The word reliable means consistent, good in quality or performance and able to be trusted. The reliability of a system can be measured in term of its authenticity, validity, time of presence and ease of access. A distributed database system considered to be reliable it must have several subsequent features
(1)Availability of data: the unavailability of data usually occurs when a particular data element is bound by another system, which is also requested by a system that is waiting for releases the data element.
(2) Grant of access: Users who are approved by the system, can access to view or modify data. Unauthorized data access results in the revelation of information to the illegal means and the users who are not supposed to have access to such information. Like in simple PC if the user is illegal or anonymous no access has been provided. In general centralized systems, it is easy to implement but it is quite complicated in case of distributed database systems. In a multilevel system, allowing access to the system is significantly much more hard and complex to apply because the DBMS has to implement the open access of the user.
(3) Guarantee of Genuineness: when the time of access is to limit the normal working time, then it helps to ensure that the anonymous user is genuine. Usage analysis is as well included to determine that if the current user is matched with the specification of a regular user. The Authenticity is just not confined with the user genuineness and how real the authentication of a user is but it also included the validation, completeness, and correctness of data information stored in a database and available at the time of the request.
The primary significant issues in the security of distributed environment are identification, verification and enforcing suitable access controls. For example, the procedure for recognizing and validate the user, or if a simple password method sufficient with respect to access control rules, data manipulation languages like SQL have built-in GRANT and REVOKE statements which are used to grant And revoke access to users. In many systems and applications, clear-cut GRANT and REVOKE statements are not enough. There can be more complex approval based on database content. Sometimes there is a need for Negative authorizations. The users who are interacting with the system can vary in their roles e-g some are clients, others are the system admins. There is been area under discussion about to provide access to Access to data based on the roles of the user. There is a number of papers have been published on voluntary security in the database environment.