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:

  1. 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.
  2. 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

  1. Federated − The heterogeneous database systems are independent in nature and integrated together so that they function as a single database system.
  2. Un-federated − The database systems employ a central coordinating module through which the databases are accessed.