Storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system.
The storage manager is responsible to the following tasks: i) Interaction with the file manager
ii) Efficient storing, retrieving and updating of data
Issues: i. Storage access
ii. File organization
iii. Indexing and hashing
iv. Storage Manager Components
It provides the interface between the low level data stored in the database and the application programs and quries submitted to the system. These components are:
(1) File manager: it manage disk space allocation and the data structure used to store the data.
(2) Buffer manager: it fetches the data from disk to main memeory and decides the caching strategy suitable for the application.
(3) Transaction manager: it ensures consistency of the database after any transaction performed on it.
(4) Authorization and integrity manager: it tests the integrity constraintants and checks authority of users to access data.
THE VARIOUS DATABASE MODELS
Databases appeared in the late 1960s, at a time when the need for a flexible information management system had arisen. There are five models of DBMS, which are distinguished based on how they represent the data contained.
The hierarchical model: The data is sorted hierarchically, using a downward tree. This model uses pointers to navigate between stored data. It was the first DBMS model.
The network model: like the hierarchical model, this model uses pointers toward stored data. However, it does not necessarily use a downward tree structure.
·the relational model (RDBMS, Relational database management system): The data is stored in two-dimensional tables (rows and columns). The data is manipulated based on the relational theory of mathematics.
·The deductive model: Data is represented as a table, but is manipulated using predicate calculus.
·The object model (ODBMS, object-oriented database management system): the data is stored in the form of objects, which are structures called classes that display the data within. The fields are instances of these classes
By the late 1990s, relational databases were the most commonly used (comprising about three-quarters of all databases).
