Chapter 22 & 23. Distributed DBMSs - Concepts and Design Transparencies. Chapter 22 - Objectives. Concepts. Advantages and disadvantages of distributed databases. Functions and architecture for a DDBMS. Distributed database design. Levels of transparency. Comparison criteria for DDBMSs.
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Distributed DBMSs - Concepts and Design
A logically interrelated collection of shared data (and a description of this data), physically distributed over a computer network.
Software system that permits the management of the distributed database and makes the distribution transparent to users.
A centralized database that can be accessed over a computer network. This is not a DDBMS
A DBMS running across multiple processors and disks designed to execute operations in parallel, whenever possible, to improve performance.
(a) shared memory
(b) shared disk
(c) shared nothing
Relation may be divided into a number of sub-relations, which are then distributed.
Each fragment is stored at site with “optimal” distribution.
Copy of fragment may be maintained at several sites.
Consists of single database and DBMS stored at one site with users distributed across the network.
Database partitioned into disjoint fragments, each fragment assigned to one site.
Consists of maintaining complete copy of database at each site.
Combination of partitioning, replication, and centralization.
If relation R is decomposed into fragments R1, R2, ... Rn, each data item that can be found in R must appear in at least one fragment.
CREATE TRIGGER StaffAfterInsRow
BEFORE INSERT ON Staff
FOR EACH ROW
INSERT INTO StaffDuplicate@Rentals.Glasgow.North.Com
VALUES (:new.staffNo, :new:fName, :new:lName, :new.position, :new:sex, :new.DOB, :new:salary, :new:branchNo);