Following a series of 'Database Modeling "this is the article related to" Architecture of a DBMS, "which shows in a graphic way the levels of architecture and sample data communications with other databases.
Architecture of a DBMS
A data model is used to describe the structure "logic" and "physics" of a database. Relationships, data types and constraints are known as the structure or level, dividing into 2 types:
- High Level - we call conceptual data model or Entity-Relationship model, its main concept is a projection of the data that gets closest to the vision that the user has data.
- Low - known as physical data model, is what provides a detailed view yet of how the data are aramazenados computer.
When using the term "description" of the database, as we understand the call of "schema of a database" that is specified for a project database.
Instances are formed when a data is saved in the database for a certain time they formed these database instances, being changed every time a change in the database is performed. DBMS ensures that all instances satisfying the schema of the database, respecting its structure and its constraints.
In a DBMS architecture has as main objective, separate user applications of physical data that are divided from the diagrams below:
- Level domestic or internal schema - uses a data model that shows the physical storage structure of the database, the details of the saved data and access paths.
- Level conceptual or conceptual scheme - performs a full description of the structure of the database but did not offer details of the data stored in the database.
- Level or external layout view - describes the views of the database to a group of users that shows which users have access to this database.
Figure 1: Architecture of a DBMS Layers
We can say that is allowed to perform schema changes or level of a database without changing a higher level. Recalling that only those levels or regimens are shown in Figure 1. Below is representing two types of data independence:
- Logical data independence: change only the conceptual level, without any change in the external or in user applications.
- Physical Data Independence: change the internally without having to change the conceptual level, externally or user applications
Languages for Data Manipulation
The use of language DDL (Data Definition Language - Data Definition Language) is defined by the conceptual and internal level. When there is a separation of internal and conceptual that is not absorbed a clear view of the user, the DBMS has to compile the DDL action, with the action, permission to execute the statements identified by their descriptions of schemes / levels where you will store them in the DBMS catalog.
Where this detachment has an understanding of the user uses the language SDL (Storage Definition Language - Storage Definition Language) for specifying schema / internally.
By the end, when the DBMS uses the architecture with three levels, the language used is VDL (Vision Definition Language) that serves to deinição of views (views) of the database
When the developer has knowledge in language SQL (Strucuture Query Language) simply just use the basic commands that form the language DDL, DML and DCL, which are described below:
- DDL (commands that create objects) - CREATE TABLE, CREATE VIEW, CREATE INDEX, CREATE PROCEDURE among thers.
- DCL (commands that help in the security of the database) - GRANT, REVOKE.
- DML (commands responsible for altering the data) - SELECT, DELETE, UPDATE, INSERT.
Classification of DBMS
- Users: single user, are used in workstations, minicomputers and large machines.
- Location: have 2 states located and distributed. When all data is located are on a single disk, if the data is distributed across multiple machines.
- Environment: it has two types, the homogeneous environment that is formed by a single DBMS and heterogeneous environment that is composed of different DBMSs. One example is to have a system running two types of database.
Figure 2: Structure of a System Manager Database
I hope you enjoyed and until the next opportunity.