Quality

Quality is a permanent and implicit requirement!

Yes, this pragmatic seems quite too obvious to be worth talking about. We all know that everyone wants high quality. So, why mention it in first place? The reason is that creating high quality software is not easy at all. Plenty of source code (including most software I have written), is not of high quality. Two main reasons are:

• writing high quality software requires a lot of knowledge, experience and discipline, especially if there is no or little support integrated in the used programming language and tools
  
  
• writing high quality software seems to reduce productivity, because the benefits only become visible much later

Let's take a very typical example. We have to add a new functionality to an existing program. After examining the source code, which may have been written by someone else, we see that it is of bad quality and/or it is not prepared to add the new functionality in an appropriate way. Or, worse, it contains programming errors. In this case we have two possibilities:

1. first we refactor the existing code (this takes time), and then we add the new functionality in a well designed, properly implemented way.
   
   
2. we accept the existing code (after all, it works and our client is impatient), and quickly change the code in a mediocre way.

It is a fact that programmers frequently choose solution 2, because they may feel themselves not responsible for the bad code, because it is often a frustrating task to change existing code, and also because it may take a lot of time (days, weeks or even months). But, as software often has to be changed in the real world (about 70% of a programmer's time is spent on maintenance), the consequence of repeatedly doing changes in such a way will undoubtedly end up in a disaster: unstable, unmaintainable spaghetti code.

Hence, we constantly have to resist against writing low quality, unmaintainable code.

Moreover, we should always keep in mind that:

high quality code always reduces total software development costs

even if at some points much more time must be invested.

But: What exactly is high quality code? Typically, when speaking about quality of software, we think of the following properties:

• correctness (accuracy)
  the ability of the software to execute the tasks exactly as defined in the specifications, without any faults
  
  examples:
  • the total amount of an invoice is the exact sum of the items' quantities multiplied by their unit price.
  • the quantity in stock displayed on screen is correct; we can absolutely rely on the data.
  • the balance sheet is always balanced (sum of all debits = the sum of all credits)
    
    
• reliability (robustness, stability, integrity)
  the ability to perform the tasks under normal conditions and to react appropriately under any abnormal conditions.
  
  examples:
  • any data entry is checked for validity and refused in the case of error.
  • the system doesn't crash and loose data, but reacts appropriately in the case of an unavailable resource (e.g. a file open error)
  • data integrity is guaranteed in the case of power failure.
    
    
• ease-of-use
  the ability of the users (end-users, administrators and developers) to quickly learn, understand and use the software
  
  
• flexibility (adaptability, extendibility, compatibility, portability)
  the ability of the software to be adapted easily to new or modified specifications and requirements of the users
  
  
• reusability
  the ability of a piece of software to be reused in the same or another domain (application) and/or to be extended for new functionalities, without having to re-implement its existing functionalities.
  
  
• efficiency (speed)
  the absence of any waiting times for users as well as the ability to use as few resources as possible.
  
  
• maintainability (readability, testability)
  the easiness for administrators and developers to detect and correct errors and to quickly understand and adapt the software to changes in requirements.
  
  
• security
  the ability of the software to protect data against unauthorized access as well as the possibility to trace actions to any desired precision.

Obviously, there are a lot of requirements to be fulfilled by high quality software. Each of the following pragmatics concerns quality and contributes to one or more of the above properties.