Adv.B.Part-III(দ্বিতীয় পর্ব )

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ADV.B.PART-III (দ্বিতীয় পর্ব PARTT-3)

Types of Distributed Databases

A)                 Homogeneous Distributed Databases

In a homogeneous distributed database, all the sites use identical DBMS and operating systems. Its properties are −

 

1. ·        The sites use very similar software.
2. ·        The sites use identical DBMS or DBMS from the same vendor.
3. ·        Each site is aware of all other sites and cooperates with other sites to process user requests.
4. ·        The database is accessed through a single interface as if it is a single database.

There are two types of homogeneous distributed database −

 

·        Autonomous − Each database is independent that functions on its own. They are integrated by a controlling application and use message passing to share data updates.

·        Non-autonomous − Data is distributed across the homogeneous nodes and a central or master DBMS co-ordinates data updates across the sites.

B)                Heterogeneous Distributed Databases

In a heterogeneous distributed database, different sites have different operating systems, DBMS products and data models. Its properties are −

 

·        Different sites use dissimilar schemas and software.

·        The system may be composed of a variety of DBMSs like relational, network, hierarchical or object oriented.

·        Query processing is complex due to dissimilar schemas.

·        Transaction processing is complex due to dissimilar software.

·        A site may not be aware of other sites and so there is limited co-operation in processing user requests.

Types of Heterogeneous Distributed Databases

·       Federated − The heterogeneous database systems are independent in nature and integrated together so that they function as a single database system.

·       Un-federated − The database systems employ a central coordinating module through which the databases are accessed.

 

C)                 Client – Server Architecture for DDBMS

This is a two-level architecture where the functionality is divided into servers and clients. The server functions primarily encompass data management, query processing, optimization and transaction management. Client functions include mainly user interface. However, they have some functions like consistency checking and transaction management.

 

The two different client – server architecture are −

 

·       Single Server Multiple Client

·       Multiple Server Multiple Client

 

 

 

 

D)             Non-replicated & Non-fragmented

In this design alternative, different tables are placed at different sites. Data is placed so that it is at a close proximity to the site where it is used most. It is most suitable for database systems where the percentage of queries needed to join information in tables placed at different sites is low. If an appropriate distribution strategy is adopted, then this design alternative helps to reduce the communication cost during data processing.

 

E)                 Fully Replicated

In this design alternative, at each site, one copy of all the database tables is stored. Since, each site has its own copy of the entire database, queries are very fast requiring negligible communication cost. On the contrary, the massive redundancy in data requires huge cost during update operations. Hence, this is suitable for systems where a large number of queries is required to be handled whereas the number of database updates is low.

 

F)                 Partially Replicated

Copies of tables or portions of tables are stored at different sites. The distribution of the tables is done in accordance to the frequency of access. This takes into consideration the fact that the frequency of accessing the tables vary considerably from site to site. The number of copies of the tables (or portions) depends on how frequently the access queries execute and the site which generate the access queries.

 

G)       Fragmented

In this design, a table is divided into two or more pieces referred to as fragments or partitions, and each fragment can be stored at different sites. This considers the fact that it seldom happens that all data stored in a table is required at a given site. Moreover, fragmentation increases parallelism and provides better disaster recovery. Here, there is only one copy of each fragment in the system, i.e. no redundant data.

 

The three fragmentation techniques are −

 

·       Vertical fragmentation

·       Horizontal fragmentation

·       Hybrid 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 high.

·       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

In vertical fragmentation, the fields or columns of a table are grouped into fragments. In order to maintain reconstructiveness, each fragment should contain the primary key field(s) of the table. Vertical fragmentation can be used to enforce privacy of data.

Horizontal Fragmentation

Horizontal fragmentation groups the tuples of a table in accordance to values of one or more fields. Horizontal fragmentation should also confirm to the rule of reconstructiveness. Each horizontal fragment must have all columns of the original base table.

Hybrid Fragmentation

In hybrid fragmentation, a combination of horizontal and vertical fragmentation techniques are used. This is the most flexible fragmentation technique since it generates fragments with minimal extraneous information. However, reconstruction of the original table is often an expensive task.

 

H)             Mixed Distribution

This is a combination of fragmentation and partial replications. Here, the tables are initially fragmented in any form (horizontal or vertical), and then these fragments are partially replicated across the different sites according to the frequency of accessing the fragments.

 

Practical

Form Design