A logically interrelated collection of shared data and a description of this data, physically distri

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Distributed Database

• A logically interrelated collection of shared
  data (and a description of this data), physically
  distributed over a computer network.
• A single logical database is split into a number
  of fragments, each stored on one or more
  computers connected by a communication network.
• DDBMS the software that manages the database
  and makes the distribution transparent to users.

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Distributed Processing

• A centralized database that can be accessed over
  a computer network.

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The Client/Server Architecture
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A Multi-Tier Architecture Environment Example
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Parallel DBMS

• A DBMS running across multiple processors and
  disks that has been designed to execute
  operations in parallel, whenever possible, in
  order to improve performance.

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Homogeneous Distributed Database
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An Oracle Distributed Database System
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Advantages of DDBMS

• Many organizations are naturally distributed over
  several locations.
• Data can be placed at the site close to the users
  who normally use that data.
• A failure at one site or communication link does
  not make the entire system inoperable.
• It costs much less to create a system of smaller
  computers with the equivalent power of a single
  large computer.
• New sites can be added to the network without
  affecting the operations of other sites.

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Disadvantages of DDBMS

• Complexity
• Cost
• Security
• Integrity control
• Lack of standards
• Lack of experience
• Database design

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One Reference Architecture for DDBMS

• Global external schemas external views
• Global conceptual schema as a centralized DB
• Fragmentation schema how the data is to be
  logically partitioned
• Allocation schema where the data is to be
  located
• Schemas for local DBMS

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Component Architecture for a DDBMS

• Local DBMS component
• Data Communication component
• Global System Catalog
• Distributed DBMS component

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Functions of a DDBMS

• Allow the transfer of queries and data among the
  sites using a network.
• System catalog should store data distribution
  details.
• Distributed query processing
• Extended concurrency control
• Extended recovery services

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Objectives of Fragmentation and Allocation

• Locality of reference
• Improved reliability and availability
• Acceptable performance
• Balanced storage capacities and costs
• Minimal communication costs

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Data Allocation

• Centralized
• Partitioned (or fragmented)
• Complete replication
• Selective replication

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Reasons for Fragmentation

• In general, applications work with views rather
  than entire relations.
• Data not required by local application needs not
  be stored efficiency and security.
• Increase degree of concurrency or parallelism in
  the system.

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Disadvantage of Fragmentation

• Lower performance of applications that require
  data from several fragments located at different
  sites.
• Difficult integrity control if data and
  functional dependencies are fragmented and
  located at different sites.

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Correctness of Fragmentation

• Completeness each data item that can be found
  in the original relation must appear in at least
  one fragment.
• Reconstruction there must exist a relational
  operation that will reconstruct the original
  relation from the fragments.
• Disjointness if a data item appears in a
  fragment, it should not appear in any other
  fragment except the primary key in the case of
  vertical fragmentation.

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Database Link
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Network Directories and Global Database Names
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Replication

• The process of copying and maintaining database
  objects, such as tables, in multiple databases
  that make up a distributed database system.

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Reasons for Replication

• Availability it provides application with
  alternative data access options. If one site
  becomes unavailable, users can continue to query
  or even update the remaining locations.
• Performance users can access data from the
  replication site that has the lowest access cost.
   
• Disconnected Computing snapshots enable users
  to work on a subset of a database while
  disconnected from the central database server.
• Network Load Reduction applications can access
  various regional servers instead of accessing one
  central server.
• Mass Deployment deployment templates enable you
  to create multiple snapshot environments quickly.

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Multimaster Replication
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Read-Only Snapshot Replication
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Updateable Snapshot Replication
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Hybrid Replication