture.

A reference architecture consists of:Set of global external schemas. (GES)Global conceptual schema (GCS).Fragmentation schemaAllocation schema.

Set of schemas for each local DBMSconforming to 3-level ANSI/SPARC.

Some levels may be missing, depending on

levels of transparency supported.

Reference Architecture for

DDBMS


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Local and Global Transactions

A local transaction accesses data in the single siteat which the transaction was initiated.

A global transaction either accesses data in a site

different from the one at which the transactionwas initiated or accesses data in several differentsites.


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Implementation Issues for Distributed

Databases Atomicity needed even for transactions that update

data at multiple sites

The two-ph

ase commit protocol (2P

C) is used toensure atomicity

Basic idea: each site executes transaction until just beforecommit, and the leaves final decision to a coordinator

Each site must follow decision of coordinator, even if there is afailure while waiting for coordinators decision

Distributed concurrency control (and deadlock detection)required

Data items may be replicated to improve data availability


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DDBMS Network Types Local-areanetworks (LANs) composed

of processors that are distributed over small

geographical areas, such as a singlebuilding or a few adjacent buildings.

Wide-areanetworks (WANs) composed of processors distributed over alarge geographical area.


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Networks Types (Cont.) WANs with continuous connection (e.g. the

Internet) are needed for implementing distributed

database systems Groupware applications such as Lotus notes can

work on WANs with discontinuous connection:

Data is replicated.

Updates are propagated to replicas periodically.

Copies of data may be updated independently.

Non-serializable executions can thus result.

Resolution is application dependent.


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Other CategoriesOpen Database Access and Interoperability

Open Group has formed a Working Group to provide specifications thatwill create database infrastructure environment where there is:

Common SQL API that allows client applications to be written that donot need to know vendor of DBMS they are accessing.

Common database protocol that enables DBMS from one vendor tocommunicate directly with DBMS from another vendor without theneed for a gateway.

A common network protocol that allows communications betweendifferent DBMSs.

Most ambitious goal is to find a way to enable transaction to span DBMSsfrom different vendors without use of a gateway.


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DDBMS in which each site maintains complete

autonomy. DBMS that resides transparently on top of existing

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

Allows users to access and share data withoutrequiring physical database integration.

Two Categories: Unfederated MDBS (no local

users) and

federate

dMDBS.

MultiDatabase System (MDBS)Other Categories Contd..

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1---DistributedDBDesign-Basic Concepts