A project’s ultimate purpose is to create a high-quality result that is exactly what the customer requested. Customers express their requirements to the project team primarily through functional requirements. Functional requirements assist in keeping the project team on track.

Uncertain requirements lead to a poorly defined scope, which introduces a slew of problems right from the start. A poorly specified scope causes the schedule to be extended and the expense to rise. Because the buyer may not have the time or money to invest, they will accept a low-quality product.

In most cases, the customer has both needs and desires. They may request a scope reduction after seeing the cost estimate. The scope is usually reduced by deleting some of the non-functional needs. A large number of non-functional needs can quickly drive-up costs, whereas a lack of non-functional requirements can lead to poor user experience.

Both the customer and the IT provider will benefit from knowing the difference between functional and non-functional requirements since they will be able to properly grasp their requirements. This results in scope refinement, cost optimization, and a satisfied customer.

A rational and complete set of both functional and non-functional requirements is the most important thing any project can have in order to avoid failure.

The requirements for every project must be well-considered, balanced, and well understood by all parties involved, but perhaps most importantly, they must not be dropped or compromised halfway through the project.

What is the distinction between ‘functional’ and ‘non-functional’ needs, though? It’s not that difficult to comprehend, and once you do, the definition will be evident.

A functional requirement, according to the official definition, is essentially a specification of what the system should accomplish.

Functional requirements usually identify a behavior or function, such as:

“Display a flash drive’s name, total size, available space, and format when connected to a USB port.” “Add customer” and “print invoice” are two more examples.

What are Functional Requirements?

A functional need is defined as follows:

“Any criterion that specifies what the system should be able to do.”

In other words, when certain conditions are met, a functional requirement will specify a specific behavior or function of the system, such as “Send email when a new customer registers up” or “Open a new account.”

“Ability to contain tea or coffee without leaking” would be a functional need for an everyday object like a cup.

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

What about Non-Functional Requirements, for example? What are they, and how do they differ from one another?

Simply expressed, non-functional requirements describe how the system operates, whereas functional requirements describe what the system should accomplish.

A non-functional requirement is defined as something that fundamentally dictates how the system should behave and is a constraint on that behavior. Non-functional requirements can alternatively be thought of as system quality attributes.

What are non-functional requirements?

A non-functional demand is defined as follows:

“Any criterion that specifies how a system performs a specific task.”

In other words, a non-functional requirement will specify how a system should behave as well as its functional limitations.

Non-functional requirements encompass all criteria that aren’t addressed by the functional requirements. They define criteria that determine a system’s operation rather than specific behaviors, such as “Modified data in a database should be updated within 2 seconds for all users accessing it.”

“Contain hot liquid without heating up to more than 45°C,” for example, is a non-functional criterion for the cup stated above.

The basic functioning will not be impaired even if the non-functional requirements are not completed.

Why are non-functional requirements necessary if the product’s functionality is not dependent on them? The answer lies in the usability of the product. Non-functional requirements have an impact on the user experience since they define the behavior, features, and overall characteristics of a system.

When non-functional needs are effectively defined and implemented, the system will be easier to use and perform better.

Non-functional requirements, like product attributes, are concerned with user expectations.

Consider the following example of a functional need. When a user hits a button, the system loads a webpage. The non-functional criterion that goes with it states how quickly the webpage must load. Even if the functional need is fully addressed, a delay in loading will result in a negative user experience and poor system quality.

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, as previously stated, define the system’s ‘quality characteristics’ or ‘quality attributes.’

Many diverse stakeholders have a vested interest in getting the non-functional needs right, especially in the case of complex systems when the system’s buyer isn’t always the system’s user.

The value of non-functional requirements should not be underestimated. Using non-functional requirement groups is one technique to ensure that as few non-functional needs as possible are left out. Read this blog post for a discussion of how to use non-functional requirement groups, as well as four of the most common groups to use.

Difference between functional and non-functional requirements:
Functional Requirements Non-Functional Requirements
They define a system or its component. They define the quality attribute of a system
It specifies, “What the system should do?” It specifies, “How should the system fulfill the functional requirements?”
User specifies functional requirement. Non-functional requirement is specified by technical peoples e.g. Architect, Technical leaders and software developers.
It is mandatory to meet these requirements. It is not mandatory to meet these requirements.
It is captured in use case. It is captured as a quality attribute.
Defined at a component level. Applied to a whole system.
Helps you to verify the functionality of the software. Helps you to verify the performance of the software.
Functional Testing like System, Integration, End to End, API testing, etc. are done. Non-Functional Testing like Performance, Stress, Usability, Security testing, etc. are done.
Usually easy to define. Usually more difficult to define.


Examples of functional and non-functional requirements:

Check out the samples of functional and non-functional requirements below:

Functional Requirements Example:
  1. Authentication of a user when he/she tries to log into the system.
  2. System shutdown in the case of a cyber-attack.
  3. Verification email is sent to user whenever he/she registers for the first time on some software system.
Non-functional Requirements Example:
  1. Emails should be sent with a latency of no greater than 12 hours.
  2. Each request should be processed within 10 seconds.
  3. The site should load in 3 seconds when the number of simultaneous users is > 10000
How to write functional and non-functional requirements?

Functional and non-functional requirements can be written in a variety of ways.

A Requirements Specification Document is the most popular approach to write functional and non-functional requirements. It is a textual description of the functionality that is required.

It defines the project goal and contains a project summary for context, as well as any limits and assumptions. Visual representations of the requirements should be included in the requirements definition document to enable non-technical stakeholders grasp the scope.

A Work Breakdown Structure, or WBS, is closely related to a requirements specification document. This “decomposes” the criteria until they can’t be broken down any further, breaking down the entire process into its components.

User stories are another option. They explain the system’s functionality from the point of view of the end-user and specify exactly what they want it to perform.

“As a type of user>, I want goal> so that reason>,” it effectively asserts. User stories have the advantage of requiring little technical knowledge to write. By assisting in the definition of user needs, user stories can also be utilized as a precursor to a requirements specification document.

Use Cases are comparable to user stories in that they don’t require any technical understanding. Simply said, use cases describe in detail what a user does when performing a task. A use case can be “buy product,” which describes each stage in the purchasing process from the user’s perspective.

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