Manipulation of information located in a heterogeneous database requires an additional software layer on the top of existing database systems. This software layer is called a multi database system. The local database systems may differ in their concurrency control and transaction management mechanisms since they employ different logical models, data-manipulation and data-definition languages, and A multi database system provides an environment in which new database applications can access data from a variety of preexisting databases located in various heterogeneous hardware and software environments.
A multi database system creates the illusion of logical database integration without requiring physical database integration.
Full integration of existing systems into a homogeneous distributed system face some of the difficulties . They are
1.Technical difficulties: – The investment in application programs based on existing database system may be huge, and cost of converting these applications may be prohibitive.
2.Organizational difficulties:- Even if integration is technically possible, it may not be practically possible due to the existing database systems belonging to different corporation or organization.
In second case, it is important for a multi database system to allow the local database systems to retain a high degree of autonomy over the local database and transactions running against that data.
For these reasons multi database systems offer significant advantages that outweigh their overhead.
A multi database system supports two types of transactions.
1. Local transactions:- These transactions are executed by each local DBMS, outside of the multi database system’s control.
2. Global transactions:- These transactions are executed under the multidatabase systems control.
The multi database system is aware of the fact that local transactions may run at the local sites, but it is not aware of what specific transactions are being executed, or of what data they may access.
Ensuring the local autonomy of each DBMS requires making no changes to the local DBMS software. A DBMS at one site thus is not able to communicate directly with a DBMS at any other sites to synchronize the execution of a global transaction active at several sites.
Since the multi database system has no control over the execution of local transactions, each local DBMS must use a concurrency-control scheme to ensure that its schedule is serializable.
There are many protocols for ensuring consistency despite concurrent execution of global and local transaction in multidatabase systems. Some are based on imposing sufficient conditions to ensure global serializability. Others ensure only a form of consistency by less restrictive means.