Lecture 1

Distributed processing

A database s logical processing is shared among two or more physically independent sites that are connected through a network. For example, the data input/output (I/O), data selection, and data validation might be performed on one computer, and a report based on the data might be created on another computer.



Distributed database stores a logically related database over two or more physically independent sites. The sites are connected via a computer network. In a distributed database system, a database is composed of several parts known as database fragments.


From the two previous figures:
? Distributed processing does not require a distributed database, but a distributed database requires distributed processing (each database fragment is managed by its own local database process).
? Distributed processing may be based on a single database located on a single computer. For the management of distributed data to occur, copies or parts of the database processing functions must be distributed to all data storage sites.
? Both distributed processing and distributed databases require a network to connect all components.

Distributed database management systems (DDBMS)
Governs the storage and processing of logically related data over interconnected computer systems in which both data and processing functions are distributed among several sites.

Problems of centralized DBMS:
• Performance degradation due to a growing number of remote locations over greater distances.
• High costs associated with maintaining and operating large central (mainframe) database systems.
• Reliability problems created by dependence on a central site.
• Scalability problems associated with the physical limits imposed by a single location
• Organizational rigidity imposed by the data might not support the flexibility required by modern global organizations.

DDBMS components:
1. Computer workstations (sites or nodes) that form the network system.
2. Network hardware and software components that reside in each workstation.
3. Communications media that carry the data from one workstation to another.
4. The transaction processor (TP), which is the software component, found in each computer that requests data. The transaction processor receives and processes the application’s data requests (remote and local). The TP is also known as the application processor (AP) or the transaction manager (TM).
5. The data processor (DP). which is the software component residing On each computer that stores and retrieves data located at the site. The DP is also known as the data manager (DM). A data processor may even be a centralized DBMS.


The protocols determine how the distributed database system will:
• Interface with the network to transport data and commands between data processors (DPs) and transaction processors (TPs).
• Synchronize all data received from DPs (TP side) and route retrieved data to the appropriate TPs (DP side).
• Ensure common database functions in a distributed system. Such functions include security, concurrency control, backup, and recovery.

Levels of data and process distribution
Current database systems can be classified on the basis of how process distribution and data distribution are supported.


Codd s Relational Database Rules:
In 1985, Dr. E. F. Codd published a list of 12 rules to define a relational database system. These rules as shown in table below: