A DBMS is a complex software system, which is made up of a set of software component modules. The following figure shows various functional modules of a DBMS:
The above figure showing the components of DBMS can be grouped into two parts: upper part and lower part. Upper part includes various users of the database system and the interfaces used by them to interact with the DBMS. Whereas the lower part of the figure shows various functional modules of the DBMS that are responsible for storage of data and processing of transactions (modication or updation of data).
At physical level, databases are stored in the form of files on hard disk. Access to the hard disk is controlled by the Operating System (OS), and hence DBMS has to make use of the services provided by OS for read/write access of data from databases.
In DBMS, the task of storage and retrieval of data from the database is handled with the help of Stored Data Manager, a higher-level module of the DBMS. Stored data manager makes use of basic operating system services for carrying out low-level read/write operations between the disk and main memory.
Moreover, many DBMS have their own buffer management module to schedule disk read/write, because management of buffer storage has a considerable effect on the performance of storage and retrieval of data from database.
Finally, the runtime database processor module is responsible for performing the following tasks:
- Executes the privileged commands
- Takes care of the executable query plans
- Runs the canned transactions with runtime parameters
It does the above mentioned activities with the help of system catalog and updates it with statistics.
Inerfaces and Components in the Upper Part of DBMS:
Users of databases make use of some form of interface to interact with DBMS directly or indirectly. The Database Administrator directly work with the DBMS and interact with it through commands - DDL Statements or Privileged Commands. DDL Statements given by a DBA will be processed by the DDL Compiler to structure the database (schema) by storing the metadata in the System Catalog.
The System Catelog also called as Data Dictionary includes information such as the names and sizes of files, names and data types of data items, storage details of each file, mapping information among schemas, and constrains.
Casual Users work with the interactive interfae to formulate queries that can be processed by the DBMS with the help of Query Compiler and Query Optimizer. Query compiler parses and validates the correctness (syntax) of the queries entered by the casual user and compiles them into an internal form. The internal query is then subjected to query optimizer for query optimization.
The query optimizer is concerned with the rearrangement and possible reordering of operations, elimination of redundancies, and makes use of efficient search algorithms during the execution of SQL queries. It consults the system catalog for statistical and other physical information about the stored data and generates executable code that performs the necessary operations for the query.
Application Programmers write programs in host languages such as C, C++ or Java that are submitted to the precompiler. The precompiler extracts DML commands from an applicaiton program written in a host programming language. These commands are sent to the DML Compiler for compilation into object code for database access. The rest of the program is sent to the host language compiler.
The object code generated by the DML Compiler and the rest of the program are linked, forming a canned transaction whose executable code includes calls to the runtime database processor. Canned transactions are executed repeatedly by parametric users via PCs or mobile apps; these users simply supply the parameters to the transactions. Each execution is considered to be a separate transaction.
Role of a DataBase Administrator (DBA)
One of the main reasons for using DBMS (back-end) is to have central control of data that can be stored and retrieved from the disk by the application programs (called frond-ends). A person who has such central control over the design and management of data in databases is called DataBase Administrator (DBA). Some of the functions of DBA include the following:
1. Defining or Modifying Database Schema: It is the role of the DBA to create the database schema (structure of data to be stored in the database) by executing a set of commands called DDL - Data Definition Language. He can also make modifications in the existing schema to reflect the changing need of the organization.
2. Granting or Revoking Authorization for Data Access: By granting different types of authorization, the database administrator can regulate which parts of the database various users can access. The authorization information is kept in a special database that the DBMS consults whenever someone attempts to access the data in the system.
3. Takes Care of the Routing Maintenance: Some of the maintenance work done by the DBA are as follows:
- Periodically backing up the database, either onto tapes or onto remote servers, to prevent loss of data in case of disasters such as flooding
- Ensuring that enough disk space is available for normal operations, and upgrading disk space as required
- Monitoring jobs running on the database and ensuring that performance is not degraded by very expensive tasks submitted by some users.
