Database System Structure

The functional components of the database system can be divided into two components: the Storage manager and the Query processor. 

1. Storage Manager

A storage manager is a program which provides interface between low – level data, the application programs and queries submitted to the system. It also interacts with the file manager. The storage manager translates various DML statements into low – level file – system commands. It is also responsible for storing, retrieving, and updating the data stored in the database. The components of storage manager include: 

• Authorization and integrity manager: checks the authority of the users to access the data and tests for the satisfaction of integrity constraints. 
• Transaction manager: ensures consistency of database despite system failures, and concurrent transaction execution without conflict. 
• File manager: manages space allocation on disk storage and data structures used to represent the information stored on disk. 
• Buffer manager: enables database to handle data sized higher than the main memory, fetches data from disk storage into main memory, and also decides what data to cache into main memory.

The storage manager implements several data structures such as: 

• Data files: stores database. 
• Data dictionary: stores metadata about the database structure, in particular database schema. 
• Indices: provides fast access to the data items having particular values. 

2. Query processor

The components of the query processor include: 

• DDL interpreter: interprets DDL statements and records definitions in the data dictionary. 
• DML Complier: translates DML statements in query language into low – level instructions that query evaluation engine understand and also performs query optimization.
• Query evaluation engine: executes low – level instructions generated by DML compiler