Global Semantic Serializability: An Approach to Increase Concurrency in Multidatabase Systems

1. Global Semantic Serializability: An Approach to Increase Concurrency in Multidatabase Systems Angelo Brayner Theo Härder CoopIS - Trento, September 2001

2. Contents • Motivation • Multidatabase System Model • Global Semantic Serializability • Concurrency Control Protocols • Conclusions CoopIS - Trento, September 2001

3. 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 CoopIS - Trento, September 2001

4. 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 CoopIS - Trento, September 2001

5. 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 CoopIS - Trento, September 2001

6. 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 CoopIS - Trento, September 2001

7. 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 CoopIS - Trento, September 2001

8. Global Recovery Manager Global Scheduler Interface Server n Interface Server 1 Log Log SUBi1 SUBj1 Local Transactions DBMS DBMS DB DB LDBS1 Multidatabase System Model (1) Global Transactions Gi Gj Global Transaction Manager MDBS Global Log SUBin SUBjn Local Transactions LDBSn CoopIS - Trento, September 2001

9. Multidatabase System Model (2) • 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 CoopIS - Trento, September 2001

10. Multidatabase System Model (3) • 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 CoopIS - Trento, September 2001

11. 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 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 CoopIS - Trento, September 2001

12. GS-Serializability Model (2) • 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) • T2= r2(G) w2(C) w2(E) r1(B) Module CoopIS - Trento, September 2001

13. GS-Serializability Model (3) • 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 CoopIS - Trento, September 2001

14. GS-Serializability Model (4) • 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 CoopIS - Trento, September 2001

15. GS-Serializability Model (5) • 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 • The Semantic Serialization Graph (SSG) CoopIS - Trento, September 2001

16. Concurrency Control in MDBSs • Concurrency Control Protocols • Conservative • Based on a locking mechanism • Aggressive • Management of an always acyclic graph • Based on the SSG CoopIS - Trento, September 2001

17. 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 CoopIS - Trento, September 2001