Project Software System Requirement

Functional and Non Functional Requirements

Software system requirement can be classified as functional and non functional requirements.

Functional Requirements

• Functional requirements should describe all the required functionality or system services.


• The customer should provide statement of service.  It should be clear how the system should react to particular inputs and how a particular system should behave in particular situation.


• Functional requirements are heavily dependent upon the type of software, expected users and the type of system where the software is used.


• Functional user requirements may be high-level statements of what the system should do but functional system requirements should describe the system services in detail.

For example

Consider a library system in which there is a single interface provided to multiple databases.  These databases are collection of articles from different libraries.  A user can search for, download and print these articles for a personal study.

From this example we can obtain functional requirements as :

1. The user shall be able to search either all of the initial set of databases or select a subset from it.
2. The system shall provide appropriate viewers for the user to read documents in the document store.
3. A unique identifier (ORDER_ID) should be allocated to every order.  This identifier can be copied by the user to the account’s permanent storage area.

Problems Associated with Requirements

·   Requirements Imprecision

1. Problems arise when the requirements are not precisely stated.


2. Ambiguous requirements may be interpreted in different ways by developers and users.


3. Consider meaning of term ‘appropriate viewers’.

·        User intention – special purpose viewer for each different document type.

·        Developer interpretation – Provide a text viewer that shows the contents of the document.

·        Requirements completeness and consistency

The requirements should be both complete and consistent.  Complete means they should include descriptions of all facilities required.  Consistent means there should be no conflicts or contradictions in the descriptions of the system facilities.

Actually in practice, it is impossible to produce a complete and consistent requirements document.

Non Functional Requirements

• The non functional requirements define system properties and constraints.  Various properties of a system can be : Reliability, response time, storage requirements.  And constraints of a system can be : Input and Output device capability, system representatives etc.,

• Process requirements may also specify programming language or developments method.


• Non functional requirements are more critical than functional requirements.  If the non
  functional requirements do not meet then the complete system is of no use.

Types of Non Functional Requirements.

The classification of non functional requirements is as given below.

• Product Requirements,


• Organizational Requirements,


• External Requirements,


• Domain Requirements.