A distributed database is a set of interconnected databases spread over a computer network. It manages these distributed databases and makes them transparent to users. Data is logically interrelated but physically stored across multiple sites connected by a network. Distributed databases provide advantages like fast processing, reliability, lower costs and easier expansion but are more complex to manage with security and concurrency issues.

1. Distributed Databases
 A distributed database is a set of
interconnected databases that is distributed
over the computer network or internet.
 It manages the distributed database and
provides mechanisms so as to make the
databases transparent to the users

2. Distributed Databases
 Features
 Databases in the collection are logically interrelated with
each other. Often they represent a single logical database.
 Data is physically stored across multiple sites.
 The processors in the sites are connected via a network.
 A distributed database is not a loosely connected file
system.

3. Distributed Databases
 Advantages:
 Fast data processing
 Reliability and availability
 Reduced operating cost
 Easier to expand
 Improved sharing ability and local autonomy.

4. Distributed Databases
 Disadvantages:
 Complex to manage and control.
 The security issues must be carefully managed
 The system require deadlock handling during the
transaction processing
 Need of standardization.

5. Distributed Databases
 Homogeneous Distributed Database:
 In this, all sites have identical database
management system software.
In such a system, local sites surrender a portion of
their autonomy in terms of their right to change
schemas or database management system software.

6. Distributed Databases
 Homogeneous Distributed Database:
 This software must also cooperate with other sites
in exchanging information about transactions, to
make transaction processing possible across
multiple sites.
It appears to user as a single system.

7. Distributed Databases
 Heterogeneous Distributed Database:
 In this, different sites may use different schemas, and
different database management system software.
 The sites may not be aware of one another, and they
may provide only limited facilities for cooperation in
transaction processing.

8. Distributed Databases
 Data Storage:
 Replication: System maintains multiple copies of
data, stored in different sites, for faster retrieval
and fault tolerance
 Fragmentation: Relation is partitioned into several
fragments stored in distinct sites

9. Distributed Databases
 Data Replication:
 The process of storing separate copies of the database
at two or more sites.
 Full Replication: Entire relation is stored at all the
sites.
 Partial Replication: Only some fragments of relation
are replicated on the sites.

10. Distributed Databases
 Data Replication – Advantages:
 Availability
 Parallelism
 Faster Accessing
 Fault Tolerance
 Reduction in Network Load

11. Distributed Databases
 Data Replication – Disadvantages:
 Increased Storage Requirements
 Increased Cost and Complexity of Data Updating

12. Distributed Databases
 Data Fragmentation:
 A division of relation r into fragments r1, r2,
r3…rn which contain sufficient information to
reconstruct relation r.

13. Distributed Databases
 Data Fragmentation – 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.

14. Distributed Databases
 Data Fragmentation – Vertical Fragmentation:
 Example: Student(RollNo, Marks, City)
 select RollNo from Student
 select City from Student.

15. Distributed Databases
 Data Fragmentation – Horizontal Fragmentation:
 In this approach, each tuple of r is assigned to one or
more fragments.
 If relation R is fragmentation in r1 and r2 fragments,
then to bring these fragments back to R we must use
union operation.

16. Distributed Databases
 Data Fragmentation – Horizontal
Fragmentation:
 Example:
Select * from student where marks>50 and
city=‘chennai’

17. Distributed Databases
 Transaction Processing:
 Transaction may access data at several sites
 Local and Global Transaction

18. Distributed Databases
 Transaction Processing – Transaction
Manager:
Maintaining a log for recovery purposes
Participating in coordinating the concurrent
execution of the transactions executing at that site

19. Distributed Databases
 Transaction Processing – Transaction
Coordinator:
 Starting the execution of transactions that
originate at the site.
Distributing subtransactions at appropriate sites for
execution

20. Distributed Databases
 Transaction Processing – Architecture: