Lecture 1 (Part 5)

Question 1

What are three models of Distributed Database?

Answer 1

1. Federated Databases
2. Warehouse Databases
3. Mediation Databases

Question 2

What are Federated Databases?

Answer 2

All local databases are independent, but one database can call on others to supply information (connecting several distinct databases together through a network)

Question 3

What is a Feature of Federated Databases?

Answer 3

Each local database site builds fixed connections to all other database sites

Question 4

What are Warehouse Databases?

Answer 4

The databases from several local sites are extracted and combined into a global schema. The combines database is then stores at the warehouse, which looks to the user like an ordinary database. User updates are forbidden. System updates are done periodically (e.g. each night)

Question 5

What are features of Warehouse Databases?

Answer 5

Copies of databases from several sites are stored in a single location. Data may be processed before storage. Data is updated periodically, normally overnight.

Question 6

What are Mediation Databases?

Answer 6

A Mediation Database uses a mediator to support virtual views by integrating several local databases. (But there is not any actual data stored physically). The user can issue a query to the mediator. The
mediator forwards the query to each of its local wrappers, which in turn transforms the query into the correct structure that be accepted by their corresponding local databases. When the results come back from the wrappers, the mediator then combines the results and returns them to the user.

Question 7

What is a feature of Mediation Databases?

Answer 7

The user’s queries are transformed into several sub-queries to access the local databases. The returned answer from each local site are synthesized and passed to the user finally.