What is the difference between functional and non-functional requirements

Question

What is the difference between functional and non-functional requirements in the context of designing a software system  Give examples for each case

User · Answer

FUNCTIONAL REQUIREMENTS  the activities the system must perform   business uses functions the users carry out use cases example if you are developing a payroll system required functions generate  electronic fund transfers calculation commission amounts calculate payroll taxes report tax deduction to the IRS

User · Answer

functional requirements are the main things that the user expects from the software for example if the application is a banking application that application should be able to create a new account  update the account  delete an account  etc  functional requirements are detailed and are specified in the system design   Non-functional requirement are not straight forward the requirement  of the system rather it is related to usability  in some way   for example for a banking application a major non-functional requirement will be available the application should be available 24 7 with no downtime if possible

User · Answer

A functional requirement describes what a software system should do  while non-functional requirements place constraints on how the system will do so   Let me elaborate   An example of a functional requirement would be     A system must send an email whenever a certain condition is met  e g  an order is placed  a customer signs up  etc     A related non-functional requirement for the system may be     Emails should be sent with a latency of no greater than 12 hours from such an activity    The functional requirement is describing the behavior of the system as it relates to the system s functionality  The non-functional requirement elaborates a performance characteristic of the system   Typically non-functional requirements fall into areas such as    Accessibility Capacity  current and forecast Compliance Documentation Disaster recovery Efficiency Effectiveness Extensibility Fault tolerance Interoperability Maintainability Privacy Portability Quality Reliability Resilience Response time Robustness Scalability Security Stability Supportability Testability   A more complete list is available at Wikipedia s entry for non-functional requirements   Non-functional requirements are sometimes defined in terms of metrics  i e  something that can be measured about the system  to make them more tangible  Non-functional requirements may also describe aspects of the system that don t relate to its execution  but rather to its evolution over time  e g  maintainability  extensibility  documentation  etc

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Functional requirements   Functional requirements specifies a function that a system or system component must be able to perform  It can be documented in various ways  The most common ones are written descriptions in documents  and use cases   Use cases can be textual enumeration lists as well as diagrams  describing user actions  Each use case illustrates behavioural scenarios through one or more functional requirements  Often  though  an analyst will begin by eliciting a set of use cases  from which the analyst can derive the functional requirements that must be implemented to allow a user to perform each use case  Functional requirements is what a system is supposed to accomplish  It may be    Calculations  Technical details  Data manipulation Data processing Other specific functionality  A typical functional requirement will contain a unique name and number  a brief summary  and a rationale  This information is used to help the reader understand why the requirement is needed  and to track the requirement through the development of the system    Non-functional requirements  LBushkin have already explained more about Non-functional requirements  I will add more    Non-functional requirements are any other requirement than functional requirements  This are the requirements that specifies criteria that can be used to judge the operation of a system  rather than specific behaviours   Non-functional requirements are in the form of  system shall be    an overall property of the system as a whole or of a particular aspect and not a specific function  The system s overall properties commonly mark the difference between whether the development project has succeeded or failed  Non-functional requirements - can be divided into two main categories    Execution qualities  such as security and usability  which are observable at run time  Evolution qualities  such as testability  maintainability  extensibility and scalability  which are embodied in the static structure of the software system   Non-functional requirements place restrictions on the product being developed  the development process  and specify external constraints that the product must meet  The IEEE-Std 830 - 1993 lists 13 non-functional requirements to be included in a Software Requirements Document          Performance requirements   Interface requirements   Operational requirements   Resource requirements   Verification requirements   Acceptance requirements   Documentation requirements   Security requirements   Portability requirements   Quality requirements   Reliability requirements   Maintainability requirements   Safety requirements      Whether or not a requirement is expressed as a functional or a non-functional requirement may depend    on the level of detail to be included in the requirements document the degree of trust which exists between a system customer and a system developer    Ex  A system may be required to present the user with a display of the number of records in a database  This is a functional requirement  How up-to-date  update  this number needs to be  is a non-functional requirement  If the number needs to be updated in real time  the system architects must ensure that the system is capable of updating the  displayed  record count within an acceptably short interval of the number of records changing   References    Functional requirement Non-functional requirement Quantification and Traceability of Requirements

User · Answer

Functional requirements are those which are related to the technical functionality of the system   non-functional requirement is a requirement that specifies criteria that can be used to judge the operation of a system in particular conditions  rather than specific behaviors   For example if you consider a shopping site  adding items to cart  browsing different items  applying offers and deals and successfully placing orders comes under functional requirements   Where as performance of the system in peak hours  time taken for the system to retrieve data from DB  security of the user data  ability of the system to handle if large number of users login comes under non functional requirements

User · Answer

I think functional requirement is from client to developer side that is regarding functionality to the user by the software and non-functional requirement is from developer to client i e  the requirement is not given by client but it is provided by developer to run the system smoothly e g  safety  security  flexibility  scalability  availability  etc

[requirements] What is the difference between functional and non-functional requirements?

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