Chapter%204%20Transaction%20Management

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Chapter 4 Transaction Management

• Title Granularity of Locks and Degrees of
  Consistency in a Shared Database
• Authors J.N. Gray, R.A. Lorie, G. R. Putzolu and
  I. L. Traiger
• Presenter Stuart Ness

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Outline

• Motivation
• Problem Definition
• Contributions
• Key Concepts
• Validation Methodology
• Assumptions
• Rewrite Changes

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Motivation

• Concurrent Transactions
• Existing Locking mechanisms
• Overhead
• large-granularity locking
• Alternatives to existing techniques
• Finer-granularity
• Less overhead

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Problem Definition

• Given
• Shared Database
• Need for Concurrent Processing
• Find Efficient DBMS Lock method
• Inherent Locking mechanism
• Fine Granularity locks
• Objectives
• Efficiency
• Low Overhead
• Constraints
• Hierarchical Database Systems

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Contributions

• Produces a technique for the implicit locking of
  an entire sub-tree
• Introduce an intention mode to tag ancestors to
  prevent explicit or implicit exclusive or share
  modes (allows for implicit locking)
• Implements a method that allows for fine
  granularity without drastically increasing
  overhead (implicit locking)

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Concepts

• Dynamic Locks
• Index interval locks index based on value
• Extends DAG adding a lock on the index intervals.
  (If moving from one index interval to another, it
  requires both)
• Scheduling and Granting Requests
• Coexisting requests per group
• Conversion modes
• Request higher permissions
• Deadlock due to higher permission requests

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Validation Methodology

• Content primarily presents informal theory
• Validated while being developed and implemented
  on IBM Research Lab System
• Shows benefits of working system / Proof of
  concept
• Some of the modes have been applied in the IMS/VS
  system
• Lacks formal proof

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Assumptions

• Database can be broken into some fashion of a
  hierarchical form
• Assumes the use of an early database model (a
  simple hierarchical or directed graph).
• This assumption does not allow for transcending
  to all DB types without additional effort
  (relational and structured, etc)
• Assumes limited was of accessing data. More
  Complex methods may complicate locking mechanism
  and add inefficiencies.

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Rewrite Changes

• Rewrite to reflect the current state of database
  systems (as article is from 1975)
• Add statistical evidence of tradeoffs between
  overhead and concurrency
• Preserve the notion of intention modes and the
  scheduling of blocking non-compatible modes.
• Within the dynamic scheduling, consider using
  some type of enhanced queue.

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Questions?
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Additional References

• SQL Server
• http//www.awprofessional.com/articles/article.asp
  ?p26890seqNum4rl1
• http//media.wiley.com/product_data/excerpt/5X/076
  45795/076457955X.pdf
• Oracle Locking Levels
• http//www.akadia.com/services/ora_locks_survival_
  guide.html