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Monday, 20 October 2014
Different options for distributing a database in a distributed database system
A database is distributed over network and stored on various sites in geographically different locations for ease of access. This we know earlier. In an actual case, a database may be stored in multiple sites as it is, or some tables of a database might be stored at one site, the others at some other site and so on. So, what are the various options available to us to distribute database over different locations? They are,
1. Data replication – it is about keeping the same copies at different sites.
· The whole database may be reproduced and maintained at all or few of the sites, or
· A particular table may be reproduced and maintained at all or few of the sites
2. Horizontal partitioning – it is about partitioning a table by records without disturbing the structure of the table. For example, if you have a table EMP which stores data according to a schema EMP(Eno, Ename, Dept, Dept_location), then horizontal partitioning of EMP on Dept_location is about breaking employee records according to the department location values and store different set of employee details at different locations. The data at different locations will be different, but the schema will be the same, ie., EMP(Eno, Ename, Dept, Dept_location).
3. Vertical partitioning – it is about partitioning a table vertically, ie., decomposition. Hence, the partition of tables at different locations will of different structure.
For example, assume the schema EMP(Eno, Ename, Dept, Dept_location). If you would like to break the above schema like one to store employee details and the other to store the department details, it can be done as follows;
EMP(Eno, Ename, Dept), and DEPT(Dept, Dept_location)
These two tables might be stored at different locations for ease of access according to the defined organization policies for example.
4. Hybrid approach – it is a combination of few or all of the above said techniques. That is, it may be a combination like the few listed below;
· Horizontal partitioning and replication of few or all horizontal partitions.
· Vertical partitioning and replication of few or all vertical partitions.
· Vertical partitioning, followed by horizontal partitioning of some vertical partitions, followed by replication of few horizontal partitions, etc.
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