Distributed Databases
A distributed database (DDB) is an integrated collection of databases that is physically distributed across sites in a computer network. A distributed database management system (DDBMS) is the software system that manages a distributed database such that the distribution aspects are transparent to the users. To form a distributed database system (DDBS), the files must be structured, logically interrelated, and physically distributed across multiple sites. In addition, there must be a common interface to access the distributed data.
Distributed database design refers to the following problem: given a database and its workload, how should the database be split and allocated to sites so as to optimize certain objective function (e.g., to minimize the resource consumption in processing the query workload).
There are two issues:
The design problem is applicable when a distributed database system has to be built from scratch. In the case when multiple existing databases are to be integrated (e.g., in multi-database context), there is no design issue.
There are two issues:
- Data fragmentation which determines how the data should be fragmented
- Data allocation which determines how the fragments should be allocated
The design problem is applicable when a distributed database system has to be built from scratch. In the case when multiple existing databases are to be integrated (e.g., in multi-database context), there is no design issue.
Features of distributed databases:
- When in a collection, distributed databases are logically interrelated with each other, and they often represent a single logical database.
- With distributed databases, data is physically stored across multiple sites and independently managed.
- The processors on each site are connected by a network, and they don't have any multiprocessing configuration.
A common misconception is that a distributed database is a loosely connected file system. In reality, it's much more complicated than that. Distributed databases incorporate transaction processing, but are not synonymous with transaction processing systems.
In general, distributed databases include the following features:
In general, distributed databases include the following features:
- Location independent
- Distributed query processing
- Distributed transaction management
- Hardware independent
- Operating system independent
- Network independent
- Transaction transparency
- DBMS independent
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