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Transcript of ADB Chapter 7
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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&le, all sitesrun Oracle or D-2, or"ybase or some otherdatabase system.
– he underlyingoperating systems canbe a mi&ture o! 5inu&,8indo', *ni&, etc.
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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&le 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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