Chapter 4 Transaction Management

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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
• Presentors Stuart Ness,
• Charles Braxmeier

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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
• Authors have used 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
  (hierarchical or directed graph in nature).
• This assumption does not allow for transcending
  to all DB types without additional effort
  (relational and structured, etc)
• Removing assumption allows for maintaining a
  basic structure, but would require other means of
  controlling the concurrency access.
• Possible problems with more complex index based
  queries

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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.