3 Level Architecture Flashcards
What are the 2 main reasons for the 3LA?
Data Abstraction and Data Independance
Define Data Abstraction…
Architecting the database in such a way that the end users can’t view or interact directly with the internal level, but can only see the views on the external level.
Define Data Independence…
The property that data is stored independently from the rest of the model. This means that data and configuration of levels can be modified without effecting other levels.
Logical Independence - The separation of the External and Conceptual level, meaning the external level can be changed without effecting the internal level.
Physical Independence - The separation of the Internal and Conceptual level, meaning the physical level can be changed without effecting the end users.
What are the main benefits of Data Abstraction?
Security - Users can only access their allocated data from the database.
Reduce Complexity - Reduces the complexity of the database for the users, only showing them relevant data.
Customisable - Views enable users to have a tailored view of the database to see data relevant to them.
Define the Internal Level
Consists of the technical structuring of the database. This includes which data structures are used as storage, which hardware to use, security measures etc.
Focuses on HOW the data is stored.
Define the Conceptual Level
Provides a conceptual mapping of the data stored in the internal level. Shows the data, data descriptions and the logical constraints. This level provides mapping to the internal and external levels, and is the enabler of the abstraction and independence properties.
Focuses on WHAT data is stored.
Define External Level
The user facing level which provides the views that users use to view and interact with the data base.
Focuses on the WAY in which the data is presented to the users.
In what way does the External Level map to the Conceptual Level?
In a many to one way.
Define Database Schema
Refers to the logical design of the database. This is relatively static and is defined the design stage.
Define Database State
Refers to the state of the database at any given time. This is dynamic, and changes in accordance with the changes of data instances held in the database.
What is a Data Model?
A collection of concepts that enable a high level interpretation of the database. This makes the database schemas more readable for users.
What are the 3 types of Data Models?
Object-orientated : Consists of Objects (entities), attributes, relationships and constraints. These include ER models and semantic models.
Record-based : Consist of tables with records and attributes, each row is known as a tuple. Each column is an attribute. These include the relational model, network model and hierarchical model.
Physical models : Physical representations of the database.
What are the 3 types of Data Models?
Object-orientated : Consists of Objects (entities), attributes, relationships and constraints. These include ER models and semantic models.
Record-based : Consist of tables with records and attributes, each row is known as a tuple. Each column is an attribute. These include the relational model, network model and hierarchical model.
Physical models : Physical representations of the database.
