UNIFOR
Global Semantic Serializability: An
Approach to Increase Concurrency in
Multidatabase Systems
Angelo Brayner
University of
Fortaleza
Brazil
 Angelo Brayner
Theo Härder
University of
Kaiserslautern
Germany
CoopIS - Trento, September 2001
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Contents
UNIFOR
Motivation
Multidatabase System Model
Global Semantic Serializability
Concurrency Control Protocols
Conclusions
 Angelo Brayner
CoopIS - Trento, September 2001
2
Motivation (1)
UNIFOR
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
 Angelo Brayner
CoopIS - Trento, September 2001
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Motivation (2)
UNIFOR
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

 Angelo Brayner
Collection of local databases
CoopIS - Trento, September 2001
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Motivation (3)
UNIFOR
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
 Angelo Brayner
CoopIS - Trento, September 2001
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Motivation (4)
UNIFOR
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
 Angelo Brayner
CoopIS - Trento, September 2001
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Motivation (5)
UNIFOR
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
 Angelo Brayner
CoopIS - Trento, September 2001
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Multidatabase System Model (1)
UNIFOR
Global
Transactions
Gi
Gj
MDBS
Global
Scheduler
Interface
Server 1
Local
Transactions
SUBi1
SUBj1
Interface
Server n
SUBin
DBMS
DB
 Angelo Brayner
Log
Global Recovery
Manager
LDBS1
DB
CoopIS - Trento, September 2001
Global
Log
Log
SUBjn
DBMS
Global Transaction
Manager
Local
Transactions
LDBSn
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Multidatabase System Model (2)
UNIFOR
MDBS
1. 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
 Angelo Brayner
CoopIS - Trento, September 2001
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Multidatabase System Model (3)
UNIFOR
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
 Angelo Brayner
CoopIS - Trento, September 2001
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GS-Serializability Model (1)
UNIFOR
Assumptions


An MDBS integrates a collection of preexisting 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
transaction G on an object of a particular semantic
unit does not depend on values of objects
belonging to other semantic units previously read
by G
 Angelo Brayner
CoopIS - Trento, September 2001
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GS-Serializability Model (2)
UNIFOR
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, C}
 SULDBS2={D, E, F, G}
T1= r1(G) w1(E) w1(C) r1(B)
Module
T2= r2(G) w2(C) w2(E) r1(B)
 Angelo Brayner
CoopIS - Trento, September 2001
12
GS-Serializability Model (3)
UNIFOR
GS-Serial Global Schedule



Local schedules are conflict serializable and
Serial execution of modules belonging to
global transactions
Example
G1=r1(G)w1(E)w1(C)r1(B); G2=r2(A)w2(B)w2(D)r2(E)
SC= r2(A)w2(B)r1(G)w1(E)w2(D)r2(E)w1(C)r1(B)
SC is GS-Serial
SC is not conflict serializable
 Angelo Brayner
CoopIS - Trento, September 2001
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GS-Serializability Model (4)
UNIFOR
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
 Angelo Brayner
CoopIS - Trento, September 2001
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GS-Serializability Model (5)
UNIFOR
Identifying GS-Serializable Schedule

Since existing DBMSs yield conflict
serializable schedules
The GTM has solely to verify the
execution order of global transactions
 A graph-based method

 Angelo Brayner
The Semantic Serialization Graph (SSG)
CoopIS - Trento, September 2001
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Concurrency Control in MDBSs
UNIFOR
Concurrency Control Protocols

Conservative
 Based on a locking mechanism

Aggressive
 Management of an always acyclic graph

 Angelo Brayner
Based on the SSG
CoopIS - Trento, September 2001
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Conclusions
UNIFOR
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
 Angelo Brayner
CoopIS - Trento, September 2001
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