Different options for distributing a database in a distributed database system

Various options for distributing databases / What are the different options for distributing databases over a distributed database system? / List the various possible options for distributing a database in a DDBS. /  List and discuss the techniques for distributing a database

 

Various options for distributing databases

A database is distributed over network and stored on various sites in geographically different locations for ease of access. This we know earlier. In an actual case, a database may be stored in multiple sites as it is, or some tables of a database might be stored at one site, the others at some other site and so on. So, what are the various options available to us to distribute database over different locations? They are,

1. Data replication – it is about keeping the same copies at different sites.

·        The whole database may be reproduced and maintained at all or few of the sites, or

·        A particular table may be reproduced and maintained at all or few of the sites

2. Horizontal partitioning – it is about partitioning a table by records without disturbing the structure of the table. For example, if you have a table EMP which stores data according to a schema EMP(Eno, Ename, Dept, Dept_location), then horizontal partitioning of EMP on Dept_location is about breaking employee records according to the department location values and store different set of employee details at different locations. The data at different locations will be different, but the schema will be the same, ie., EMP(Eno, Ename, Dept, Dept_location).

3. Vertical partitioning – it is about partitioning a table vertically, ie., decomposition. Hence, the partition of tables at different locations will of different structure.

For example, assume the schema EMP(Eno, Ename, Dept, Dept_location). If you would like to break the above schema like one to store employee details and the other to store the department details, it can be done as follows;

EMP(Eno, Ename, Dept), and DEPT(Dept, Dept_location)

These two tables might be stored at different locations for ease of access according to the defined organization policies for example.

4. Hybrid approach – it is a combination of few or all of the above said techniques. That is, it may be a combination like the few listed below;

·        Horizontal partitioning and replication of few or all horizontal partitions.

·        Vertical partitioning and replication of few or all vertical partitions.

·        Vertical partitioning, followed by horizontal partitioning of some vertical partitions, followed by replication of few horizontal partitions, etc.

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Reasons for using ORDBMS over RDBMS

Why do we need ORDBMS / Reasons for using ORDBMS / Advantages of ORDBMS

 

Reasons for having ORDBMSs over conventional RDBMSs

• Storage requirements of newer applications - Conventional RDBMSs cannot handle the data like engineering designs, scientific experiments, GIS, and multimedia data.

• Handling of data stored for newer applications – retrieve and manipulate data stored for newer applications like other data with RDBMS functionalities.

• Ability to define user defined data types – to handle new type of data as and when required.

• Reusability nature of objects – One of the main features of using objects, and methods.

• Database extensibility – easy addition of data types and operations

And finally, the inability of RDBMS to handle the above said things.

Object-based Database Systems - Overview

Object Based Database Systems Overview

It is a type of Database Management System where the information that is stored and manipulated is all in the form of objects. Through Object based database systems, we are really incorporating the features of Object Oriented Programming like objects, classes, and inheritance.

Object Relational DBMS (ORDBMS)

The conventional RDBMS is extended with new functionalities of storing and manipulating data in the form of objects. We mention that type RDBMS as ORDBMS.

The RDBMS combines its conventional capabilities such as,

• Secondary storage management
• Schema management
• Concurrency control
• Transaction management, recovery
• Query processing
• Access authorization and control, safety, security

with the capabilities of object oriented programming such as,

• Complex objects
• Object identities
• User-defined types (ADTs)
• Encapsulation
• Type/class hierarchy with inheritance
• Overloading, overriding, polymorphism

Figure 1 - ORDBMS = RDBMS + OOP

4 necessary conditions for occurrence of deadlock

Four necessary conditions for occurrence of deadlock in databases / Four necessary conditions for deadlock to occur / Necessary and sufficient conditions for deadlock / List and discuss four conditions for deadlock / Necessary conditions that ensures a deadlock occurrence state

 

Necessary conditions for deadlock to occur in database

A deadlock can arise if the following 4 conditions hold simultaneously in a system;

• Mutual exclusion: At least one resource is held in a non-sharable mode. For example, among transactions if there is any Exclusive lock (Write lock) request, that data item cannot be shared with others.

• Hold and wait: There is a transaction which acquired and held lock on a data item, and waits for other data item.

• No preemption: A situation where a transaction releases the locks on data items which it holds only after the successful completion of the transaction. Not on voluntarily.

• Circular wait: A situation where a transaction say T1 is waiting for another transaction T2 to release lock on some data items, in turn T2 is waiting for another transaction T3 to release lock, and so on.

During transaction processing, if all the said conditions are held then there occurred a deadlock.

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List of necessary conditions for deadlock to happen in database