Concurrency and Transaction Management in an Object Oriented Database

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Concurrency and Transaction Management in an
Object Oriented Database

• Initial Presentation
• Author Jeremy Torres
• Date 5/30/2003

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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

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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

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OODB and Relational DB Differences2,6

• 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
• 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

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Some OODBMSs

• Commercial
• FastObjects (formerly Poet)
• GemStone
• Versant
• Ontos
• Objectivity/DB

• Open Source
• Ozone
• XL2
• FramerD
• Zope
• Academic
• ObjectStore

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Definitions4 1 of 3

• Data Itemscollection of objects representing a
  database
• Granularitysize of a data item
• Concurrencymultiple users accessing a database
  instance at the same time
• Transactiona logical unit of database processing
  that includes one or more database access
  operations
• Insert, Delete, Modify, Retrieve operations
• SerializabilityInterleaving execution of a set
  of concurrent transactions without giving up any
  correctness

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Definitions, 2 of 3

• Concurrency Control Protocolsset of rules for
  defining the execution of concurrent transactions
  (ultimately to ensure serializability)
• Optimistic Concurrency Controlvalidation or
  certification of a transaction AFTER it executes
• If interference is detected, the transaction is
  aborted and restarted at a later time

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Definitions, 3 of 3

• Pessimistic Concurrency ControlLocks 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 2PLa transaction does not release any of
  its exclusive (write) locks until after it
  commits or aborts

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Database Management Systems Should1 of 2

• Provide Concurrency Control4
• The DBMS should allow more than one user to
  access/manipulate data concurrently
• When there is concurrency, Transaction
  Management must be addressed
• The Lost Update Problemtwo transactions have
  their operations interleaved in such a way that
  some database item is incorrectinconsistent
  state!
• The Temporary Update (Dirty Read) ProblemOne
  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

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Database Management Systems Should2 of 2

• Provide Transactions that have ACID properties4
• Atomicitya transaction is an atomic unit of
  work its performed in its entirety or not at
  all
• Consistency preservationa transaction takes
  database from one consistent state to another
• Isolationa transaction should appear as though
  it is being executed in isolation from other
  transactions
• Durability or permanencythe changes applied to
  the database by a committed transaction must
  persist in the database. The changes must not be
  lost because of any failure

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Concurrency and Transaction Management OODBMs
vs. RDBMs

• Both DBMSs must deal with Concurrency and
  Transaction Management issues
• Many concurrency protocols can be applied to both
  DBMSs
• 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

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Many OODBsVarying Frameworks

• There are many OODBMs 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 OODMS as there was for a
  Relational DBMS (such as Codds 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.

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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

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Research Objectives/Goals

• To design a flexible and extendable (pluggable)
  framework for concurrency and transaction
  management within the context of an object
  oriented database using 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

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Research Objectives/Goals Contd

• Provide various implementations of the framework,
  each providing different transactional properties
  (e.g., optimistic versus pessimistic locking),
  for the ObjectStore

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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

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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

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High-Level Logical Diagram
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Issues To Be Addressed

• Object Store is distributed via Java RMI
• The framework proposed will have to account for
  each clients state

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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/Transact
  ional 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.

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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

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Questions?