1. Distributed Databases

2. Introduction
In a distributed database system, the database is stored on several computers. The computers in a distributed system communicate with one another through various communication media, such as high-speed networks or telephone lines. They do not share main memory or disk.

5. Database systems that run on each site are independent of each other.
The computers in distributed system are referred by names such as sites or nodes.
Distributed database system consists of loosely coupled sites that share no physical component.
Database systems that run on each site are independent of each other.
Transactions may access data at one or more sites.

6. Types of Transactions Local Transaction Global transaction
It is one that accesses data only from site where that transaction was initiated.
Example
Consider a transaction to add Rs. 50 to account number A102. It is called as local transaction.
Global transaction
It is one that either accesses data from a site other than the site where that transaction was initiated or accesses data from several different sites.
Consider a transaction which transfers Rs. 50 from account number A102 to account number A201, which is at different branch, is a global transaction, since accounts in two different sites are accessed.

7. Types of distributed databases
Homogeneous
Heterogeneous
All sites (nodes) have identical database management software, are aware of one another.
They are agree to cooperate in processing user’s requests.
Local sites a portion of their autonomy in terms of their right to change schemas or “DBMS software”. That software must also cooperate with other sites in exchanging information about transactions to make transaction processing possible across multiple sites.
It appears to user as a single system.

8. Heterogeneous
Different sites may use different schemas and different management system software. The sites may not be aware of one another.
They provide only limited facilities for cooperation in transaction processing.
The differences in schemas are often a major problem for query processing, while the divergence in software becomes a hindrance for processing transactions that access multiple sites.

9. Problems of Distributed System
Query processing
Catalog management
Update propagation
Recovery
Concurrency