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Chapter 7

Distributed Databases system

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Distributed DatabaseConcepts

• A transaction can be e&ecuted by multiplenet'or(ed computers in a uni)ed manner.

• A distributed database (DDB) processes*nit o! e&ecution +a transaction in adistributed manner. A distributed database+DD- can be de)ned as – A distributed database +DD- is a collection o!

multiple logically related database distributed oer

a computer net'or(, and a distributed databasemanagement system as a so!t'are system thatmanages a distributed database 'hile ma(ing thedistribution transparent to the user.

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Distribution ransparency

• An important goal o! a distributedsystem is to hide the !act that itsprocesses and resources are physically

distributed across multiple computers.• A distributed system that is able to

present itsel! to users and applications

as i! it 'ere only a single computersystem is said to be transparent.

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Distribution ransparency

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Distributed Database"ystem

• Adantages +transparency, contd. – he 40%5O644, %RO4C, and

8OR9":O; tables may be !ragmented

hori

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Distributed Database"ystem

• Adantages +transparency, contd. – Distribution and Network

transparency

• *sers do not hae to 'orry aboutoperational details o! the net'or(.

 – here is 5ocation transparency, 'hich re!ers to!reedom o! issuing command !rom any location'ithout aecting its 'or(ing.

 – hen there is ;aming transparency, 'hich allo'saccess to any names ob=ect +)les, relations, etc.!rom any location.

>

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Distributed Database"ystem

• Other Adantages – Increased reliability and

availability

• Reliability re!ers to system lie time, that is,system is running eBciently most o! thetime. Aailability is the probability that thesystem is continuously aailable +usable or

accessible during a time interal.• A distributed database system has multiple

nodes +computers and i! one !ails thenothers are aailable to do the =ob.

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Data Fragmentation, Replication andAllocation

• Data Fragmentation – "plit a relation into logically related and

correct parts. A relation can be

!ragmented in t'o 'ays• !ori"ontal Fragmentation

• #ertical Fragmentation

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Data Fragmentation, Replication andAllocation

• #ertical fragmentation

 – ?t is a subset o! a relation 'hich is created by asubset o! columns. hus a ertical !ragment o! arelation 'ill contain alues o! selected columns.

 here is no selection condition used in ertical!ragmentation.

 – Consider the 4mployee relation. A ertical!ragment o! can be created by (eeping the alueso! ;ame, -date, "e&, and Address.

 – -ecause there is no condition !or creating a ertical!ragment, each !ragment must include the primary(ey attribute o! the parent relation 4mployee. ?nthis 'ay all ertical !ragments o! a relation are

connected. 1#

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Data Fragmentation, Replication andAllocation

• Representation

 – !ori"ontal fragmentation

• 4ach hori

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Data Fragmentation, Replication andAllocation

• Fragmentation sc%ema

 – A de)nition o! a set o! !ragments +hori

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Data Fragmentation, Replication andAllocation

• Data Replication

 – Database is replicated to all sites.

 – ?n !ull replication the entire database is replicatedand in partial replication some selected part is

replicated to some o! the sites. – Data replication is achieed through a replication

schema.

• Data Distribution (Data llocation)

 –  his is releant only in the case o! partialreplication or partition.

 –  he selected portion o! the database is distributedto the database sites.

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 ypes o! Distributed Database "ystems

• Homogeneous – All sites o! the

database system haeidentical setup, i.e.,same database system

so!t'are. –  he underlying

operating system maybe dierent.

• For e&ample, all sitesrun Oracle or D-2, or"ybase or some otherdatabase system.

 –  he underlyingoperating systems canbe a mi&ture o! 5inu&,8indo', *ni&, etc.

2

Site 5

Site 1

Site 2Site 3Oracle Oracle

Oracle

Oracle

Site 4

Oracle

LinuxLinux

Window

Window

Unix

Communications

network 

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 ypes o! Distributed Database "ystems

• Heterogeneous – Federated 4ach site may run dierent database system

but the data access is managed through a singleconceptual schema.

•  his implies that the degree o! local autonomy is minimum.

4ach site must adhere to a centrali

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$uery %rocessing in DistributedDatabases

• ?ssues

 – Cost o! trans!erring data +)les and results oer thenet'or(.

•  his cost is usually high so some optimi

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$uery %rocessing in DistributedDatabases

• Result – he result o! this @uery 'ill hae 1,

tuples, assuming that eery employee is

related to a department. – "uppose each result tuple is # bytes

long. he @uery is submitted at site 3and the result is sent to this site.

 – %roblem 4mployee and Departmentrelations are not present at site 3.

2#

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$uery %rocessing in DistributedDatabases

• "trategies

1. rans!er 4mployee and Department to site 3.•  otal trans!er bytes 1,, I 3/ 1,3,/ bytes.

2. rans!er 4mployee to site 2, e&ecute =oin at site 2 and

send the result to site 3.• $uery result si

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$uery %rocessing in DistributedDatabases

• Consider the @uery – $K For each department, retriee the

department name and the name o! the

department manager• Relational Algebra e&pression

 –   ΠFname,5name,Dname +4mployee 0grssn ""; 

Department

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$uery %rocessing in DistributedDatabases

•  he result o! this @uery 'ill hae 1 tuples,assuming that eery department has a manager,the e&ecution strategies are1. rans!er 4mployee and Department to the result

site and per!orm the =oin at site 3.•  otal bytes trans!erred 1,, I 3/

1,3,/ bytes.

2. rans!er 4mployee to site 2, e&ecute =oin at site 2and send the result to site 3. $uery result si

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$uery %rocessing in DistributedDatabases

• ;o' suppose the result site is 2.%ossible strategies 1. rans!er 4mployee relation to site 2,

e&ecute the @uery and present theresult to the user at site 2.•  otal trans!er si

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$uery %rocessing in DistributedDatabases

• "emi=oin – Ob=ectie is to reduce the number o! tuples in a

relation be!ore trans!erring it to another site.• 4&ample e&ecution o! $ or $K

1. %ro=ect the =oin attributes o! Department at site 2,and trans!er them to site 1. For $, # J 1 #bytes are trans!erred and !or $K, J 1 bytes are trans!erred.

2. oin the trans!erred )le 'ith the 4mployee relationat site 1, and trans!er the re@uired attributes !rom

the resulting )le to site 2. For $, 3# J 1, 3#, bytes are trans!erred and !or $K, 3 J 1 3 bytes are trans!erred.

3. 4&ecute the @uery by =oining the trans!erred )le'ith Department and present the result to the userat site 2.

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