Sunday, March 15, 2015

Date's Twelve Rules for Distributed Database Systems - Fragmentation independence

Date's Twelve Rules for Distributed Database Systems - Fragmentation independence / Fragmentation Transparency

5. Fragmentation independence (Fragmentation Transparency)
Data fragmentation is transparent to the user. The user does not need to know the name of the database fragment in order to retrieve them.
For example, assume that the table Emp is fragmented into three fragments and stored in Chennai, Mumbai, and New Delhi sites. Assume a user query that demands for data from Chennai site Emp table. The name of the Emp table at Chennai site may be Chennai.Emp, at Mumbai site may be Mumbai.Emp, and at New Delhi site may be Newdelhi.Emp. According to the fragmentation independence rule, the name of the table at different sites should be hidden from the end user. Hence, user needs to request data differently from what does the system interprets.
User view. [What does the user see?]
In figure 1, user request for data from Emp table with certain conditions;
The user query : SELECT * FROM Emp WHERE condition;
Figure 1 - User view

System view. [How does the system interpret?]
The DDBS should interpret the query and find the exact fragment and location of data, accordingly it will translate the query as follows;
System translated query : SELECT * FROM Chennai.Emp WHERE condition;

Figure 2 shows the system translated query reaching the right fragment at the right site.
Figure 2 - System interpretation

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