Multi-database System

• It is defined as an integrated data system composed of collection of two or more autonomous datasets or databases.

• A multi-database system (MDBS) isadistributed system designate to integrate data and provide access to collection of pre-existing local databases managed by heterogeneous database management system (DDBMS).

Need of Multi-database System

• With the rapid development of computer networks, the need for auniform access to information stored in different databases has grown increasingly during the last decade.
• In order to solve these problems, the integration of different data management systems is needed, making differences between the existing data management systems invisible and providing users withauniform and transparent access to all the databases.
• This integration of several existing heterogeneous database systems and file systems is multi-database system (MDBS).

Features of MDBS

• MDBS have common characteristics of general DBS.
• MDBS possess ACID properties of transaction.
• MDBS have special characteristics such as pre-existence, autonomy, distribution and heterogeneity.
• A MDBS constructs global system management level and provides interface towards global users so those users can access all heterogeneous transparently database systems and file systems

Taxonomy for MDBS

• Several prototypes and approaches exist for management of MDBS. The taxonomy classifies the Multi-database system along two axis:

1. The degree of system integration

2. The degree of datatype heterogeneity between component databases.

Architecture of Multi-database System

• The five-level schema architecture of an MDBS includes the following:
• Local Schema:Alocal schema is expressed in the native data model of the component DBMS, and hence different local schema may be expressed in different data model.

• Component Schema: This schema describes the divergent local schemas usingasingle representation and semantics that are missing inalocal schema can be added to its component schema.

• Export Schema: Not all data ofacomponent DBMS may be available to the federation and its users. An export schema representsasubset ofacomponent schema that is available to the MDBS.

• Federated Schema:Afederated schema is an integration of multiple export schemas. This schema also includes the information on data distribution that is generated when integrating export schema.
• External Schema: This level consists of multiple views representing subset of integrated Multi-databases to the end user.