Concurrency and Transaction Management in an Object Oriented Database

1. Concurrency and Transaction Management in an Object Oriented Database Initial Presentation Author: Jeremy Torres Date: 5/30/2003

2. Agenda • Brief introduction to Object Oriented Database • Definitions required for the discussion of concurrency and transaction management • Desired properties of a database management system • Research Goals and Objectives • Work Plan • Q and A

3. An Object Oriented Database… • Is the coupling of Object Oriented (OOP) Programming principles with Database Management System (DBMS) principles • Provides access to persisted objects using the same OO-programming language

4. OODB Uses an OO data model Data is a collection of objects whose behavior, state, and relationships are stored as a physical entity Language dependence (OO-Language Specific) No impedance mismatch in application using OODB Relational DB Uses record-oriented model Data is a collection of record types (relations), each having collection of records or tuples stored in a file Language independence (via SQL) Impedance mismatch in an OO application. Mapping must be performed OODB and Relational DBDifferences2,6

5. Commercial FastObjects (formerly Poet) GemStone Versant Ontos Objectivity/DB Open Source Ozone XL2 FramerD Zope Academic ObjectStore Some OODBMS’s

6. Definitions4 1 of 3 • Data Items—collection of objects representing a database • Granularity—size of a data item • Concurrency—multiple users accessing a database instance at the same time • Transaction—a logical unit of database processing that includes one or more database access operations • Insert, Delete, Modify, Retrieve operations • Serializability—Interleaving execution of a set of concurrent transactions without “giving up any correctness”

7. Definitions, 2 of 3 Concurrency Control Protocols—set of rules for defining the execution of concurrent transactions (ultimately to ensure serializability) • Optimistic Concurrency Control—validation or certification of a transaction AFTER it executes • If interference is detected, the transaction is aborted and restarted at a later time

8. Definitions, 3 of 3 • Pessimistic Concurrency Control—Locks are used to prevent conflicting transactions • 2-Phase Locking Protocol (2PL): Best known locking protocol for guaranteeing serializability • Phase 1: Expanding/Growing. New locks can be acquired but none can be released • Phase 2: Shrinking. Existing locks can be released but no new locks can be acquired • Strict 2PL—a transaction does not release any of its exclusive (write) locks until after it commits or aborts

9. Database Management Systems Should…1 of 2 • Provide Concurrency Control4 • The DBMS should allow more than one user to access/manipulate data concurrently • When there is concurrency, TransactionManagementmust be addressed • The Lost Update Problem—two transactions have their operations interleaved in such a way that some database item is incorrect—inconsistent state! • The Temporary Update (Dirty Read) Problem—One transaction updates a database item and then the transaction fails; the updated item is access by another transaction before it is changed back to its original value

10. Database Management Systems Should…2 of 2 • Provide Transactions that have ACID properties4: • Atomicity—a transaction is an atomic unit of work; it’s performed in its entirety or not at all • Consistency preservation—a transaction takes database from one consistent state to another • Isolation—a transaction should appear as though it is being executed in isolation from other transactions • Durability or permanency—the changes applied to the database by a committed transaction must persist in the database. The changes must not be lost because of any failure

11. Concurrency and Transaction Management: OODBM’s vs. RDBM’s • Both DBMS’s must deal with Concurrency and Transaction Management issues • Many concurrency protocols can be applied to both DBMS’s • Optimistic and Pessimistic protocols are relevant to both • However, semantically different: • Example: Data Item Granularity • In traditional RDBMS, fine granularity data item would be a record field value of a record • In an OODBMS, fine granularity data item may be an Object or data member (field) of an Object

12. Many OODB’s…Varying Frameworks • There are many OODBM’s existing and emerging in both commercial and open source areas • Implementations vary differently • Distributed database model • Centralized database model • “hybrid” implementation, such as Object-Relational Databases • Use Relational DBMS Engine • Use Structured Query Language (SQL) or an extension of it for providing access to “Objects” • Currently, there is no consensus or clear specification for an OODMSas there was for a Relational DBMS (such as Codd’s original specification for a relational data model and query language)5 • NOTE: The Object Data Management Group (ODMG) has a specification for portability; however, ODMG-specific object models and query languages are used.

13. Study Rationale • My research has not found a generic, reusable design pattern for a transactional engine, or “layer”, geared specifically toward object oriented databases.  • Even though there are many varying implementations of object oriented databases, there are very few research papers (as found in the initial research phase) describing an complete transactional engine implementation

14. Research Objectives/Goals • To design a flexible and extendable (“pluggable”) framework for concurrency and transaction management within the context of an object oriented databaseusing the Java programming language • Ideally, the framework will be presented as a design pattern specifically for concurrency and transaction management of an Object Oriented database

15. Research Objectives/Goals Cont’d • Provide various implementations of the framework, each providing different transactional properties (e.g., optimistic versus pessimistic locking), for the ObjectStore

16. Research Design • The generic framework proposed, designed, and implemented by this research will be compared against the strengths and weaknesses of other alternative implementations (e.g., the Gemstone and Ozone object oriented databases) found during the research process • The comparison will focus on the design and implementation of the concurrency and transactional software “layers” of an object oriented database only; the remaining “layers” are out of the scope of this research

17. ObjectStore Project Current State • The ObjectStore Object Oriented Database is geared specifically for Java • Does not contain transaction management • Accessible by remote clients via RMI

18. High-Level Logical Diagram

20. Issues To Be Addressed • Object Store is distributed via Java RMI • The framework proposed will have to account for each client’s state

21. Work Plan • Phase I: First Presentation, May 30th, 2003: The research for this first phase will be completed. • Phase II: Summer I and II, 2003: Development and implementation of generic Concurrency/Transactional Model via the ObjectStore will be completed • Phase III: Second Presentation, September 2003. Results of developed concurrency model and implementations via the object store presented. • Completion: Final Presentation, November 2003. Demonstration of implemented framework via ObjectStore. All results of framework and implementations, including changes since second presentation.

22. References • Maier et al, Development of an Object-Oriented DBMS, ACM Press, 1986. • 2Zand et al, A Survey of Current Object-Oriented Databases, ACM Press, 1995 • 3Thomasian, Alexander, Concurrency Control: Methods, Performance, and Analysis, ACM Computing Surveys, 1998 • 4Elmasri, R., and Navathe, S, Fundamentals of Database Systems, Addison-Wesley, 2000. • 5Atkinson, et al, The Object-Oriented Database System Manifesto • 6Obasanjo, Dare, An Exploration of Object Oriented Database Management Systems • McClure, Steve, Object Database vs. Object-Relational Databases, International Data Corporation

23. Questions?