Distributed database, Study notes of Distributed Database Management Systems

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Distributed DatabasesUnit 1 - IntroductionUnit 1^ Introduction

Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

Distributed^ Databases • Distributed^ database^ is^ a^ collection

of^ data which belong logically to the same system butwhich^ belong^ logically^

to^ the^ same^ system^ but are^ spread^ over^ the^ sites

of^ computer^ network. 1 Distribution^ : The fact that the data are not1.^ Distribution^ :‐^ The^ fact

that^ the^ data^ are^ not resident^ at^ the^ same^ site. 2 L^ i^ l^ l ti^ Th

f^ t th t th^ d t^ h

2.^ Logical^ correlation^ :

‐^ The^ fact^ that^ the^ data

have some^ properties^ which

tie^ them^ together. Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

DB1^ DB2T

T

Branch^1

Branch^2 Computer^ Center T T T Comp 1 Comp^2

T T p^ p Local^ N/W Comp^3 DB3 A distributed databaseA distributed database on a local networkT^ T^ T Branch^3 Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

Features^ of^ Distributed

VS^ Centralized^ Databases

-^ Centralized^ ControlD t^ i d^ d •^ Data^ independence •^ Reduction^ of^ redundancy •^ Complex^ physical^ structures

and^ efficient^ accessExample

-^ Integrity,^ Recovery^ and

Concurrency^ Control

-^ Privacy & Security •^ Privacy^ &^ Security

Next^5 Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

1. At^ site^1 Send^ sites^2 and^3 the

supplier^ number^ SN

2. At sites 2 and 32)^ At^ sites^2 and^3 execute^ in^ parallel,^

upon^ receipt^ of^ the^ supplier number the following program:number, the following^ program: Find all PARTS records^ having^ SUP#=SN;Send result to site^ 1. 3)^ At^ site^1 Merge^ results^ from^

sites^2 and^ 3; BACKBACK Output the result (^) Output the result. Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

Why Distributed DatabasesWhy^ Distributed^

Databases

-^ Organizational and economic reasonsOrganizational^ and^ economic

reasons.

-^ Interconnection^ of^ existing

databases.l G h

-^ Incremental^ Growth. •^ Reduced^ communication

overhead.

-^ Performance^ considerations. •^ Reliability and AvailabilityReliability^ and^ Availability .^8 Vijaykumar Mantri, Assoc. Prof.^ in^ IT,^ BVRIT

Global SchemaSiteFragmentation^ Independent Schema^ SchemaAllocationReference^ Schema

Architecture^ for Distributed Databases Local Local Mapping M i Distributed^ Databases Mapping Mapping Schema2 Schema1DBMS of Site 1 DBMS^ of^ Site^2 Local^ DB^ atLocal DB at^ site^2 site 1 Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

(^1) (R^1 R 1 )^1 R 1 (^1) (R^2 R (Site^ 1)) (^) R^2 R (^2) (R)^1 2 R (Site^ 2)(^ ) (^2) (R)^2 R 3 Physical ImagesPhysical^ ImagesR 34 (R)^2 33 R (Site^ 3)^3 (R)^3 Global^ Relation^ Fragments

(^3) (R)^4^11 Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

Following^ are^ the^ rules

which^ must^ be^ followed C l C di i^ ll h^ d^ f h

-^ Completeness^ Condition

:‐^ All^ the^ data^ of^ the global^ relation^ must^ be

mapped^ into^ fragments.

-^ Reconstruction^ Condition

:‐^ It^ must^ always possible^ to^ reconstruct

each^ global^ relation from^ its^ fragments. • Disjointness^ Condition

:‐^ It^ is^ convenient^ that j fragments^ be^ disjoint,^

so^ that^ the^ replication^

of data^ can^ be^ controlled^

at^ each^ allocation^ level. Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

Horizontal^ Fragmentation • Horizontal^ Fragmentation

:‐^ Horizontal Fragmentation^ consists

of^ partitioning^ the tuples of a global relation into subsets, whereeach^ subset^ can^ contain

data^ which^ have common geographical properties. • This^ can^ be^ defined

by^ using^ selectiony^ g operation on the global relation. Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

Derived^ Horizontal

Fragmentation

S^ ti^ H^ i^ t l F

t ti^ b

-^ Sometimes^ Horizontal

Fragmentation^ may^ be derived from Horizontal^ Fragmentation^ of th l tianother relation Eg:‐^ Consider^ the^ global

relationSUPPLY(SNUM, PNUM,^ DEPTNUM,^ QUAN) We can fragment the tuples for suppliers which areWe^ can^ fragment^ the^ tuples

for^ suppliers^ which^ are in^ given^ city.^ The^ derived

fragmentation^ can^ be defines^ asde^ es as SUPPLY=^ SUPPLY^ SJ^1 SNUM

SUPPLIER = SNUM 1

SUPPLY^ SUPPLY^ SJ^

SUPPLIER

SUPPLY=^ SUPPLY^ SJ^2 SNUM

SUPPLIER = SNUM 2^16 Vijaykumar Mantri, Assoc. Prof. in IT, BVRIT

Vertical^ Fragmentation • The Vertical Fragmentation of a global relation isthe subdivision of its attributes into groups. • The fragmentation is correct if each attributes ismapped^ into^ at^ least^ one^

attribute^ of^ the ppfragments. • It must^ be^ possible^ to^ reconstruct

the^ original It^ must^ be^ possible^ to

reconstruct^ the^ original relation by joining fragments. Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

Mixed^ FragmentationEg:‐ Consider^ the^ global^ schema.EMP(EMPNUM,^ NAME,^ SAL,^ TAX,^ MGRNUM,

DEPTNUM)

-^ The^ following^ is^ a^ mixed

fragmentation^ obtained by^ vertical^ fragmentation

followed^ by^ horizontal y^ g^

y fragmentation Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT

•^ EMP=^ SL^1 DEPTNUM<=

PJ^ EMPNUM,^ NAME,^ MGRNUM, EMPEMP DEPTNUM

•^ EMP=^ SL^2 10 DEPTNUM

PJ^ EMP SL PJ^20 EMPNUM NAME MGRNUM 2 10 <DEPTNUM<=20 EMPNUM,^ NAME,^ MGRNUM

, EMP DEPTNUM (^) • EMP= SL^ PJ 3 DEPTNUM>^ EMPNUM,^ NAME,^ MGRNUM, EMPEMP DEPTNUM (^) DEPTNUM (^) • EMP= PJ^4 EMPNUM NAME SAL TAX

EMP

4 EMPNUM,^ NAME,^ SAL,^ TAX Vijaykumar^ Mantri,^ Assoc.^ Prof.^ in^ IT,^ BVRIT