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Sunday, January 6, 2013

1.1 DISTRIBUTED DATABASES VS CONVENTIONAL DATABASES




Ø mimics organisational structure with data
Ø local access and autonomy without exclusion
Ø cheaper to create and easier to expand
Ø improved availability/reliability/performance by removing reliance on a central site
Ø Reduced communication overhead
§  Most data access is local, less expensive and performs better
Ø Improved processing power
§  Many machines handling the database rather than a  single server

Ø more complex to implement
Ø   more costly to maintain
Ø security and integrity control
Ø standards and experience are lacking
Ø Design issues are more complex


1.2 DISTRIBUTED   DATABASES ARCHITECTURE 

Ø Defines the structure of the system
o   components identified
o   functions of each component defined
o   interrelationships and interactions between components defined

What is a distributed database?

l“A logically interrelated collection of shared data (and a description of this   data), physically distributed over a computer network”
Implicit Assumptions

Ø Data stored at a number of sites ï each site logically consists of a single processor.
Ø Processors at different sites are interconnected by a computer network ï no multiprocessors
o   parallel database systems
Ø Distributed database is a database, not a collection of files ï data logically related as exhibited in the users’ access patterns
o   relational data model
Ø D-DBMS is a full-fledged DBMS
o   not remote file system, not a TP system

Advantages of distributed databases
Ø Capacity and incremental growth
Ø Increase reliability and availability
Ø Modularity
Ø Reduced communication overhead
Ø Protection of valuable data
Ø Efficiency and Flexibility

Disadvantages of distributed databases
Ø DDB design more complex, fragmentation & replication; extra work must be done by the DBAs to ensure that the distributed nature of the system is transparent.
Ø Economics,
Ø Concurrency control,
Ø Inexperience,
Ø Security,
Ø Difficult to maintain integrity

Applications
Ø Manufacturing - especially multi-plant manufacturing
Ø Military command and control

Ø Electronic fund transfers and electronic trading
Ø Corporate MIS
Ø Airline restrictions
Ø Hotel chains
Ø Any organization which has a decentralized organization structure


1 comment:

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