What is distributed database by Prof. Abbas Muhamamd

What is distributed database

A distributed database (DDB) is a collection of multiple, logically interrelated databases distributed over a computer network
 Some systems may seem to be distributed database system, but in actual they are not. For example: shared memory system, shared disk, shared nothing and central database.

Distributed DBMS (DDBMS)

A distributed database management system (DDBMS) is the software that manages the DDB and provides transparent access to the users.

Applications

Airlines
Hotel chains
Any organization which has a decentralized organization structure

Advantages

Higher reliability
Improved performance
Easier system expansion
Transparency of distribution and replication
Higher reliability
Replication of components
No single points of failure
a broken communication link does not bring down the entire system
Distributed transaction processing guarantees the consistency of the database and concurrency
Improved performance
Data is placed near to its points of use that reduces remote access delays
DDBMS supports two types of parallelism during execution
Inter-query  parallelism: multiple queries run at same time
Intra-query parallelism: a query is split into number of threads, each thread runs in parallel.
Easier system expansion
Issue is database scaling
Emergence of microprocessor and workstation technologies
Network of workstations much cheaper than a single mainframe computer
Data communication cost versus telecommunication cost
Increasing database size
Transparency
A transparent system “hides” the implementation details from the users.
User does not know the replication or fragmentation details of data
It hides network concerns from the user.

Disadvantages

Complexity:  database is physical distributed over multiple locations. Several users may access it at a time. Different users may have different privileges (rights or permissions). Therefore proper implementation of such a huge system is very complex.
Cost: Replication adds an extra demand for memory and faster processing element. Also a communication infrastructure is required which raises the overall cost of the system.
Security: System is highly insecure due to decentralized nature. Unlike there is no centralized control over data. Replication may invite unauthorized persons to misuse the data. Thus system security is a big challenge.
Integrity control more difficult: Due to distributed nature of data, its integrity control is not an easy job. Updates coming from various concurrent users are a big threat to data integrity.
Lack of standards: Each site has its own system. No common standards exist for DDBS that are acceptable to all. Each site works well in isolated manner (separately) but issues arise when their integration is required.
Complex DBMS: Distributed database is logically a single database but physically it is distributed over multiple sites. This distribution factor of DDB can’t be handled by simple database management system; rather it required a complex DBMS.
To a user, DDBS seems very simple and single because of transparency but from programmer’s point of view it is very complex. Programmer is the person that knows the inner details of DDBS and writes code to facilitate user as if he/she is using a local and centralized system.
<By Prof. Abbas Muhammad>

Posted By Shahzad Younas