Data models: Representing data with different modeling tools
Query Languages: Not only involves the use of completely different data access paradigms in different data models, but also covers difference in languages, even when the individual systems use the same data model.
Global Directory is an extension of the normal directory, including information about the location of the fragments as well as the makeup of the fragments, for cases of distributed DBMS or a multi-DBMS, that uses a global conceptual schema,
Global Directory Issues
Relevant for distributed DBMS or a multi-DBMS that uses a global conceptual schema
Includes information about the location of the fragments as well as the makeup of fragments.
Directory is itself a database that contains meta-data about the actual data stored in database.
A directory may either be global to the entire database or local to each site.
Directory may be maintained centrally at one site, or in a distributed fashion by distributing it over a number of sites.
If system is distributed, directory is always distributed
Replication, may be single copy or multiple copies.
Example Used Table Name :Employee D3 30K C 300 D2 20K B 200 D1 10K A 100 Dept Sal Name Id D2 20K B 200 D1 10K A 100 Dept Sal Name Id D3 30K C 300 Dept Sal Name Id C 300 B 200 A 100 Name Id 300 200 100 Id D3 30K D2 20K D1 10K Dept Sal
If a relation instance R is decomposed into fragments R 1 ,R 2 …. R n , each data item that can be found in R can also be found in one or more of R i ’s.
If a relation R is decomposed into fragments R 1 ,R 2 …. R n , it should be possible to define a relational operator such that
R = ▼R i , ¥R i ε F R ,
Please note the operator would be different for the different forms of fragmentation
If a relation R is horizontally decomposed into fragments R 1 ,R 2 …. R n , and data item d i is in R j , it is not in any other fragment Rk (k != j).
Comparison of Replication Alternatives Possible application Realistic Possible Application Reality Low High Very High Reliability Easy Difficult Moderate Concurrency Control Difficulty Same Easy or nonexistent Directory Management Difficulty Same Easy Query Processing Partitioning Partial Replication Full Replication