Hitchhiker's Guide to Software Architecture and Everything Else - by Michael Stal

Saturday, April 07, 2007

Non-functional requirements

Many software projects don't fail because of functional aspects. In general, experts know very well how to build a system in their domain. The number one technical reason for failure are non-functional requirements. Issues such as reliability, availability or performance have to be addressed in addition to all the domain logic. Unfortunately, many domain experts are not necessarily experts in these non-functional topics. And the efforts to get consistency or availability right can easily outnumber those for functionality. Ask what the CTO of Amazon thinks about this challenge. Thus, my first rule could be described as "always define key developers for the requirements with the highest risks and relevance". These key developers are in charge of teams that explicitely address one or a small number of these issues. The size of these teams and the number of teams depends on your overall project size.
Another important point in this context is desribed as requirements elicitation and requirements engineering. It is interesting how unusable many requirement specifications are. Statements such as "System should provide flexibility" are worthless. What exactly do these stakeholders mean by flexibility? Which parts of the systems should be flexible and when? Of course, you are also doomed to fail if you get a bunch of requirements with no priorization. The same application would look completely different in case security is the number one requirement compared to the same application when performance had the highest priority. So, my second rule simply says "Architects and key developers are in charge to clarify all requirements including their priorities. For each pair of requirements, it should be clear which one has precedence in concrete design decisions". As many requirements might contradict each other, this is inevitable to survive. If your management does not agree to detail requirements or prioritize them, either refuse to be in the project or, more realistic, document this fact and possible implications so that all stakeholders know they might encounter project failure. Unfortunately, non-functional requirements are incredibly diverse. Performance is totally different to security. Even security might mean a lot of various things such as authentication, authorization, confidentiality, name it. Thus, my rule 3 is: "Detail all non-functional requirements in the beginning of the project, maybe establish checklists, and use these information to clarify requirements".
Developmental requirements such as flexibility (e.g., changeability) have to be addressed early only if you are going to design a product line where a Commonality/Variability analysis is of foremost importance. If this is not your primary setting, developmental issues should be treated as a tactical refinement of your software architecture. Thus, the strategic base line architecture is primary influenced by the domain logic and by operational requirements. The integration of developmental issues is a further refinement activity. Considering issues such as extensibility in early phases typically leads to generic and inperformant architectures with instantiations of patterns such as Observer, Interceptor or Strategy. In addition, your job as an architect tends to continue forever in such environments. This is where I'd like to add my fourth rule "Domain logic dominates infrastructure; infrastructure dominates operational requirements; operational requirements dominate developmental requirements". Of course, it is also important to specify when operational requirements appear in the running system (run-time, install time, compile time, ...) and who is in charge to inject them (operator, user, developer, ...).
As of the huge diversity of requirements it is not easy to come up with general rules of thumb for all of them. Some observations might be interesting in tbis context: A given requirement might be invasive (i.e., cross cutting through other parts of your system) or non-invasive (i.e., it is sufficient to add some additional infrastructural components to the architecture). The same requirement might be invasive for one system (security in all its aspects must be supported everywhere) or non-invasive (we just can add a firewall and don't have to care about security at all). A requirement might me measurable such as the response time to a method request or not (such as removability). Requirements that can be measured can be covered by testing, all other requirements must be covered by architecture and code analysis. Last, but not least, my rule 5: "Classify your requirements and provide strategies how to deal with them in architecture design activities".

Of course the overall goal of software engineering must be to
  • Let engineers focus on their domain by strong separation of domain aspects from other aspects.
  • Automate where possible and useful.

There are several efforts to support these goals such as Model-Driven-Software Development, Product Line Engineering, Aspect-Oriented Software Development, SOA.

One way could be to provide DSLs for the domain, additional domains for the most important non-functional requirements, a technology that easily lets project teams integrate these DSLs under one umbrella (e.g., by leveraging AOSD techniques, Semantic Integration). A Product Line Engineering approach could define core assets which become then part of platforms or frameworks. With SOA, platform dependencies could be overcome. Is this just Science Fiction or a reasonable future vision? Let's work on this :-;


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