Flexibility and Extensibility
Flexibility is the ability of software to add/modify/remove functionality without damaging the current system. Extensibility is the ability of software to add functionality without damaging system, so it may be thought of as a subset of flexibility. Those functionality changes may occur according to changing requirements, or an obligation if the development process is one of the iterative methods. Change is inevitable in software development and so, this is one of the most important properties of quality software
Maintainability and Readability
Maintainability is a little similar with flexibility but it focuses on modifications about error corrections and minor function modifications, not major functional extensibilities. It can be supported with useful interface definitions, documentation, and self-documenting code and/or code documentation. The more correct and useful documentation exists, the more maintainability can be performed.
Performance and Efficiency
Performance is mostly about the response time of the software. This response time should be in acceptable intervals (e.g. max. a few seconds), and should not increase if transaction count increases. And also, resources are expensive. Efficiency must be supported with resource utilization. As an exaggerated example, the ability to perform a simple function only by using a 32 processor machine or 1 TB disk space is not acceptable. Optimal source/performance ratio must be aimed.
Scalability
A scalable system responds user actions in an acceptable amount of time, even if load increases. Of course more hardware may be added for handling increasing user transaction, but the architecture should not change while doing this. This is called vertical scalability. Ability of running on multiple, increasing count of machines is multiple processing. If the software can perform that type of processing, this is called horizontal scalability. A preffered scalable system should suit both of these methods.
Availability, Robustness, Fault Tolerance and Reliability:
A robust software should not lose its availabilty even in most failure states. Even if some components are broken down, it may continue running. Besides, even if whole application crashes, it may recover itself using backup hardware and data with fault tolerance approaches. There should always be B and even C, D .. plans. Reliability also stands for the integrity and consistency of the software even under high load conditions. So it is relevant with availability and scalability. An unreliable system is also unscalable.
Usability and Accessability
User interfaces are the only visible parts of software according to the viewpoint of user. So, simplicity, taking less time to complete a job, fast learnability etc. are very important in this case. The most well known principle for this property is KISS (Keep It Simple Stupid). Simple is always the best. A usable software should also support different accessibility types of control for people with disabilities.
Platform Compatibility and Portability
A quality software should run on as much various platforms as it can. So, more people can make use of it. In different contexts we may mention different platforms, this may be OS platforms, browser types etc. And portability is about adapting software that can run on different platforms, for being more platform compatible. In this sense, portability is also related with flexibility
Testability and Manageability
Quality software requires quality testing. Source code should be tested with the most coverage and with the most efficient testing methods. This can be performed by using encapsulation, interfaces, patterns, low coupling etc. techniques correctly. Besides testability, a qualified software should be manageable after deployment. It may be monitored for e.g. performance or data usage status, or may enable developer to configure system easily. Creating a successful logging system is another very important issue about managability.
Security
Security is a very important issue on software development, especially for web or mobile based ones which may have millions of users with the ability of remote accessing to system. You should construct a security policy and apply it correctly by leaving no entry points. This may include authorization and authentication techniques, network attack protections, data encryption and so on. all possible types of security leaks should be considered, otherwise one day only one attack may crash your whole applicaion and whole company.
Functionality and Correctness
Functionality (or correctness) is the conformity of the software with actual requirements and specifications. In fact this is the precendition attribute of an application, and maybe not a quality factor but we wanted to point that as the last quality factor, for taking attention: Quality factors are not meaningful when we are talking about unfunctional software. First, perform desired functionality and produce correct software, then apply quality factors on it. If you can perform both paralelly, it is the best.
