A discussion at the Agile Modelling Group put this picture into my head.

The original question was about how to go an document requirements for an existing system. My initial response was why would you want to reverse engineer a system back to requirements?

It makes sense to me to get a full picture of what a system can do - and so document features, but I can't get why you would take things back to requirements.

After all, requirements are a statement around a particular problem or opportunity, not a design specification.

The diagram below came to me care of a comment made by Jon Kern.

Jon notes that not all formally accepted requirements make it in to the final release. Sometimes they are too hard, sometimes you run out of time and money before you get there.

Additionally extra non-formal requirements often make their way into the final release. Non-formal requirements arise from a number of places including uncontrolled requirements changes, requirements elaboration and developers realizing that a neat feature is just 30 minutes of code away.

So at the end of the day, a system doesn't match any particular set of requirements anyway.

Now, there is a case for re-using requirements. After all, if you have a set of standard requirements for say, logging into a system, why not re-use them. It saves time and builds consistency.  But requirements re-use isn't the same thing as reverse engineering a system to establish requirements.

I ask again; what value can there be in this? Any suggestions out there?

Orinally posted to www.betterprojects.net

Hi Craig,

There are a number of instances when value is achieved from reverse engineering:

• Convert a legacy system to a new platform therefore you need to know what the existing system does.  I realize that many folks would say that in this case we should go back to the users and elicit requirements rather than reverse engineering the system.  The reality is that if the current system works just fine it is much more efficient to get a baseline list of features from the existing system as compared to starting a formal requirements process.  Gathering requirements from stakeholders is not cheap but very expensive due to the cost of people's time.
• The system will need to be maintained for much longer therefore it is a good idea to gradually reverse engineer and document the existing behavior of the system so that when future changes come the next analyst does not have to re-engineer again.  Here the result of reverse engineering would probably be just functional specifications.

- Adrian

Hi:

Why the need to reverse engineer a system back to its requirements?

Core essential requirements are largely static.   Example:  A retail store calculates sales tax.   The store has always had this as an essential - unchanging - requirement, and it probably always will.

 Now the mechanism used to accomplish this fucntional requirement can be anything from a person with pencil and paper, a calculator, or a computer.   The mechanism may change, but the essential requirement (to calculate sales tax) remains.

What we need when we develope a new system is a comprehensive understanding of all the largely unchanging essential requirements.   In the past, some of these were implemented in older systems.   Some were not. (maybe, for example, they were done manually ).  We need them all.

Granted when we reverse engineer the existing system for the essential, we have to pold through alot of non-essential (i.e., implementation specific) requirements.  And we really don't need the non-essential requirements from the old system to develop a new system.  But there really is no way to avoid analyzing them - the essential is typically buried deep within the non-essential.

So, we need to reverse engineer the old system to get those essenital requirements that are currently accomplished by the system.

Tony