Features of DBMS

A very important advantage of using a DBMS is that it offers data independence.Application programs are insulated from changes in the way the data is structured and stored.users can be shielded from changes in the logical structure of the data, or changes in the choice of relations to be stored. This property is called logical data independence.the conceptual schema insulates users from changes in physical storage details. This property is referred to as physical data independence.

A DBMS provides a specialized language, called the query language, in which queries can be posed.

Query languages:

• Relational calculus- based on mathematical logic, and queries in this language have an intuitive, precise meaning. 
• Relational algebra-based on a collection of operators for manipulating relations

A DBMS enables users to create, modify, and query data through a data manipulation language (DML). Thus, the query language is only one part of the DML, which also provides constructs to insert, delete, and modify data. Let's discuss the DML features of SQL in a later blog post.

An important task of a DBMS is to schedule concurrent accesses to data so that each user can safely ignore the fact that others are accessing the data concurrently.A locking protocol is a set of rules to be followed by each transaction.A lock is a mechanism used to control access to database objects.Two kinds of locks are commonly supported by a DBMS: 

• Shared locks on an object can be held by two different transactions at the same time
• Exclusive lock on an object ensures that no other transactions hold any lock on this object

A DBMS must ensure that the changes made by incomplete transactions(system interruption,crashes) are removed from the database.To do so, the DBMS maintains a log of all writes to the database.A crucial property of the log is that each write action must be recorded in the log (on disk) before the corresponding change is reflected in the database itself.This property is called Write-Ahead Log, or WAL.