Distributed databases can be broadly classified into homogeneous and heterogeneous distributed database environments, each with further sub-divisions, as shown in the following illustration.
1. Homogeneous Distributed Databases
In a homogeneous distributed database, all the sites use identical DBMS and operating systems. Its properties are
- The sites use very similar software.
- The sites use identical DBMS or DBMS from the same vendor.
- Each site is aware of all other sites and cooperates with other sites to process user requests.
- The database is accessed through a single interface as if it is a single database.
Types of Homogeneous Distributed Database
There are two types of homogeneous distributed database are:
- Autonomous − Each database is independent that functions on its own. They are integrated by a controlling application and use message passing to share data updates.
- Non-autonomous − Data is distributed across the homogeneous nodes and a central or master DBMS co-ordinates data updates across the sites.
2. Heterogeneous Distributed Databases
In a heterogeneous distributed database, different sites have different operating systems, DBMS products and data models. Its properties are −
- Different sites use dissimilar schemas and software.
- The system may be composed of a variety of DBMSs like relational, network, hierarchical or object oriented.
- Query processing is complex due to dissimilar schemas.
- Transaction processing is complex due to dissimilar software.
- A site may not be aware of other sites and so there is limited co-operation in processing user requests.
Types of Heterogeneous Distributed Databases
- Federated − The heterogeneous database systems are independent in nature and integrated together so that they function as a single database system.
- Un-federated − The database systems employ a central coordinating module through which the databases are accessed.