Q. Differentiate between "Centralized Database Approach" and "Distributed Database Approach".
Database Management System supports different type of database system for data organization. Distributed and Centralized database approaches are the systems which are use for data organization. Decentralization and Parallel are other approach for data organization.
DISTRIBUTED DATABASE APPROACH
A single logical database that is spread physically across computers in multiple locations that are connected by a data communications link.
A distributed database system is a collection of logically related database that co-operate in a transparent manner.
In a distributed database scheme, smaller pieces of the overall database are stored at various physical locations. Each piece is controlled by a local database server that satisfies requests for data valid over its local domain only. All local data servers are connected by a common network so that any application connected to this network has access to every server and its local data.
- Distributed World Wide Web (WWW) Database.
- Mobile Database.
Functions of Distributed Database:
- Keeping Track of Data: The ability to keep track of the data distribution, fragmentation, and replication by expanding the DDBMS catalog.
- Distributed Query Processing: The ability to access remote sites and transmit queries and data among the various sites via a communication network.
- Distributed transaction management: The ability to devise execution strategies for queries and transactions that access data from more than one site and to synchronize the access to distributed data maintain integrity of the overall database.
- Distributed Data Recovery: The ability to recover from individual site crashes and from new type of failures such as the failure of a communication links.
- Security: Distributed transactions must be executed with the proper management, security of the data and the authorization/access privileges of users.
- Distributed Catalog Management: A catalog contains information about data in the database. The catalog may be global for the entire Distributed database or local for each site. The placement and distribution of the catalog are design and policy issues.
Advantages and Disadvantages of Distributed Database Approach:
- Increased Reliability and Availability: when a centralized system fails, the database is unavailable to all users. A distributed system will continue to function at some reduced level, however even when a component fails. The reliability and availability will depend on how the data are distributed.
- Lower Communication costs: With a distributed system data can be located closer to their point of use. This can reduce communication costs, compared to a central system.
- Faster Response: Depending on how data are distributed, most requests for data by user at a particular site can be satisfied by data stored at that site. This speed up query processing since communication and central computer delays are minimized.
- Local Control: The database is brought nearer to its user. This can effect a cultural change as it allows potentially greater control over local data.
- Easier Expansion: In a distributed environment, expansion of the system in terms of adding more data, increasing database size, or adding more processors is much easier.
- Data Integrity: A by-product of the increased complexity and need for coordination is the additional exposure to improper updating and other problems of data integrity.
- Slow Response: If the data are not distributed properly according to their usage, or if queries are not formulated correctly, response to request for data can be extremely slow.
- Processing Overhead: The various sites must exchange messages and perform additional calculations to ensure proper coordination among data at the different sites.
- Architectural Complexity.
- Lack of Standards.
- Lack of Experience.
- Database design more complex.
Factors for adoption Distributed Database Approach:
- When the data is divided into different sites distributed database is suitable.
- Distributed Database is suitable for accessing data and transmits queries among different sites.
- Distributed database is suitable for proper management and security of the data and the authorized/access privileges of user.
- For achieving local control over data for each user/site Distributed database is suitable.
CENTRALIZED DATABASE APPROACH
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A centralized database consists of a single data server into which all data are stored and from which all data are retrieved. All the data reside at a single location and all applications must retrieve all data from that location.
The centralized approach consists of a central server into which all forecast data are stored. At some predefined time, software on this central server requests data from each of the local data servers scattered throughout the country. These data are received, processed and stored, possibly at lower spatial and temporal resolution than the data from which it was derived.
- DBMS it self was a centralized DBMS where all the DBMS functionality, application program execution and user interface processing carried out in one machine.
- User management system.
- Central Documents management system.
Advantages and Disadvantages of Centralized Approach:
- Decreased Risk: With Centralized data management, all edits and manipulation to core data are housed and stored centrally. This model allows for staunch controls, detailed audit trails, and enables business users to access consistent data.
- Data Consistency: When data feeds are managed in a central repository, an organization can achieve consistent data management and distribution throughout its global offices and internal systems.
- Data Quality: A data-centric approach enables the establishment of a data standard across an enterprise, allowing organizations to make better business assessments.
- Operational Efficiency: When one business unit controls an organization data centrally, the resources previously devoted to data management can be redirected back to core business needs.
- Single Point of Entry: By introducing single point of entry for data, this allows changes from data vendors to be implemented once, rather than in multiple instances.
- Cost Saving: With data management centralized, costs attributed to vendor relationships are better controlled, minimizing any redundancy in market data contracts and their associated costs.
- Response Time: The size of a centralized database could cause data retrieval delays.
- Equipment Cost: Depend on architecture, probably expensive if main frames are used.
- Incremental Growth: Updating a centralized system is likely to be more difficult and more costly particularly if is it build on main frames. Large systems are typically more problematic to upgrade.
- Single Point of Failure: Entire data is sorted at a single point (central server), if the data failed or corrupted then all the data will be lose.
Factors for Adoption of Centralized Approach:
- Data can be organized in single point, by introducing single point of entry for data Database Administrator can implement the data only once instead of in multiple sites.
- Data consistency can achieve by introducing data-centric approach.
- Centralized database approach is suitable for establishment of data standards across an enterprise.
- For batter security purpose Centralized database approach is suitable.
- For quick efficient searching Centralized approach is good one.
- For controlled access to the database repository.
In my point of view we must insure that the system we design is flexible enough to take advantage of the latest available technology not only to solve today's problems but to have ability to offer new products and services we can't even imagine. In both of the systems Centralized approach is more reliable as compare to distributed database system.
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