What is Distributed Database?
A logically interrelated collection of shared data (and a description of this data), physically distributed over a computer network.
What is Distributed DBMS?
Software system that permits the management of the distributed database and makes the distribution transparent to users.
What is Distributed processing?
A centralized database that can be accessed over a computer network
What is Parallel DBMS?
A DBMS running across multiple processors and disks that is designed to execute operations in parallel, whenever possible, in order to improve performance.
3 main architectures for parallel DBMS:
- shared memory is a tightly coupled architecture in which multiple processors within a single system share system memory.
- shared disk is a loosely couple architecture optimized for applications that are inherently centralized and require high availability and performance.
- shared nothing is a multiple-processor architecture in which each processor is part of a complete system, with its own memory and disk storage.
Advantages of Distributed DBMS?
- Reflects organizational structure
- Improved shareability and autonomy
- Improved availability
- Improved reliability
- Improved performance
- Economics
- Modular growth
- Integration
- Remaining competitive
Disadvantages of Distributed DBMS?
- Complexity
- Cost
- Security
- Integrity control more difficult Lack of standards
- Lack of experience
- Database design more complex
Types of DDBMS?
- Homogeneous DDBMS
3. Heterogeneous DDBMS
What is Multidatabase system?
A distributed DBMS in which each site maintains complete autonomy
Functions of a DDBMS?
- Extended communication services.
- Extended Data Dictionary.
- Distributed query processing.
- Extended concurrency control.
- Extended recovery services.
3 key factors in Distributed Database Design?
- Fragmentation
Relation may be divided into a number of sub-relations, which are then distributed. - Allocation
Each fragment is stored at site with “optimal” distribution. - Replication
Copy of fragment may be maintained at several sites.
Why fragment?
- Usage
Applications work with views rather than entire relations. - Efficiency
Data is stored close to where it is most frequently used. Data that is not needed by local applications is not stored. - Parallelism
With fragments as unit of distribution, transaction can be divided into several subqueries that operate on fragments. - Security
Data not required by local applications is not stored and so not available to unauthorized users.
Disadvantages
- Performance, may be slower
- Integrity, because data and functional dependencies are fragmented and located at different sites.
3 correctness rules in Fragmentation?
- Completeness,
- Reconstruction,
- Disjointness.
4 types of fragmentation?
- Horizontal,
- Vertical,
- Mixed,
- Derived.
4 main types of transparency in a DDBMS?
- distribution transparency
a. Fragmentation Transparency
b. Location Transparency
c. Replication Transparency
d. Local Mapping Transparency
e. Naming Transparency - transaction transparency
a. Concurrency transparency
b. Failure transparency - performance transparency
- DBMS transparency
Date’s 12 Rules for a DDBMS?
- Local Autonomy
- No Reliance on a Central Site
- Continuous Operation
- Location Independence
- Fragmentation Independence
- Replication Independence
- Distributed Query Processing
- Distributed Transaction Processing
- Hardware Independence
- Operating System Independence
- Network Independence
- Database Independence
last four rules are ideals
