UNIFOR University of Fortaleza Brazil University of Kaiserslautern Germany Angelo Brayner CoopIS - Trento, September 20011 Global Semantic Serializability:

UNIFOR

University of

FortalezaBrazil

Angelo Brayner CoopIS - Trento, September 2001 1

University of Kaiserslautern

Germany

Global Semantic Serializability: An Approach to Increase Concurrency in

Multidatabase Systems

Angelo Brayner

Theo Härder


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Contents

MotivationMultidatabase System ModelGlobal Semantic SerializabilityConcurrency Control ProtocolsConclusions


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Motivation (1)

Integration of heterogeneous databases is a strategic requirement Integration of heterogeneous databases

in a enterprise Integration of heterogeneous web

databases Web as a large collection of distributed

autonomous and heterogeneous databases Integration of ubiquitous databases

mobile heterogeneous databases providing data everywhere


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Motivation (2)

Multidatabase technology Efficient solution for integrating a

collection of autonomous and heterogeneous databases Local databases

Created independently without considering the possibility of being integrated in the future

Operate autonomously Local autonomy is a key feature

Multidatabase Collection of local databases


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Motivation (3)

Multidatabase System (MDBS) Software component to manage a

multidatabase Provides DBMS functionalities

Multidatabase environment Global transactions

Submitted to the MDBS Access and update local database objects

Local transactions Submitted to local database systems


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Motivation (4)

Classical transaction-processing Model "Syntactic" serializability

Serialization order of all active transactions must be known

For identifying correct execution of concurrent transactions

Efficient criterion for synchronizing operations of short transactions


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Motivation (5)

Concurrency control problem in MDBSs Global transactions

Involve operations on multiple local databases Long-living transactions

MDBS does not have any information about the execution (serialization) order of local transactions

Classical transaction model is inefficient for solving the CC problem in MDBSs


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Multidatabase System Model (1)

LDBSn

LocalTransactions

SUBjnSUBin

Global RecoveryManager

GlobalLog

Global TransactionManagerMDBS

GjGi

Global Transactions

Global Scheduler

InterfaceServer 1

Log LogInterfaceServer n

DB

DBMS

LocalTransactions

SUBi1 SUBj1

LDBS1DB

DBMS


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MDBS1. A set LD={LDBS1, LDBS2, … ,

LDBSn} of local database systems

2. A set L={L1, L2, … , Ln} Each LK represents a set of local

transactions executed at LDBSK

3. A set G={G1, G2, … , Gn} of global transactions


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Local Schedule SK Models the execution of interleaved

operations belonging to local and global transactions Executed at LDBSK

Global Schedule SG

Models the execution of all local schedules


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GS-Serializability Model (1)

Assumptions An MDBS integrates a collection of pre-

existing local databases (LDBs) A collection of disjoint sets of objects

Each set represents a single local database Semantic Unit

An update operation executed by a global An update operation executed by a global transaction G on an object of a particular transaction G on an object of a particular semantic unit does not depend on values of semantic unit does not depend on values of objects belonging to other semantic units objects belonging to other semantic units previously read by Gpreviously read by G


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Module-structured Transaction Operations are grouped into subsequences

Modules Encompasses operations on objects of only one

semantic unit Example

DB={A, B, C, D, E, F, G} SULDBS1={A, B, CA, B, C} SULDBS2={D, E, F, GD, E, F, G}

T1= r1(G) w1(E) w1(C) r1(B)

T2= r2(G) w2(C) w2(E) r1(B)Module


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GS-Serial Global Schedule Local schedules are conflict serializable

and Serial execution of modules belonging to

global transactions ExampleG1=r1(G)w1(E)w1(CC)r1(BB); G2=r2(AA)w2(BB)w2(D)r2(E)

SC= r2(AA)w2(BB)r1(G)w1(E)w2(D)r2(E)w1(CC)r1(BB)

SSCC is is GSGS-Serial-Serial SSCC is not conflict serializable is not conflict serializable


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GS-Serial Schedules preserve multidatabase consistency Correctness criterion for MDBSs

GS-Serializable Schedule S Local schedules are conflict serializable

and The execution order of global

transactions in S is conflict equivalent to the execution of a GS-Serial schedule over the same set of transactions


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Identifying GS-Serializable Schedule Since existing DBMSs yield conflict

serializable schedulesThe GTM has solely to verify the

execution order of global transactions

A graph-based method The Semantic Serialization Graph (SSG)


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Concurrency Control in MDBSs

Concurrency Control Protocols Conservative

Based on a locking mechanism Aggressive

Management of an always acyclic graph Based on the SSG


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Conclusions

GS-Serializability Model Increases concurrency in MDBSs

More permissive than syntactic serializability Increases concurrency in mediator-

based systems Each web database can be seen as a

semantic unit Can be applied to control concurrency in

ubiquitous database Mobile database can be defined as a

semantic unit

Documents

UNIFOR University of Fortaleza Brazil University of Kaiserslautern Germany Angelo Brayner CoopIS - Trento, September 20011 Global Semantic Serializability: