Distributed DBMS - Database Control


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Database control refers to the task of enforcing regulations so as to provide correct data to authentic users and applications of a database. In order that correct data is available to users, all data should conform to the integrity constraints defined in the database. Besides, data should be screened away from unauthorized users so as to maintain security and privacy of the database. Database control is one of the primary tasks of the database administrator (DBA).

The three dimensions of database control are −

  • Authentication
  • Access rights
  • Integrity constraints

Authentication

In a distributed database system, authentication is the process through which only legitimate users can gain access to the data resources.

Authentication can be enforced in two levels −

  • Controlling Access to Client Computer − At this level, user access is restricted while login to the client computer that provides user-interface to the database server. The most common method is a username/password combination. However, more sophisticated methods like biometric authentication may be used for high security data.

  • Controlling Access to the Database Software − At this level, the database software/administrator assigns some credentials to the user. The user gains access to the database using these credentials. One of the methods is to create a login account within the database server.

Access Rights

A user’s access rights refers to the privileges that the user is given regarding DBMS operations such as the rights to create a table, drop a table, add/delete/update tuples in a table or query upon the table.

In distributed environments, since there are large number of tables and yet larger number of users, it is not feasible to assign individual access rights to users. So, DDBMS defines certain roles. A role is a construct with certain privileges within a database system. Once the different roles are defined, the individual users are assigned one of these roles. Often a hierarchy of roles are defined according to the organization’s hierarchy of authority and responsibility.

For example, the following SQL statements create a role "Accountant" and then assigns this role to user "ABC".

CREATE ROLE ACCOUNTANT; 
GRANT SELECT, INSERT, UPDATE ON EMP_SAL TO ACCOUNTANT; 
GRANT INSERT, UPDATE, DELETE ON TENDER TO ACCOUNTANT; 
GRANT INSERT, SELECT ON EXPENSE TO ACCOUNTANT; 
COMMIT; 
GRANT ACCOUNTANT TO ABC; 
COMMIT;

Semantic Integrity Control

Semantic integrity control defines and enforces the integrity constraints of the database system.

The integrity constraints are as follows −

  • Data type integrity constraint
  • Entity integrity constraint
  • Referential integrity constraint

Data Type Integrity Constraint

A data type constraint restricts the range of values and the type of operations that can be applied to the field with the specified data type.

For example, let us consider that a table "HOSTEL" has three fields - the hostel number, hostel name and capacity. The hostel number should start with capital letter "H" and cannot be NULL, and the capacity should not be more than 150. The following SQL command can be used for data definition −

CREATE TABLE HOSTEL ( 
   H_NO VARCHAR2(5) NOT NULL, 
   H_NAME VARCHAR2(15), 
   CAPACITY INTEGER, 
   CHECK ( H_NO LIKE 'H%'), 
   CHECK ( CAPACITY <= 150) 
); 

Entity Integrity Control

Entity integrity control enforces the rules so that each tuple can be uniquely identified from other tuples. For this a primary key is defined. A primary key is a set of minimal fields that can uniquely identify a tuple. Entity integrity constraint states that no two tuples in a table can have identical values for primary keys and that no field which is a part of the primary key can have NULL value.

For example, in the above hostel table, the hostel number can be assigned as the primary key through the following SQL statement (ignoring the checks) −

CREATE TABLE HOSTEL ( 
   H_NO VARCHAR2(5) PRIMARY KEY, 
   H_NAME VARCHAR2(15), 
   CAPACITY INTEGER 
); 

Referential Integrity Constraint

Referential integrity constraint lays down the rules of foreign keys. A foreign key is a field in a data table that is the primary key of a related table. The referential integrity constraint lays down the rule that the value of the foreign key field should either be among the values of the primary key of the referenced table or be entirely NULL.

For example, let us consider a student table where a student may opt to live in a hostel. To include this, the primary key of hostel table should be included as a foreign key in the student table. The following SQL statement incorporates this −

CREATE TABLE STUDENT (  
   S_ROLL INTEGER PRIMARY KEY, 
   S_NAME VARCHAR2(25) NOT NULL, 
   S_COURSE VARCHAR2(10), 
   S_HOSTEL VARCHAR2(5) REFERENCES HOSTEL 
); 

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