Lecture 1 (Part 3)

Question 1

What is Transparency?

Answer 1

Transparency is the separation of the higher level semantics of a system from the lower level implementation issues. Fundamental issue is to provide data independence in the distributed environment.

Question 2

What is Schema?

Answer 2

Schema shows the definition of the available information at each level.

Question 3

What are the four levels of Schema?

Answer 3

1. The Global Schema
2. The Fragmentation Schema
3. The Allocation Schema
4. Local Mapping Schema

Question 4

What the Global Schema?

Answer 4

Defines all the data which is contained in the distributed database. (same as in the nondistributed database). Each global relation can be split into several non-overlapping portions which are called fragments. (only keys are overlapped)

Question 5

What is the Fragmentation Schema?

Answer 5

Defines the mapping between global relations and fragments. The mapping is a one-to-many relationship. (A global relation corresponds to several fragments.)

Question 6

What is the Allocation Schema?

Answer 6

Defines at which sites a fragment is located

Question 7

Redundant:

Answer 7

fragments are physically located at several sites of the network

Question 8

Non-redundant:

Answer 8

fragments are physically located only at one site of the network

Question 9

What is the Local Mapping Schema?

Answer 9

Maps the physical images to the objects at local DBM. Physical images: defines all fragments which correspond to the same local site.