Software Twist – Adrian Tosca Blog

What is software quality? Depends who is asking

Posted in Patterns and Practices, Software Architecture by Adrian Tosca on 2011, January 3

Sometimes the software quality is an elusive subject. Defining quality is complex because a different perspective will, most of the time, give a whole new definition.

Take for example what the user perceives as quality attributes of a system. When thinking about the user perspective, usability is the first to come to mind. Most users just muddle through an application functions without taking time to learn it. If the user doesn’t figure out what to do he will just leave. If the user really needs to do a task, such as at work using a company system, he will be very angry if he cannot find its way through the system functions.

Another one of the first thing a user notices about an application is the performance. The performance is a show stopper if is not enough. On the other hand, trying to have extreme performance is not needed most of the time and can have a negative impact on other quality attributes. The availability of the system is also an important aspect from the user perspective. A system must be there when needed or is useless.

Security is also perceived as fundamental from the user point of view, especially in the recent years more and more emphasis is put on securing personal data.

On the other hand if looking from the developer perspective quality of a system looks a bit different. The developer will (or should ) think about maintainability as the system will more likely change in one form or the other as a result of new functions or corrections of the ones already implemented. Some components of the system might be reused if the proper thinking is applied and reusability can have a big impact on future implementations.

One of the latest trends in software development has been to use tests from the inception phases of the project. Techniques such as test driven development are applied with great success for a large class of problems. But to be effective the testability of the system must be build into the product from the begging not as an after thought.

From the business perspective the cost of building the system or the time to marked can be very important as these can decide if it is build at all. The projected life time of a system can be also important because a system needs to be operated and maintained, maybe for several years, and this will incur some king of operating costs. A new system will most likely not exists in isolation and certainly will not change over night what a company does, so integration with legacy systems might be an important aspect. Other important aspects for the business can also be thinks like roll-out time or robustness.

So, in conclusion, what is software quality? Well, it seems it depends who is asking!

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Why Performance is Bad for your Software System

Posted in Software Architecture by Adrian Tosca on 2010, January 16

Performance is one of the so called non-functional requirements (or quality attributes) of a system and you can find it in most analysis documents. The fact that the analyst has written it in the analysis document is a good thing, but is it correctly specified most of the time?

When talking about performance there are many terms that are used, not every time in a consistent way. For example  one can talk about performance and mean ‘response time’ while other can understand ‘throughput’. Let’s see a list of terms that are associated with performance and their most common meaning (1):

  • Response time is the amount of time it takes for the system to completely process a request
  • Responsiveness is about how quickly the system acknowledges a request without actually processing it
  • Latency is the minimum amount of time to get a response even if there is no processing
  • Throughput is  how much processing can be done in a fixed amount of time

There are even more terms related to performance like load, load sensitivity and efficiency but these are what people generally understand when talking about performance. It is important to distinguish between these terms when talking about performance because they can have very different impact on the system. For example to improve the responsiveness of the system you most likely need to change the architecture maybe by implementing something like asynchronous processing.

But the system architecture should take into consideration a lot more than just performance. These are the most common quality attributes that need planned for a software system (2):

  • Agility is the ability of a system to be both flexible and undergo change rapidly
  • Flexibility is the ease with which a system or component can be modified for use in applications or environments other than those for which it was specifically designed
  • Interoperability is the ability of two or more systems or components to exchange information and to use the information that has been exchanged
  • Maintainability is the ease with which a software system or component can be modified
  • Reliability is the ability of the system to keep operating over time
  • Reusability is the degree to which a software module or other work product can be used in more than one computing program or software system
  • Supportability is the ease with which a software system can be operationally maintained
  • Security is a measure of the system’s ability to resist unauthorized attempts at usage and denial of service
  • Scalability is the ability to maintain or improve performance while system demand increases
  • Testability is the degree to which a system or component facilitates testing
  • Usability is the measure of a user’s ability to utilize a system effectively

Usually, modifying one of this attributes affects other attributes as well. For example improving agility will most of the time also improve maintainability. But how about performance? How does the performance improvement, such as modifying the system response time, will affect the other attributes?

Let’s take some examples of things that someone can do to improve performance and what are the effects:

  • A change from a web service interface to remote method invocation to improve the latency will negatively affect reusability
  • A change from an object oriented processing to a stored procedure to improve the response time will negatively affect flexibility and maintainability
  • A change from a clean implementation of an algorithm to an optimized one will negatively affect maintainability and testability

The following diagram illustrates the likely effects of improving the performance of a system to other attributes:

Performance vs Other Quality Attributes

Performance vs Other Quality Attributes

The performance improvement affects negatively every one of the quality attributes described above. So, it is better to carefully analyse the trade-offs of the performance improvement over the other quality attributes. Most of the time ‘enough’ performance is just the right thing to do when thinking in perspective.

1. Patterns of Enterprise Application Architecture by Martin Fowler
2. Implementing System Quality Attributes

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