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Types of Testing in the V-Model

As you get involved in the development of a new system a vast number of software tests appear to be required to prove the system. While they are consistent in all having the word "test" in them, their relative importance to each other is not clear. This advice article gives an outline of the various types of software testing and how they fit into the V-Model.

The main software testing types are:

To put these types of software testing in context requires an outline of the development process.

Development Process

The development process for a system is traditionally as a Waterfall Model where each step follows the next, as if in a waterfall. This shows how various products produced at each step are used in the process following it. It does not imply that any of the steps in a process have to be completed, before the next step starts, or that prior steps will not have to be revisited later in development. It is just a useful model for seeing how each step works with each of the others.

Business Case

The first step in development is a business investigation followed by a "Business Case" produced by the customer for a system. This outlines a new system, or change to an existing system, which it is anticipated will deliver business benefits, and outlines the costs expected when developing and running the system.

User Requirements

The next broad step is to define a set of "User Requirements", which is a statement by the customer of what the system shall achieve in order to meet the need. These involve both functional and non-functional requirements. Further details are in the requirements article.

System Specification

"Requirements" are then passed to developers, who produce a "System Specification". This changes the focus from what the system shall achieve to how it will achieve it by defining it in computer terms, taking into account both functional and non-functional requirements.

System Design

Other developers produce a "System Design" from the "System Specification". This takes the features required and maps them to various components, and defines the relationships between these components. The whole design should result in a detailed system design that will achieve what is required by the "System Specification".

Component Design

Each component then has a "Component Design", which describes in detail exactly how it will perform its piece of processing.

Component Construction

Finally each component is built, and then is ready for the test process.

Types of Testing

V-Model diagram


The level of test is the primary focus of a system and derives from the way a software system is designed and built up. Conventionally this is known as the "V-Model", which maps the types of test to each stage of development.

Component Test

Starting from the bottom the first test level is "Component Test", sometimes called Unit Testing. It involves checking that each feature specified in the "Component Design" has been implemented in the component.

In theory an independent tester should do this, but in practise the developer usually does it, as they are the only people who understand how a component works. The problem with a component is that it performs only a small part of the functionality of a system, and it relies on co-operating with other parts of the system, which may not have been built yet. To overcome this, the developer either builds, or uses special software to trick the component into believing it is working in a fully functional system.

Interface Test

As the components are constructed and tested they are then linked together to check if they work with each other. It is a fact that two components that have passed all their tests, when connected to each other produce one new component full of faults. These tests can be done by specialists, or by the developers.

Interface Testing is not focussed on what the components are doing but on how they communicate with each other, as specified in the "System Design". The "System Design" defines relationships between components, and this involves stating:

Tests are constructed to deal with each of these.

The tests are organised to check all the interfaces, until all the components have been built and interfaced to each other producing the whole system.

System Test

Once the entire system has been built then it has to be tested against the "System Specification" to check if it delivers the features required. It is still developer focussed, although specialist developers known as systems testers are normally employed to do it.

In essence the System Test is not about checking the individual parts of the design, but about checking the system as a whole. In effect it is one giant component.

System testing can involve a number of specialist types of test to see if all the functional and non-functional requirements have been met. In addition to functional requirements these may include the following types of testing for the non-functional requirements:

There are many others, the needs for which are dictated by how the system is supposed to perform.

Acceptance Test

Acceptance Testing checks the system against the "User Requirements". It is similar to systems testing in that the whole system is checked but the important difference is the change in focus:

The customer, and not the developer should always do acceptance testing. The customer knows what is required from the system to achieve value in the business and is the only person qualified to make that judgement. The forms of the tests may follow those in system testing, but at all times they are informed by the business needs.

Release Test

Even if a system meets all its requirements, there is still a case to be answered that it will benefit the business. The linking of "Business Case" to Release Testing is looser than the others, but is still important.

Release Testing is about seeing if the new or changed system will work in the existing business environment. Mainly this means the technical environment, and checks concerns such as:

These tests are usually run the by the computer operations team in a business. The answers to their questions could have significant a financial impact if new computer hardware should be required, and adversely affect the "Business Case".

It would appear obvious that the operations team should be involved right from the start of a project to give their opinion of the impact a new system may have. They could then make sure the "Business Case" is relatively sound, at least from the capital expenditure, and ongoing running costs aspects. However in practise many operations teams only find out about a project just weeks before it is supposed to go live, which can result in major problems.

Regression Tests

With modern systems one person's system, becomes somebody else's component. It follows that all the above types of testing could be repeated at many levels in order to deliver the final value to the business. In fact every time a system is altered.

More on the V-Model and Testing