distributed databases
DESCRIPTION
This gives the idea of distributed databases and its related conceptsTRANSCRIPT
SUSHIL SUSHIL
KULKARNIKULKARNI
DISTRIBUTED DISTRIBUTED DBMSDBMS
DDBMS ConceptsDDBMS Concepts ApplicationsApplications Characteristics, Properties of DDBMSCharacteristics, Properties of DDBMS Distributed ProcessingDistributed Processing Advantages & Disadvantages DDBMSAdvantages & Disadvantages DDBMS Types & Functions of DDBMSTypes & Functions of DDBMS Main Issues of DDBMSMain Issues of DDBMS Component Architecture for DDBMSComponent Architecture for DDBMS Data Allocation & FragmentationData Allocation & Fragmentation TransparenciesTransparencies
CONCEPTSCONCEPTS
CONCEPTSCONCEPTSCONCEPTSCONCEPTS• So far, we assume a centralized database
Data are stored in one location (e.g. a single
hard disk) A centralized database management system to
handle transaction To handle multiple requests, a client-server
system is used
- Client send requests for data to server
- Server handle query, transaction management etc.
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• This is not the only possibility• In many cases, it may be advantageous
for data to be distributed– Branches of a bank– Different part of the government storing
different kind of data about a person– Different organizations sharing part of their
data
• Thus, distributed databases
CONCEPTSCONCEPTSCONCEPTSCONCEPTS
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• Data spread over multiple machines (also referred to as sites or nodes.
• Network interconnects the machines• Data shared by users on multiple machines
CONCEPTSCONCEPTSCONCEPTSCONCEPTS
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CONCEPTSCONCEPTSCONCEPTSCONCEPTS
Distributed database
Logical interrelated collection of shared data, along with description of data, physically distributed over a computer network.
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CONCEPTSCONCEPTSCONCEPTSCONCEPTS
Distributed DBMS
The software system that permits the management of the distributed database and makes the distribution transparent to users
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CONCEPTSCONCEPTSCONCEPTSCONCEPTS
Applications
• User access distributed database via
applications
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CONCEPTSCONCEPTSCONCEPTSCONCEPTS
TWO types of Applications
• Local application : Application that do not
required data from other sites.
• Global application : Application that required
data from other sites.
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• In a homogeneous distributed database:
– All sites have identical software.– Are aware of each other and agree to
cooperate in processing user requests.– Each site surrenders part of its autonomy
in terms of right to change schemas or software.
– Appears to user as a single system.
TYPES OF DDBMSTYPES OF DDBMSTYPES OF DDBMSTYPES OF DDBMS
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• In a heterogeneous distributed database:
– Different sites may use different schemas and software.• Difference in schema is a major problem for
query processing.• Difference in software is a major problem for
transaction processing.– Sites may not be aware of each other and may
provide only limited facilities for cooperation in
transaction processing.
TYPES OF DDBMSTYPES OF DDBMSTYPES OF DDBMSTYPES OF DDBMS
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Identical DBMSs
TYPE: HOMOGENEOUS DBMSTYPE: HOMOGENEOUS DBMSTYPE: HOMOGENEOUS DBMSTYPE: HOMOGENEOUS DBMS
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Non-identical DBMSs
TYPE: HETROGENEOUS DBMSTYPE: HETROGENEOUS DBMSTYPE: HETROGENEOUS DBMSTYPE: HETROGENEOUS DBMS
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• Location Transparency – User does not have to know the location of the data.– Data requests automatically forwarded to appropriate
sites
• Local Autonomy – Local site can operate with its database when
network connections fail– Each site controls its own data, security,
logging, recovery
OBJECTIVES : DISTRIBUTED OBJECTIVES : DISTRIBUTED ARCHITECTUREARCHITECTURE
OBJECTIVES : DISTRIBUTED OBJECTIVES : DISTRIBUTED ARCHITECTUREARCHITECTURE
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Synchronous Distributed Database• All copies of the same
data are always identical
• Data updates are immediately applied to all copies throughout network
• Good for data integrity• High overhead slow
response times
• Asynchronous Distributed Database• Some data inconsistency
is tolerated• Data update propagation
is delayed• Lower data integrity• Less overhead faster
response time
NOTE: all this assumes replicated data (to be discussed later)
SIGNIFICANT TRADE -OFFSIGNIFICANT TRADE -OFFSIGNIFICANT TRADE -OFFSIGNIFICANT TRADE -OFF
Advantages & DisadvantagesAdvantages & Disadvantages
Advantages• Increased reliability
& availability• Local control• Modular growth• Lower
communication costs
• Faster response
Disadvantages• Software cost &
complexity• Processing overhead• Data integrity• Slow response
DISTRIBUTED PROCESSINGDISTRIBUTED PROCESSINGDISTRIBUTED PROCESSINGDISTRIBUTED PROCESSING
A centralized database that can be accessed over a computer network.
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DISTRIBUTED PROCESSINGDISTRIBUTED PROCESSINGDISTRIBUTED PROCESSINGDISTRIBUTED PROCESSING
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T T T
COM 1
T T T
COM 2
Communication Network
T T T
COM 3
DB
FUNCTIONS OF DDBMSFUNCTIONS OF DDBMSFUNCTIONS OF DDBMSFUNCTIONS OF DDBMSFunctions of a centralized DBMS plus:
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
optimization
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FUNCTIONS OF DDBMSFUNCTIONS OF DDBMSFUNCTIONS OF DDBMSFUNCTIONS OF DDBMS
extended concurrency control to maintain
consistency of replicated data.
extended recovery services to take account
of failures of individual sites and common
links
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TWO MAIN ISSUES IN DDBMSTWO MAIN ISSUES IN DDBMSTWO MAIN ISSUES IN DDBMSTWO MAIN ISSUES IN DDBMS
Making query from one site to the same or
remote site.
Logical database is partitioned in to different
data streams and located at different sites.
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COMPONENT ARCHITECTURE FOR COMPONENT ARCHITECTURE FOR DDBMSDDBMS
COMPONENT ARCHITECTURE FOR COMPONENT ARCHITECTURE FOR DDBMSDDBMS
• Local DBMS
• Data Communication Component
• Global System Catalog
• Distributed DBMS component
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DATA DATA ALLOCATIONALLOCATION
DATA ALLOCATIONDATA ALLOCATIONDATA ALLOCATIONDATA ALLOCATION
• Centralized
• Fragmented
• Complete replication
• Selective replication
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Distributed Data StorageDistributed Data Storage
• Assume relational data model.• Replication:
– System maintains multiple copies of data, stored in different sites, for faster retrieval and fault tolerance.
• Fragmentation:– Relation is partitioned into several fragments stored in
distinct sites
• Replication and fragmentation can be combined:– Relation is partitioned into several fragments: System
maintains several identical replicas of each such fragment.
Data ReplicationData Replication
• A relation or fragment of a relation is replicated if it is stored redundantly in two or more sites.
• Full replication of a relation is the case where the relation is stored at all sites.
• Fully redundant databases are those in which every site contains a copy of the entire database.
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Data Replication (Cont.)
• Advantages of Replication:– Availability: failure of site containing relation r
does not result in unavailability of r is replicas exist.
– Parallelism: queries on r may be processed by several nodes in parallel.
– Reduced data transfer: relation r is available locally at each site containing a replica of r.
Data ReplicationData Replication
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Data Replication (Cont.)
• Disadvantages of Replication– Increased cost of updates: each replica of
relation r must be updated.
– Increased complexity of concurrency control: concurrent updates to distinct replicas may lead to inconsistent data unless special concurrency control mechanisms are implemented.
• One solution: choose one copy as primary copy and apply concurrency control operations on primary copy.
Data ReplicationData Replication
Data FragmentationData Fragmentation
• Division of relation r into fragments r1, r2, …, rn which contain sufficient information to reconstruct relation r.
• Horizontal fragmentation: each tuple of r is assigned to one or more fragments.
• Vertical fragmentation: the schema for relation r is split into several smaller schemas.– All schemas must contain a common candidate key (or superkey)
to ensure lossless join property.– A special attribute, the tuple-id attribute may be added to each
schema to serve as a candidate key.
• Example : relation account with following schema.• Account-schema = (branch-name, account-number,
balance).
-Fragments contain subsets of complete tuples (all attributes at all sites)
How to reconstruct R= Rs1 Rs2 ……. Rsn
HORIZONTAL FRAGMENTATIONHORIZONTAL FRAGMENTATION
Original relation
A1 A2 ………. An1
1
1
2
2
3
3
3
T1T1
T2T2
T3T3
..
.T60.T60
T61T61
..
..
TnTn
A1 A2 ………. An
A1 A2 ………. AnT1
T2
T3
.
.T60
T61
.
.
Tn
Site 1
Site 2
A1 A2 A3 A4
A1 A2 A3 A4
Original Relation (R) t1
t2
tn
RS1
RS2
t1
t2
tn
t1
t2
tn
SITE1 SITE2
How to Reconstruct:
R=Rs1 Rs2 Rsn
TID –Tuple ID Hidden Attribute to
ensure account and simple join reconstruction
RS1.TID=RS2.TID
Join condition
1
2
n
1
2
n
TID TID
VERTICAL FRAGMENTATIONVERTICAL FRAGMENTATION
A1 A2 A3 A4
A1 A2 A3 A4
Original Relation (R) t1
t2
tn
RS1
RS2
t1
t2
tn
t1
t2
tn
SITE1 SITE2
How to Reconstruct:
R=Rs1 Rs2 Rsn
TID –Tuple ID Hidden Attribute to
ensure account and simple join reconstruction
RS1.TID=RS2.TID
Join condition
1
2
n
1
2
n
TID TID
VERTICAL FRAGMENTATIONVERTICAL FRAGMENTATION
usa
Europe
A1 A2 A3
A1 A2 A3
A4 A5
A4 A5
A1 A2 A3 A4 A5
(Salary Attributes)
(Benefit Attributes)
Rs1
Rs2
Rs3
Rs4
R
MIXED FRAGMENTATIONMIXED FRAGMENTATION
A1 A2 A3 A4
A1 A2 A3 A4
Original Relation (R) t1
t2
tn
RS1
RS2
t1
t2
tn
t1
t2
tn
SITE1 SITE2
How to Reconstruct:
R=Rs1 Rs2 Rsn
TID –Tuple ID Hidden Attribute to
ensure account and simple join reconstruction
RS1.TID=RS2.TID
Join condition
1
2
n
1
2
n
TID TID
MIXED FRAGMENTATIONMIXED FRAGMENTATION
Horizontal Fragmentation of Horizontal Fragmentation of accountaccount RelationRelation
branch-name account-number balance
HillsideHillsideHillside
A-305A-226A-155
50033662
account1=branch-name=“Hillside”(account)
branch-name account-number balance
ValleyviewValleyviewValleyviewValleyview
A-177A-402A-408A-639
205100001123750
account2=branch-name=“Valleyview”(account)
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branch-name customer-name tuple-id
HillsideHillsideValleyviewValleyviewHillsideValleyviewValleyview
LowmanCampCampKahnKahnKahnGreen
deposit1=branch-name, customer-name, tuple-id(employee-info)
1234567
account number balance tuple-id
50033620510000621123750
1234567
A-305A-226A-177A-402A-155A-408A-639
deposit2=account-number, balance, tuple-id(employee-info)
Vertical Fragmentation of Vertical Fragmentation of employee-info employee-info RelationRelation
Advantages of FragmentationAdvantages of Fragmentation
• Horizontal:– allows parallel processing on fragments of a relation– allows a relation to be split so that tuples are located where
they are most frequently accessed
• Vertical: – allows tuples to be split so that each part of the tuple is stored
where it is most frequently accessed– tuple-id attribute allows efficient joining of vertical fragments– allows parallel processing on a relation
• Vertical and horizontal fragmentation can be mixed.– Fragments may be successively fragmented to an arbitrary
depth.
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Partition of Attributes/tuples need not be disjoint
REPLICATION and FRAGMENTATION
A1 A2 A3 A4 A5
A1 A2 A3 A4 A2 A3 A4 A5
Overlap
(replication of attributes)
TRANSPARENCIESTRANSPARENCIES
TRANSPARENCIES IN DDBMSTRANSPARENCIES IN DDBMSTRANSPARENCIES IN DDBMSTRANSPARENCIES IN DDBMS
• Transparencies hide implementation details from the user
• Example in Centralized databases : Data independence
• Main types of transparencies in DDBMS:
o Distributed Transparency
o Transaction Transparency
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DISTRIBUTED TRANSPARENCYDISTRIBUTED TRANSPARENCYDISTRIBUTED TRANSPARENCYDISTRIBUTED TRANSPARENCY
Allows the user to see the database as a
single, logical entity.
If this transparency is exhibited then the
user does not need to know that
1. The data are partitioned.
2. Data can be replicated at several
sites.
3. Data location.
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EXAMPLEEXAMPLEEXAMPLEEXAMPLE
Staff (staffNo, position, sex, dob, salary,
fName, lName, branchNo)
Vertical fragmentation:
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(Staff)dbranchNo,lNamefName,staffNo,Π2S
(Staff)salarydob,,sexposition,staffNo,Π1S
EXAMPLEEXAMPLEEXAMPLEEXAMPLE
Fragment S 2 according to branch number.
Assume that there are only three branches.
Horizontal fragmentation:
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(Staff)'007'23
S
(Staff)'005'22
S
(Staff)'003'21
S
BbranchNo
BbranchNo
BbranchNo
EXAMPLEEXAMPLEEXAMPLEEXAMPLE
Assume that :
S 1 and S 2 are at site 5,
S 21 at site 3
S 22 at site 5
S 23 at site 7
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FRAGMENTATION TRANSPARENCYFRAGMENTATION TRANSPARENCYFRAGMENTATION TRANSPARENCYFRAGMENTATION TRANSPARENCY
If it is provided then the user does not need
to know the data is fragmented.
Example:
SELECT fName, lName
FROM Staff
WHERE position = ‘ Manager ’
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LOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCY
If it is provided then the user must know
how the data has been fragmented but still
does not have know the location of the data.
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LOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCY
Example:SELECT fName, lName
FROM S21
WHERE staffNo IN (SELECT staffNO FROM S1 where
position = ‘ Manager ’)
UNION
SELECT fName, lName
FROM S22
WHERE staffNo IN (SELECT staffNO FROM S1 where
position = ‘ Manager ’)SUSHIL KULKARNISUSHIL KULKARNI
LOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCY
Example:
UNION
SELECT fName, lName
FROM S23
WHERE staffNo IN (SELECT staffNO FROM S1 where
position = ‘ Manager ’ )
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LOCAL MAPPING TRANSPARENCYLOCAL MAPPING TRANSPARENCYLOCAL MAPPING TRANSPARENCYLOCAL MAPPING TRANSPARENCY
If it is provided then the user must know
how the data has been fragmented as well
as the location of the data.
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LOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCY
Example:SELECT fName, lName
FROM S21 AT SITE 3
WHERE staffNo IN (SELECT staffNO FROM S1 AT SITE 5
where position = ‘ Manager ’)
UNION
SELECT fName, lName
FROM S22 AT SITE 5
WHERE staffNo IN (SELECT staffNO FROM S1 AT SITE 3
where position = ‘ Manager ’)SUSHIL KULKARNISUSHIL KULKARNI
LOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCYLOCATION TRANSPARENCY
Example:
UNION
SELECT fName, lName
FROM S23 AT SITE 7
WHERE staffNo IN (SELECT staffNO FROM S1 AT SITE 3
where position = ‘ Manager ’ )
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TRANSACTION TRANSPARENCYTRANSACTION TRANSPARENCYTRANSACTION TRANSPARENCYTRANSACTION TRANSPARENCY
It maintains distributed database’s integrity
and consistency.
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Issues 1:
Parallel Processing across Fragments
LNameLName((salary>40,000salary>40,000((EmployeeEmployee))))
LNameLName( ( salary>40,000(Emp1))salary>40,000(Emp1)) UU LName( LName( salary>40,000(Emp2))salary>40,000(Emp2))
QUERY PROCESSING IN DDMSQUERY PROCESSING IN DDMS
=Emp1 U Emp2
2 Fragments
Site 1 Site 2
Execution in Parallel on fragments
and union results togetherand union results together
Horizontal Horizontal fragmentationsfragmentations
(A B) C
A (B C)
Site1 Site2 Site3 Joins- symmetric and associative
Parallel Processing
(xx(A)) (B C)
QUERY PROCESSING IN DDMSQUERY PROCESSING IN DDMS
R= Fnames, Cnames, Dnames (Employee Department)
Strategies:
1)Ship both relations to the result site and join there
2)Ship employee to 2, join at 2, results to 3
3)Ship Department to 1, join at 1, results to 3
minimize total communication cost of data transfer
1,003,000 1,003,000 bytes bytes
transferedtransfered1,002,000 1,002,000
bytes bytes transferedtransfered5,000 bytes 5,000 bytes transferedtransfered
Join StrategiesJoin Strategies
Site 3Site 3
100 records, 2000 bytes100 records, 2000 bytes
Site 1Site 1
10,000 records, 10,000 records, 1,000,000 bytes1,000,000 bytes
Site 2Site 2
100 records, 3000 100 records, 3000 bytesbytes
Mg rssn to ssn
QUERY PROCESSING IN DDMSQUERY PROCESSING IN DDMS
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