Strategies for Implementing Non-Functional Requirements

Non-functional requirements, also known as quality of service (QoS) or technical requirements, are typically system-wide thus they apply to many, and sometimes all of your functional requirements.  Part of ensuring that your solution is potentially consumable each iteration is ensuring that it fulfill its overall quality goals, including applicable NFRs.  This is particularly true with life-critical and mission-critical solutions.  Good sources for NFRs include your enterprise architects and operations staff, although any stakeholder is a potential source for NFRs.

Chapter 8 in the Disciplined Agile Delivery book, written by Mark Lines and myself, overviews several strategies for capturing and then implementing NFRs.  As your stakeholders tell you about functional requirements they will also describe non-functional requirements (NFRs).  These NFRs may describe security access rights, availability requirements, performance concerns, or a host of other issues as saw in my blog regarding initial architecture envisioning.  There are three basic strategies, which can be combined, for capturing NFRs:

  1. Technical stories.  A technical story is a documentation strategy where the NFR is captured as a separate entity that is meant to be addressed in a single iteration.  Technical stories are in effect the NFR equivalent of a user story. For example “The system will be unavailable to end users no more than 30 seconds a week” and “Only the employee, their direct manager, and manager-level human resource people have access to salary information about said employee” are both examples of technical stories.
  2. Acceptance criteria for individual functional requirements.  Part of the strategy of ensuring that a work item is done at the end of an iteration is to verify that it meets all of its acceptance criteria.  Many of these acceptance criterions will reflect NFRs specific to an individual usage requirement, such as “Salary information read-only accessible by the employee,”, “Salary information read-only accessible by their direct manager”, “Salary information read/write accessible by HR managers”, and “Salary information is not accessible to anyone without specific access rights”.  So in effect NFRs are implemented because they become part of your “done” criteria.
  3. Explicit list.  Capture NFRs separately from your work item list in a separate artifact.  This provides you with a reminder for the issues to consider when formulating acceptance criteria for your functional requirements.  In the Unified Process this artifact was called a supplementary specification.

Of course a fourth option would be to not capture NFRs at all.  In theory I suppose this would work in very simple situations but it clearly runs a significant risk of the team building a solution that doesn’t meet the operational needs of the stakeholders.  This is often a symptom of a teams only working with a small subset of their stakeholder types (e.g. only working with end users but not operations staff, senior managers, and so on)

So what are the implications for implementing NFRs given the three previous capture strategies?    Although in the book we would make this sort of comparison via a table to improve consumability, in this blog posting I will use prose due to width constraints.  Let’s consider each one:

  1. Technical stories.  The advantages of this approach are that it is a simple strategy for capturing NFRs and that it works well for solutions with a few NFRs or simple NFRs.  But, the vast majority of NFRs are cross-cutting aspects to several functional stories and as a result cannot be implemented within a single iteration.  This strategy also runs the risk of teams leaving NFRs to the end of the construction phase, thereby pushing technical risk to the end of the lifecycle where it is most difficult and expensive to address.
  2. Acceptance criteria. This is a quality focused approach which makes the complexity of an individual functional requirement apparent, working well with test driven approaches to development.  NFR details are typically identified on a just in time (JIT) basis during construction, fitting in well with a disciplined agile approach.  But, because many NFRs are cross cutting the same NFR will be captured for many functional requirements.  It requires the team to remember and consider all potential NFR issues (see Figure in my previous posting) for each functional requirement.  You will still need to consider NFRs as part of your initial architecture efforts otherwise you risk a major rework effort during the Construction phase because you missed a critical cross-cutting concern).
  3. Explicit list.  This strategy enables you to explore NFRs early in the lifecycle and then address them in your architecture.  The list can be used to drive identification of acceptance criteria on a JIT basis.  But, NFR documents can become long for complex systems (due to the large number of NFRs).  This can be particularly problematic when you have a lot of NFRs that are specific to a small number of functional requirements.  Teams lacking in discipline may not write down the non-functional requirements and trust that they will remember to address them when they’re identifying acceptance criteria for individual stories.


The advice that Mark and I give in the book is that in most situations you should maintain an explicit list and then use that to drive identification of acceptance criteria as we’ve found that it’s more efficient and lower risk in the long run.  Of course capturing NFRs is only one part of the overall process of addressing them.  You will also need to implement and validate them during construction, as well as address them in your architecture.

An important issue which goes to NFRs such as consumability, supportability, and operability, is that of deliverable documentation.  At the start of the project is the best time to identify the required documentation that must be created as part of the overall solution.  This potentially includes operations manuals, support manuals, training materials, system overview materials (such as an architecture handbook), and help manuals to name a few.  These deliverable documents will be developed and kept up to date via the continuous documentation practice.

In my next blog posting, the fourth in this three-part series, I will describe strategies for verifying non-functional requirements.

8 thoughts on “Strategies for Implementing Non-Functional Requirements

  1. Nigel H

    I like the concept, but I’m not sure of your examples being ‘NFRs’. Both of the examples that you use I believe are functional requirements. I would suggest that something along the lines of architectural limits or for a web based service moving from one OS to another are better examples and are probably better referred as where the user sees no difference to the product and it apparently is working the same before and after the change.

  2. Shiv K Singh

    I concur with Scott and the examples are clearly NFR. What additional thing I would recommend is “establishing relationship between these requirements and functional requirements”. Many a times, a NFR applies to the whole system/ solution and in that case, it doesn’t make sense to show the relationship of the NFR to all FRs, however there are many NFRs which can be tied to some FRs only and in that case establishing the relationship would help — Developers, testers and users — do their task better. For example the NFR related to Employee Salary should be tied to the FR describing this functionality. That would help Developers, testers and users during development, testing and using the system.

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  5. Soheil

    Nice article,
    I think also we have to go back to the old-fashion 3C principle and use the back side of the story cards to keep it short and SMART 😉


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