IT ELECTIVE 1 - MOD 1 Introduction to Distributed Database Management System

Database

• is an ordered collection of related data that is built for a specific purpose.

• may be organized as a collection of multiple tables, where a table represents a real world element or entity

Database management system

• is a collection of programs that enables creation and maintenance of a database 

• is available as a software package that facilitates definition, construction, manipulation and sharing of data in a database.

Construction of a database

involves actual storing of the data in any storage medium.

Manipulation

refers to the retrieving information from the database, updating the database and generating reports.

Database Schema

is a description of the database which is specified during database design and subject to infrequent alterations.

ANSISPARC architecture.

Databases represented through the three-schema architecture

3 Levels of ANSISPARC architecture

• Internal Level having Internal Schema

• Conceptual Level having Conceptual Schema

• External or View Level having External Schemas or Views

Internal Level having Internal Schema

It describes the physical structure, details of internal storage and access paths for the database.

Conceptual Level having Conceptual Schema

It describes the structure of the whole database while hiding the details of physical storage of data.

External or View Level having External Schemas or Views

It describes the portion of a database relevant to a particular user or a group of users while hiding the rest of database.

Types of DBMS

• Hierarchical DBMS

• Network DBMS

• Relational DBMS

• Object Oriented DBMS

• Distributed DBMS

Hierarchical DBMS

In ________ DBMS, the relationships among data in the database are established so that one data element exists as a subordinate of another.

Network DBMS

________ DBMS in one where the relationships among data in the database are of type many-to-many in the form of a network.

Relational DBMS

In ________ databases, the database is represented in the form of relations.

Object Oriented DBMS

_______ DBMS is derived from the model of the object-oriented programming paradigm.

Distributed DBMS

is a set of interconnected databases that is distributed over the computer network or internet.

Factors Encouraging DDBMS

• Distributed Nature of Organizational Units

• Need for Sharing of Data

• Support for Both OLTP and OLAP

• Database Recovery

• Support for Multiple Application Software

Distributed Nature of Organizational Units

Most organizations in the current times are subdivided into multiple units that are physically distributed over the globe.

Need for Sharing of Data

The multiple organizational units often need to communicate with each other and share their data and resources.

Support for Both OLTP and OLAP

Online Transaction Processing (OLTP) and Online Analytical Processing (OLAP) work upon diversified systems which may have common data.

Database Recovery

One of the common techniques used in DDBMS is replication of data across different sites. Replication of data automatically helps in data recovery if database in any site is damaged.

Support for Multiple Application Software

Most organizations use a variety of application software each with its specific database support.

Advantages of Distributed Databases

• Modular Development

• More Reliable

• Better Response

• Lower Communication Cost

Modular Development

In distributed databases, the work simply requires adding new computers and local data to the new site and finally connecting them to the distributed system, with no interruption in current functions.

More Reliable

In distributed systems, when a component fails, the functioning of the system continues may be at a reduced performance. Hence DDBMS is more reliable.

Better Response

If data is distributed in an efficient manner, then user requests can be met from local data itself, thus providing faster response.

Lower Communication Cost

In distributed database systems, if data is located locally where it is mostly used, then the communication costs for data manipulation can be minimized.

Disadvantages of Distributed Databases

• Need for complex and expensive software

• Processing overhead

• Data integrity

• Overheads for improper data distribution

Need for complex and expensive software

DDBMS demands complex and often expensive software to provide data transparency and co ordination across the several sites.

Processing overhead

Even simple operations may require a large number of communications and additional calculations to provide uniformity in data across the sites.

Data integrity

The need for updating data in multiple sites pose problems of data integrity.

Overheads for improper data distribution

Responsiveness of queries is largely dependent upon proper data distribution.

Data replication

is the process of storing separate copies of the database at two or more sites. It is a popular fault tolerance technique of distributed databases.

Advantages of Data Replication

• Reliability

• Reduction in Network Load

• Quicker Response

• Simpler Transactions

Reliability

In case of failure of any site, the database system continues to work since a copy is available at another site(s).

Reduction in Network Load

Since local copies of data are available, query processing can be done with reduced network usage, particularly during prime hours.

Quicker Response

Availability of local copies of data ensures quick query processing and consequently quick response time.

Simpler Transactions

Transactions require less number of joins of tables located at different sites and minimal coordination across the network. Thus, they become simpler in nature.

Disadvantages of Data Replication

• Increased Storage Requirements

• Increased Cost and Complexity of Data Updating

• Undesirable Application – Database coupling

Increased Storage Requirements

Maintaining multiple copies of data is associated with increased storage costs.

Increased Cost and Complexity of Data Updating

Each time a data item is updated, the update needs to be reflected in all the copies of the data at the different sites. This requires complex synchronization techniques and protocols.

Undesirable Application – Database coupling

If complex update mechanisms are not used, removing data inconsistency requires complex co-ordination at application level.

commonly used replication techniques

• Snapshot replication

• Near-real-time replication

• Pull replication

Fragmentation

is the task of dividing a table into a set of smaller tables

fragments

The subsets of the table are called ?

Types of Fragmentation

• horizontal

• vertical

• hybrid (combination of horizontal and vertical).

2 techniques of Horizontal fragmentation

• primary horizontal fragmentation

• derived horizontal fragmentation

Advantages of Fragmentation

• Since data is stored close to the site of usage, efficiency of the database system is increased.

• Local query optimization techniques are sufficient for most queries since data is locally available.

• Since irrelevant data is not available at the sites, security and privacy of the database system can be maintained.

Disadvantages of Fragmentation

• When data from different fragments are required, the access speeds may be very low. 

• In case of recursive fragmentations, the job of reconstruction will need expensive techniques.

• Lack of back-up copies of data in different sites may render the database ineffective in case of failure of a site.

Vertical Fragmentation

the fields or columns of a table are grouped into fragments.

Horizontal Fragmentation

groups the tuples of a table in accordance to values of one or more fields.

Hybrid Fragmentation

a combination of horizontal and vertical fragmentation techniques are used.

2 alternative ways of Hybrid Fragmentation

• At first, generate a set of horizontal fragments; then generate vertical fragments from one or more of the horizontal fragments.

• At first, generate a set of vertical fragments; then generate horizontal fragments from one or more of the vertical fragments.