Sunday, February 24, 2008

The Purpose of Modeling

My client wants a complete domain model, including every field and relationship...

I just finished a laborious, and somewhat exhaustive review of a relatively elaborate and extensive domain model with a set of business stakeholders last week. I know that a lot of upfront documentation is never considered a good thing on an agile project, but sometimes, when the scale of a project is incredibly large (think $20 million budget, and we inherited several thousand pages of requirements documentation before I came on board) it is necessary to capture business concepts in a form capable of providing effective guidance for future development.

The primary goal of this domain model was to take the vast ream of requirements given to us, and present them in a form that would be digestible by both business and developer/techie resources. In terms of total volume, the model consisted of approximately 16 diagrams, each describing a different domain concept, with approximately a page of text going along each diagram. In my opinion, this is not an awful lot of documentation compared to the source requirements we inherited which were thousands and thousands of pages long. When creating the domain model, we relied on many of the domain driven design approaches that can be found at

What exactly is the purpose of modeling?
While going over our models with business stakeholders, the biggest problem that I kept running into was that the client wished to see every single field and every single relationship on every class. They were under the impression that if the model wasn't perfect/complete it wasn't going to be useful. I did my best to explain that models did not need to be 100% perfect/complete in order to be useful, quite the contrary in fact. I'm not sure I did a perfect job in doing this, so I thought I would write up a quick blurb on what I believe are the primary purposes for modeling in the hopes that this will help me to defend my position better on in the future.

I should also point out that Scott ambler is an excellent job of describing modeling principles and best practices on his agile modeling method site

  1. Modeling enables Understanding

    IMHO this is the most important part of modeling. Modeling enables people to understand what has gone on before their project, and also allows them to get a better understanding of what needs to happen next.

    As an example of the "gone on before", take the all to typical situation where a developer team inherits requirements; sometimes of good quality, sometimes of not so good quality. Developers are asked to design or develop based on these requirements, and frequently don't even know where to start. Modeling and diagramming is an excellent way to comb through these artifacts and apply some critical thinking around them so as to make some initial directional setting decisions.

    As an example of the "what needs to happen next", the same example also applies. The modeling done by developer type resources can also be used to apply critical thinking around what the data structures will look like, what the interface screens will look like, what services will be required, et cetera.

    This modeling exercise is also the best way for these developer resources to actually understand any legacy documentation. Simply asking someone to read a pound of documentation will rarely result in comprehension of that documentation. Giving someone some documentation and asking them to perform a discrete activity will result in exponentially more understanding. Examples include reading requirements to do initial analysis/domain modeling, initial vendor analysis, usability/interface modeling, actual development, et cetera.

  2. Modeling Enables Communication

    Simple models that abstract and structure a large amount of requirements (or any other system information for that matter) are worth their weight in gold. The maxim "a picture is worth a thousand words" is no truer than in the system development space. Being able to illustrate a vast and incredibly complex system with 10 to 20 pictures (or 1 or 2 depending on the level of abstraction) goes a long way toward ensuring that a lot of distributed teams have the same understanding of what the system needs to do.
Complete models, be careful what you wish for...

So what does this have to do with my original clients desire to "have every field and every relationship described". You'll note that nowhere in these above two points did I mention "modeling to specify". Creating a specification means detailing every field, detailing every constraint, and every relationship, regardless of whether its fundamental to the point you are trying to make on a specific diagram or model. Creating a "complete" model actually makes it much more difficult to achieve the above two objectives. And here IMHO are the reasons why...

  1. Important concepts are obscured: if every relationship, every field, and every business constraint is modeled and presented how can we tell the important ones from the unimportant ones? A model that is too detailed has the unintended consequence of intermingling important concepts with unimportant concepts. The art of actually hiding unimportant information is crucial to creating good models, it's probably harder to create models that only display relevant information than to create models that show everything. But the result is certainly a better model, one that enables understanding of the system across a variety of stakeholders.

  2. The model is impossible to maintain: having been on several projects with a model oriented bent, I've had plenty of hands-on experience on the overhead required to maintaining readable and accurate models. In my experience, it is simply impossible to create maintainable models that are 100% precise down to the field level. At best, models are created that are "true when created" and get more and more irrelevant as time goes by. There are various experts out there who claim that modern tools make it easy to keep detailed diagrams and technical code in sync with each other. These experts need more actual hands-on practice with some of these tools, because this notion is still the stuff of ivory tower dreams, do not underestimate the effort involved in maintaining these types of models, it will equal, and often surpass your actual coding effort.

  3. Complete models are really just "code-umation”: code-umation” is documentation that approximate the detail level of actual code itself. This type of documentation is no easier to navigate than code, it is certainly harder to change than code, and is not executable or testable. Run, run, run as far away from this approach as you can. It doesn't matter if you're in sourcing, outsourcing, working on a large or small projects, this type of detailed documentation does nothing to help developers do their work, try picking up the phone, having a face-to-face conversation, or any other method to communicate detail of this nature. Better yet, follow some of the excellent implementation patterns found in Kent Beck's excellent text here to ensure that your code is readable and self-documenting.

  4. Drudgery versus innovation: creating detailed, field level, specification oriented models is mindnumbing, monotonous work. Good models are the result of critical thinking, innovative solutioning, and thoughtful analysis. These two styles of work are the opposite of each other. The more you focus on low-level details the less you'll focus on figuring out what the right things to do are, if you had the choice of receiving a high quality modeling backed by creative intelligent thinking or a complete model that was a result of monotonous repetitive effort what would you rather have. Don't ask for both, because it's simply not realistic.

    So in short, when modeling, focus on what's important, focus on what you need to communicate, and focus modeling on helping you understand what you need to do next. In most cases the simple advice will go a long way in streamlining your modeling efforts in ensuring that your modeling and models stay relevant.


  1. Jeff,

    This posting resonated with me and I think you outlined the "true value" of modeling. It is an invaluable tool for communication and analysis. Unfortunately, like you pointed out it is too often abused in design work as there seems to be a natural tendency to over-design.

    With my experiences as well, I have yet to find a practical and manageable way to keep detailed design models in sync with the requirements churn that every project experiences.

    An approach I find practical and valuable is high-level modeling of key concepts like you pointed out and then proceed directly to implementation. It's just as fast to code up the model as it is to detail a model fully. And it's compilable, executable and testable.

  2. A lot of people might be interested in the Agile Model Driven Development (AMDD) lifeycle. In it I describe how agile teams do some initial high-level requirements envisioning and architecture envisioning to think through some of the bigger issues. Detailed modeling, if required, can be done on a JIT basis throughout the lifecycle.

    Modeling is clearly an important technique for scaling agile software development.

    - Scott

  3. Scott,
    your is definitely an inspiration for many of my views on modeling and documentation, and SDLC in general.

    It does an excellent job of highlighting the value and limitations of modeling without discounting the value of modeling altogether whic which I've sometines seen other agile practitioners do.

  4. I'm sure all of us have been in similar situations with other clients who are not quite sure of how much analysis goes into modelling. Some of the best modelling experiences I have had involved both functional and technical resources throughout the requirements gathering and analysis phases.

    The common disagreement with clients is that, "You can't be sure you've modelled it correctly if you can't find a home for all of the fields." IMO, this is an issue of trust. Once you nail the trust, you can use effective AGILE methodologies as you please.