A system in which each server is autonomous and centralized DBMS that has its own local users. The term Federated Database system or in short FDS is basically used when there is some global view or schema of the Federation of the database which is basically shared by the applications. These systems are hybrid between distributed and centralized systems.
Issues in DBMS –
In heterogeneous FDBMS one server may be network DBMS another an object DBMS and a third a relational or hierarchical DBMS in such cases we may need to have canonical language system and which include language translators to translate subqueries from the canonical language to the language of the server. The type of heterogeneity present in FDBMS may arise basically from several sources. Following types of Heterogeneity or Issues will occur in FDBMS.
- Differences in data model –
In an organization, we may have different types of the data model for databases such as relational, file, object data model and modeling capabilities of these models vary from one another. Hence to deal with them uniformly in a single language is too challenging. Hence Difference in the data model is the basic issue in FDBMS.
- Difference in Constraints –
Constraints facilities and its implementation vary from one system to another. There are basically comparable features that must be reconciled in the basic construction of global schema. And this global schema also has to deal with potential conflicts among constraints. For example, the relationship from the ER model is represented as referential integrity constraint in the relational model.
- Difference in Query Language –
For the same data model, we have so many languages and their version also varies. For example, even in SQL, we have so many versions such as SQL-89, SQL-92 and SQL-99 and these versions have their own set of data types, comparison operators, string manipulation and so on.