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What are functional requirements? Differences from non-functional requirements related to system quality improvement

functional requirements
functional requirements

Functional requirements are an important process required in software and system development. Appropriately ask the client what functions you want to incorporate into the system you are creating, and clearly define what kind of functions you need. In addition, there is “non-functional requirement” as a word contrary to the functional requirement.
Non-functional requirements are non-functional requirements such as response speed and security, not the features presented by the client. This time, we will introduce important functional requirements and non-functional requirements in the system development and production process.


table of contents

  • 1. Differences between functional and non-functional requirements
  • 2. What are the functional requirements?
  • 3. “Non-functional requirements” that increase client satisfaction
  • 4. Non-functional requirement grade
  • 5. Importance and precautions of non-functional requirements
  • 6. Non-functional testing that enhances the appeal of software
  • 7. Accurate requirement definition is indispensable for functional and non-functional requirements
  • 8. How to smoothly find the best supplier for system development


Differences between functional and non-functional requirements

The functional requirement is “the” function “required by the client in software and system development.” On the other hand, non-functional requirements refer to all quality-related items such as “non-functional” usability, performance, extensibility, and security.

Satisfying the “functions” requested by clients in software and systems is the minimum requirement for development. If the non-functional requirements “other than functions” are not provided in general, users may complain that they are not easy to use, or they may develop problems such as inadequate security and information leakage. Therefore, quality can only be guaranteed by having a good balance of non-functional requirements as well as functional requirements.

Each role is explained below.


What are functional requirements?

Both functional and non-functional requirements are the first steps in system development. First, let’s look at the functional requirements that are central to development.

Generally, in system development / construction, production proceeds in the order of “requirements definition”-> “design”-> “manufacturing”-> “inspection”.

In the requirements definition, the functions required by the client are confirmed for the software and system to be produced. Of the requirement definitions, “features that must be installed” are called “functional requirements”. Functional requirements are matters that the client wants to be installed, so they can be picked up relatively easily by direct hearing. For example, functional requirements are the basic parts that must be achieved, such as “I want you to always include the functions you are currently using in the current system” and “I am in trouble if you cannot do ◯◯”.

Functional requirements are necessary functions for clients, and if the functions specified at the time of delivery are not implemented, the project will fail. Implementation of functional requirements is a matter of course for clients, and since it is the minimum required function, the fact that it is installed does not greatly increase satisfaction.


“Non-functional requirements” that increase client satisfaction

On the other hand, of the requirement definitions, those other than those that do not meet the functional requirements are called “non-functional requirements”. Non-functional requirements are directly linked to increased client satisfaction.


● What are non-functional requirements?

Non-functional requirements are functions other than the main purpose functional requirements and refer to all non-functional requirements. It refers to features such as usability, performance, scalability, and security, and is an essential “quality” part of the product. For example, even if you develop a high-performance sales management system, if it takes more than 30 minutes from the start of execution to aggregate sales in a day, customer satisfaction will be below. High-quality non-functional requirements will increase client satisfaction.


● Non-functional requirements are difficult to determine

The contents of non-functional requirements are diverse, and many secondary items are difficult to cover, such as conditions and security in the operation process, manageability, and performance.

Non-functional requirements are not certain requests from clients but are requirements that the development side considers based on the contents of the hearing. Including all possible non-functional requirements will not fit the budget, so having to decide how many non-functional requirements to include is also one of the factors that make non-functional requirements difficult.


Non-functional requirement grade

“Non-functional requirements grade” is a tool created to prevent misunderstandings between clients (users) and developers about “non-functional requirements” and unintended understanding of each other. Items related to non-functional requirements are comprehensively listed and further classified. Since the items are arranged in order from the most important items, both parties can check and set the request level while checking non-functional requests.

The non-functional requirements grade classifies non-functional requirements into the following 6 types and 238 items.

The six categories of “availability”, “performance / expandability”, “operation / maintainability”, “migration”, “security”, and “system environment / ecology” are explained below.


● Availability

Availability is the requirement for the continuous availability of systems and services. Specifically, it refers to requests related to operation schedules and recovery in the event of a failure. To achieve availability, it is necessary to establish a backup system and a recovery method in the event of a failure.

For example, in the case of an EC site, in principle, it operates 24 hours a day, 364 days. If you want to keep the service as low as possible during maintenance and minimize downtime in the event of a disaster, you need to state that fact.


● Performance and expandability

Performance/expandability is a requirement for system performance and future system expansion.

To achieve performance and expandability, it is necessary to improve the processing efficiency inside the software and consider the layout of network devices and install new devices and processing speeds.

For example, it often happens that there is no problem at the beginning of the operation, but the system slows down about 3 months after the operation, which significantly reduces the usability of the user. From the development side, “I didn’t hear that it would increase so much”, but if you show how much the amount of data and the number of users will increase from the beginning, the responsibility will shift to the development side. Therefore, the development side also plans and designs the platform and system configuration with proper scalability in mind, so such troubles are less likely to occur.


● Operation and maintainability

Operability and maintainability are requirements related to services in system operation and maintenance. Specifically, the requirements regarding the operating rate during system operation and the response when problems occur refer to operation/maintainability.

To achieve operation and maintainability, it is necessary to take measures by enhancing monitoring for normal operation and expanding operation manuals.

For example, we often see announcements on websites and in-house systems such as “We will stop the site from 3:00 to 6:00 tomorrow for system maintenance.” If the system allows some downtime for maintenance, it is natural to divide it to reduce costs. However, even in that case, it is safer to state in the non-functional requirements to what extent maintenance outages can be tolerated. On the other hand, if you want the system to operate continuously even during maintenance, you will need to specify that fact in the non-functional requirements and prepare a mechanism such as hot standby.


● Migration

Migrating is a requirement for the migration of current system assets.

To achieve migration, it is necessary to take measures by adjusting the schedule until migration and conducting rehearsals.

For example, the requirements include the migration period and method for the new system, the type and amount of assets to be migrated, and so on. Addressing this requirement involves planning a migration schedule, developing migration tools, establishing a migration system, and conducting migration rehearsals.


● Security

Security is a requirement for ensuring the security of information systems. Specifically, requests for user restrictions and prevention of unauthorized access refer to security. To achieve security, for example, access restrictions, data confidentiality, fraud tracking/monitoring/detection, and information security education for operators are required.


● System environment / ecology

The system environment/ecology is a requirement regarding the system installation environment and ecology. Specifically, there are matters related to the system environment such as earthquake resistance / seismic isolation, weight/space, temperature/humidity, and noise, and matters related to ecologies such as CO2 emissions and energy consumption. To realize the system environment and ecology, it is necessary to select equipment that meets the standards and electrical equipment and to configure it to reduce the environmental load.

Non-functional requirement grades are formulated by the Information-technology Promotion Agency (IPA) to ensure the quality of requirement definitions and provide guidelines. Non-functional requirements grades can be downloaded from the IPA official website for anyone to use. For those who want to know more detailed contents or who want to use the non-functional requirement grade in practice, download the material from ” Strengthening the upstream process of system construction (non-functional requirement grade): IPA Information-technology Promotion Agency “. Please use it.


Importance and precautions of non-functional requirements

Non-functional requirements are not the requirements directly requested by the client. But why is it so important in system development?


● Why are non-functional requirements important?

Non-functional requirements are requirements that are installed after satisfying the functional requirements. In system development, if functional requirements are the main function, non-functional requirements are functions like options, and the more non-functional requirements are met, the more satisfied the client is.

A system company does not end with receiving an order for one system from one client and delivering it. Continuous interaction with clients is essential, such as system expansion, system replacement after a few years, and ordering and operation of new systems. Therefore, it is very important to meet non-functional requirements and increase client satisfaction.


● Notes on non-functional requirements

Non-functional requirements are heard through hearings about what the client is having trouble with and wants to improve. Prepare the required items that are expected to some extent in advance, and make proposals while listening to the client’s request. If the functions that you think are good and are not included in your way are not in line with the actual workflow, even if the functions are created over time, the client’s satisfaction will drop.

Non-functional requirements that the client has not made a definite request must be agreed upon in advance before installation. Find out how to tell the client about an item, such as “which item of non-functional requirements indicates what” and “how important it is”. For example, in the case of a system that strengthens security items, it is necessary not only to explain its importance but also to convince the client of the development budget for security functions.

It is not necessary to explain all non-functional requirements to the client, but there is a risk that it will lead to complaints after delivery unless the parts that the user directly touches, such as usability and performance, are agreed upon in advance. there is.


● Check non-functional requirements on both sides and avoid troubles

Non-functional requirements are prone to trouble in system development projects. This is not the case for all clients, but in general, when developing a system for something, the client can consider “what he wants to do”. However, in most cases, it is not possible to grasp the “actual usability” at the next stage.

For example, when it comes to developing a system that automatically calculates payroll, a function that takes a month to process the system is useless in the actual field. Therefore, the client-side complains that “I haven’t heard that it will take so long”, but the subcontractor may say “I have installed the functions as requested”.

Therefore, in order not to cause trouble with non-functional requirements, both parties need to discuss the non-functional requirements properly and consider them from a realistic operational perspective.


Non-functional testing that enhances the appeal of software

Let’s check with non-functional tests to deliver high-quality software and systems that are directly linked to client satisfaction.


● What is a non-functional test?

A non-functional test that corresponds to the inspection process of system production is a test that verifies parts other than the main purpose of the system. There are many types of test items, as everything but functionality is subject to verification. The more non-functional tests you pass, the more satisfied your clients will be.


● Examples of non-functional tests

Here are some of the non-functional tests.


<Performance test>

Performance tests, also called performance tests, are tests related to performance parts such as software and system processing speed and response speed. In particular, a system that does not pass the performance test due to the content that affects the user’s operability will cause operational stress and cause complaints.


<Stress test>

A stress test is a test in which a pre-defined limit value or a load higher than that is applied. For example, we will test and confirm whether the server can withstand the expected number of simultaneous accesses so that the response speed of the server does not slow down or stop due to a large number of simultaneous accesses.


<Usability test>

Usability testing is a test of the ease of use of the system, such as whether it can be operated intuitively. Even if it is a perfect system that has all the functions, is highly responsive, and has perfect security, if the design is such that the user does not know where and what function is, the function will not be fully used and the satisfaction will be reduced.


<Maintainability test>

This is a test to check whether the software or system is designed to be easy to fix or add when it is defective and needs to be repaired. Inadequate maintainability testing can require a larger budget than the client expects when refurbishing the system, which can lead to problems later.


Accurate requirement definition is indispensable for functional and non-functional requirements

Both requirement definitions are important, and accurate requirements are essential for software and system development.

Of course, the most important is the functional requirement, and if there is a leak, it can cause big trouble.

However, for clients, the features defined in the functional requirements are implemented and taken for granted, and it is the non-functional requirements that improve the quality and satisfaction of the system. Functional and non-functional requirements will need to be accurate and polite over time to provide clients with a satisfying system.


How to smoothly find the best supplier for system development

Are you having trouble choosing a system development company?
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