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Distributed DBs (DDBMS Key Characteristics (sites are linked by a comms…
Distributed DBs
Distributed DB - logically interrelated collection of shared data, physically distributed over a computer network
Distributed (D)DBMS - software system that permits the management of the distributed DB and makes the distribution transparent to the user
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Parallel DBMS - running across multiple processors and disks designed to execute operations in parallel when possible - improve performance
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Heterogenous DDBMS
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sites may run different DBMS products, with the possibility of different underlying data models
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Fragmentation
Advantages
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Security - data not required b local apps is not stored, so not available to unauthorised users
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Vertical Fragmentation - a projection of the columns, where we store different attributes of the same tuple at different sites
Horizontal Fragmentation - the tuples that belong to a given horizontal fragment are identified by a selection
Allocation
Centralized - consists of a single DB and DBMS stored at one site with users distributed across the network
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Partitioned - Db partitioned into disjoint fragments, each fragment assigned to one site