III. Current Trends

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III. Current Trends
3C13/D6

• Distributed Databases and DBMSs
• Concepts and Design

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12.0 Content
Content
12.1 Objectives 12.2 Overview of Networking 12.3
Introduction to DDBMSs - Concepts - Advantages
and Disadvantages - Homogeneous and
Heterogeneous 12.4 Functions and Architecture -
Functions of a DDBMS - Reference Architecture
for a DDBMS/ Federated MDBS 12.5 Distributed
Relational Database Design - Data Allocation -
Fragmentation
12.6 Transparency in a DDBMS - Distribution
Transparency - Transaction Transparency -
Performance Transparency 12.7 Dates 12 Rules for
DDBMs 12.8 Summary
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Objectives
12.1 Objectives

• In this Lecture you will learn
• Concepts.
• Advantages and disadvantages of distributed
  databases.
• Functions and architecture for a DDBMS.
• Distributed database design.
• Levels of transparency.
• Comparison criteria for DDBMSs.

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Overview of Networking
12.2 Overview of Networking

• Network interconnected collection of autonomous
  computers, capable of exchanging information.
• Local Area Network (LAN) intended for connecting
  computers at same site.
• Wide Area Network (WAN) used when computers or
  LANs need to be connected over long distances.
• WAN relatively slow
• Less reliable than LANs.
• DDBMS using LAN provides much faster response
  time than one using WAN.

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Overview of Networking
12.2 Overview of Networking

• Network interconnected collection of autonomous
  computers, capable of exchanging information.
• Local Area Network (LAN) intended for connecting
  computers at same site.
• Wide Area Network (WAN) used when computers or
  LANs need to be connected over long distances.
• WAN relatively slow
• Less reliable than LANs.
• DDBMS using LAN provides much faster response
  time than one using WAN.

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Concepts
12.3 Introduction

• Databases and networks
• A centralized DBMS could be physically processed
  by several computers distributed across a network
• There could be several separate DBMS on several
  computers distributed across a network
• There may be a Distributed DBMS (DDBMS)
• made up of several DBMSs distributed across a
  network
• each with local autonomy
• Each participates in at least one global DBMS
  action
• The DDBMS therefore can operate as a single
  global DBMS

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Concepts
12.3 Introduction

• DDBMS to Avoid islands of information problem
• A Distributed Database is a logically
  interrelated collection of shared data (and a
  description of this data), physically distributed
  over a computer network.
• A Distributed DBMS (DDBMS) is a Software
  system that permits the management of the
  distributed database and makes the distribution
  transparent to users.
• Fundamental Principle make distribution
  transparent to user.
• The fact that fragments are stored on different
  computers is hidden from the users

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Concepts
12.3 Introduction

• DDBMS has following characteristics
• Collection of logically-related shared data.
• Data split into fragments.
• Fragments may be replicated.
• Fragments/replicas allocated to sites.
• Sites linked by a communication network.

• Data at each site is under control of a DBMS.
• DBMSs handle local applications autonomously.
• Each DBMS participates in at least one global
  application.

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Important difference between DDBMS and
distributed processing !
12.3 Introduction
Distributed processing of centralised DBMS
DDBMS
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Distributed Processing
12.3 Introduction

• Distributed processing of a centralised DBMS has
  following characteristics
• Much more tightly coupled than a DDBMS.
• Database design is same as for standard DBMS
• No attempt to reflect organizational structure
• Much simpler than DDBMS
• More secure than DDBMS
• No local autonomy

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Important difference between DDBMS and parallel
database
12.3 Introduction
Parallel Database Architectures Shared
a)memory b)disk c)nothing
DDBMS
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Why use a DDBMS? (!)
12.3 Introduction

• Advantages
• Reflects organizational structure
• Improved shareability and
• local autonomy
• Improved availability
• Improved reliability
• Improved performance
• Economics
• Modular growth

• Disadvantages
• Complexity
• Cost
• Security
• Integrity control more difficult
• Lack of standards
• Lack of experience
• Database design more complex

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Homogeneous Heterogeneous DDBMSs
12.3 Introduction

• Homogeneous All sites use same DBMS product.
• Much easier to design and manage.
• Approach provides incremental growth
• Allows increased performance.
• Heterogeneous Sites may run different DBMS
  products, underlying data models.
• Sites implemented their own databases -
  integration considered later.
• Translations required to allow for
• Typical solution is to use gateways.

• Different hardware.
• Different DBMS products.
• Different hardware and DBMS products.

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Open Database access and interoperability
12.3 Introduction

• The Open Group formed Specification Working
  Group (SWG)
• to provide specifications that create database
  infrastructure environment where there is
• Common SQL API allows client applications to be
  written that do not need to know vendor of DBMS
  they are accessing.
• Common database protocol enables DBMS from one
  vendor to communicate directly with DBMS from
  another vendor without need for a gateway.
• Common network protocol allows communications
  between different DBMSs.

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Multidatabase system (MDBS)!
12.3 Introduction

• MDBS DDBMS where each site maintains complete
  autonomy
• Resides transparently on top of existing
  database and file systems
• presents a single database to its users.
• Allows users to access and share data without
  requiring physical database integration.
• 2 types
• Federated MDBS looks like a DDBMS for global
  users and a centralized DBMS for local users.
• Unfederated MDBS has no local users

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Functions and Architecture of a DDBMS
12.4 Functions and Architecture of a DDBMS
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Functions of a DDBMS
12.4 Functions and Architecture of a DDBMS

• Expect DDBMS to have at least the functionality
  of a DBMS.
• Also to have following functionality
• Extended communication services.
• Extended Data Dictionary.
• Distributed query processing.
• Extended concurrency control.
• Extended recovery services.

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DDBMS Reference Architecture
12.4 Functions and Architecture of a DDBMS

• A reference architecture consists of
• Set of global external schemas.
• Global conceptual schema (GCS).
• Fragmentation schema and allocation schema (see
  later )
• Set of schemas for each local DBMS conforming to
  3-level ANSI/SPARC.
• Comparison with federated MDBS
• In DDBMS GCS is union of all local conceptual
  schemas.
• In FMDBS GCS is subset of local conceptual
  schemas (LCS), consisting of data that each local
  system agrees to share.
• GCS of tightly coupled system involves
  integration of either parts of LCSs or local
  external schemas.
• FMDBS with no GCS is called loosely coupled.

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Distributed Relation Database Design
12.4 Functions and Architecture of a DDBMS
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Data Allocation !
12.5 Distributed Relational Database Design

• Four alternative strategies regarding placement
  of data
• Centralized single database and DBMS stored at
  one site with users distributed across the
  network.
• Partitioned Database partitioned into disjoint
  fragments, each fragment assigned to one site.
• Complete Replication Consists of maintaining
  complete copy of database at each site.
• Selective Replication Combination of
  partitioning, replication, and centralization.

Comparison of strategies
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Data Allocation
12.5 Distributed Relational Database Design

• Four alternative strategies regarding placement
  of data
• Centralized single database and DBMS stored at
  one site with users distributed across the
  network.
• Partitioned Database partitioned into disjoint
  fragments, each fragment assigned to one site.
• Complete Replication Consists of maintaining
  complete copy of database at each site.
• Selective Replication Combination of
  partitioning, replication, and centralization.

Comparison of strategies
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Fragmentation
12.5 Distributed Relational Database Design
Why fragment? Usage - Apps work with views
rather than entire relations. Efficiency -
Data stored close to where most frequently used.
- Data not needed by local applications is not
stored. Security - and so not available to
unauthorized users. Parallelism - With
fragments as unit of distribution, T can be
divided into several subqueries that operate
on fragments.
Disadvantages Performance Integrity.
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Fragmentation !
12.5 Distributed Relational Database Design

• Three Correctness of fragmentation rules
• Completeness If relation R decomposed into
  fragments R1, R2, ... Rn, each data item that
  can be found in R must appear in at least one
  fragment.
• Reconstruction Must be possible to define a
  relational operation that will reconstruct R from
  the fragments.
• - for horizontal fragmentation Union
  operation
• - for vertical Join
• 3. Disjointness If data item di appears in
  fragment Ri, then should not appear in any other
  fragment.
• - Exception vertical fragmentation.
• - For horizontal fragmentation, data item is a
  tuple.
• - For vertical fragmentation, data item is an
  attribute.

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Fragmentation !
12.5 Distributed Relational Database Design

• Four types of fragmentation
• Horizontal Consists of a subset of the tuples
  of a relation.
• - Defined using Selection operation
• - Determined by looking at predicates used by
  Ts.
• - Involves finding set of minimal (complete and
  relevant) predicates.
• - Set of predicates is complete, iff, any two
  tuples in same fragment are referenced with same
  probability by any application.
• - Predicate is relevant if there is at least
  one application that accesses fragments
  differently.

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Fragmentation !
12.5 Distributed Relational Database Design

• Other possibility is no fragmentation
• If relation is small and not updated frequently,
  may be better not to fragment.

• Four types of fragmentation
• 2. Vertical subset of atts of a relation.
• - Defined using Projection operation
• - Determined by establishing affinity of one
  attribute to another.
• 3. Mixed horizontal fragment that is vertically
  fragmented, or a vertical fragment that is
  horizontally fragmented.
• - Defined using Selection and Projection
  operations
• 4. Derived horizontal fragment that is based on
  horizontal fragmentation of a parent relation.
• - Ensures fragments frequently joined together
  are at same site.
• - Defined using Semijoin operation

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Transparency in a DDBMS
12.6 Distributed Relational Database Design

• Transparency hides implementation details from
  users.
• Overall objective equivalence to user of DDBMs
  to centralised DBMS
• - FULL transparency not universally accepted
  objective
• Four main types
• Distribution transparency
• Transaction transparency
• Performance transparency
• DBMS transparency (only applicable to
  heterogeneous)

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1. Distribution Transparency
12.6 Distributed Relational Database Design

• Distribution transparency allows user to
  perceive database as single, logical entity.
• If DDBMS exhibits distribution transparency, user
  does not need to know
• fragmentation transparency data is fragmented
• Location transparency location of data items
• otherwise call this local mapping transparency
• replication transparency user unaware of
  replication of fragments

• Naming transparency each item in a DDB must have
  a unique name.
• One solution create central name server - loss
  of some local autonomy. - central site
  may become a bottleneck. - low availability if
  the central site fails.
• Alternative solution prefix object with
  identifier of creator site, each fragment and its
  copies. Then each site uses alias.

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2. Transaction Transparency
12.6 Distributed Relational Database Design

• Transaction transparency Ensures all distributed
  Ts maintain distributed databases integrity and
  consistency.
• Distributed T accesses data stored at more than
  one location.
• Each T is divided into no. of subTs, one for each
  site that has to be accessed.
• DDBMS must ensure the indivisibility of both the
  global T and each of the subTs.

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2. Transaction Transparency
12.6 Distributed Relational Database Design

• Concurrency transparency All Ts must execute
  independently and be logically consistent with
  results obtained if Ts executed in some arbitrary
  serial order.
• Replication makes concurrency more complex
• Failure transparency must ensure atomicity and
  durability of global T.
• Means ensuring that subTs of global T either all
  commit or all abort.
• Classification transparency In IBMs Distributed
  Relational Database Architecture (DRDA), four
  types of Ts
• Remote request
• Remote unit of work
• Distributed unit of work
• Distributed request.

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3. Performance Transparency
12.6 Distributed Relational Database Design

• DDBMS - no performance degradation due to
  distributed architecture.
• - determine most cost-effective
  strategy to execute a request.
• Distributed Query Processor (DQP) maps data
  request into ordered sequence of operations on
  local databases.
• - Must consider fragmentation, replication, and
  allocation schemas.
• DQP has to decide
• which fragment to access
• which copy of a fragment to use
• which location to use.
• - produces execution strategy optimized with
  respect to some cost function.
• Typically, costs associated with a distributed
  request include I/O cost
• CPU cost, communication cost.

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Dates 12 Rules for DDBMS
12.7 Dates 12 Rules for DDBMS

• Fundamental Principle To the user, distributed
  system should look exactly like a nondistributed
  system.
• 1. Local Autonomy
• 2. No Reliance on a Central Site
• 3. Continuous Operation
• 4. Location Independence
• 5. Fragmentation Independence
• 6. Replication Independence
• 7. Distributed Query Processing
• 8. Distributed Transaction Processing

Ideals 9. Hardware Independence 10. Operating
System Independence 11. Network Independence 12.
Database Independence
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Summary
12.8 Summary

• 12.1 Objectives
• 12.2 Overview of Networking
• 12.3 Introduction to DDBMSs
• Concepts
• Advantages and Disadvantages
• Homogeneous and Heterogeneous
• 12.4 Functions and Architecture
• Functions of a DDBMS
• Reference Architecture for a
• DDBMS/ Federated MDBS
• 12.5 Distributed Relational Database Design
• Data Allocation
• Fragmentation

• 12.6 Transparency in a DDBMS
• - Distribution Transparency
• - Transaction Transparency
• - Performance Transparency
• 12.7 Dates 12 Rules for DDBMs

NEXT LECTURE III Current Trends Part 2
Distributed DBMSs- Advanced concepts - advanced
concepts - protocols for distributed deadlock
control - X/Open Distributed Transaction Processin
g Model - Oracle.