Saturday, 13 May 2017

What are the correctness rules for verifying fragmentation

What are the correctness rules for verifying fragmentation? How do we verify the correctness of fragmentation?


Correctness rules of fragmentation

Fragmentation is the major concept in distributed database. We fragment a table horizontally, vertically, or both and distribute the data to different sites (servers at different geographical locations). While we perform the fragmentation process, as a result we expect the following as outcomes;

  • We should not lose data because of fragmentation
  • We should not get redundant data because of fragmentation
Hence, to ensure these properties we need to verify that whether we performed the fragmentation correctly or not. For this verification we use the correctness rules. The rules are as follows;

  • Completeness - To ensure that there is no loss of data due to fragmentation. Completeness property ensures this by checking whether all the records which were part of a table (before fragmentation) are found in at least one of the fragments after fragmentation.
  • Reconstruction - This rule ensures the ability to re-construct the original table from the fragments that are created. This rule is to check whether the functional dependencies are preserved or not.
If a table R is partitioned into fragments R1, R2, …, Rn, then Reconstruction insists the following;
R = R1 U R2 U … U Rn

  • Disjointness - This rule ensures that no record will become a part of two or more different fragments during the fragmentation process.
If a table R is partitioned into fragments R1, R2, …, Rn, then Disjointness insists the following;
R1 ∩ R2 ∩ … ∩ Rn = Null set

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