The difference between functional and nonfictional requirements:-
Khalid A. Elzairy, PfMP, PgMP, PMP, RMP,PBA,ACP,SP?,MSc, H.D
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The official definition of ‘a functional requirement’ is that it essentially specifies something the system should do.
Typically, functional requirements will specify a behavior or function, for example: “Display the name, total size, available space and format of a flash drive connected to the USB port.” Other examples are “add customer” and “print invoice”.
Some of the more typical functional requirements include:
Business Rules
Transaction corrections, adjustments and cancellations
Administrative functions
Authentication
Authorization levels
Audit Tracking
External Interfaces
Certification Requirements
Reporting Requirements
Historical Data
Legal or Regulatory Requirements
Example: A functional requirement for a milk carton would be “ability to contain fluid without leaking”.
The definition for a non-functional requirement is that it essentially specifies how the system should behave and that it is a constraint upon the systems behavior. One could also think of non-functional requirements as quality attributes for of a system.
Non-functional requirements cover all the remaining requirements which are not covered by the functional requirements. They specify criteria that judge the operation of a system, rather than specific behaviors, for example: “Modified data in a database should be updated for all users accessing it within 2 seconds.”
Some typical non-functional requirements are:
Performance – for example Response Time, Throughput, Utilization, Static Volumetric
Scalability
Capacity
Availability
Reliability
Recoverability
Maintainability
Serviceability
Security
Regulatory
Manageability
Environmental
Data Integrity
Usability
Interoperability
non-functional requirements specify the system’s ‘quality characteristics’ or ‘quality attributes’.
Example: A non-functional requirement for a hard hat might be “must not break under pressure of less than 10,000 PSI”
Simply, the difference is that non-functional requirements describe how the system works, while functional requirements describe what the system should do.