Everything Changes ... but some things change faster than others

It's a simple enough proposition.

Some Business Requirements are Enduring: 
Others are Volatile;
For enduring requirements the priority is efficiency,
For volatile requirements the priority is flexibility.

And the trick is to know which is which.

When dealing with requirements and change a current "best practice" approach could be summarised as:

• Gather current requirements
• Establish known points of variation
• Investigate likely future requirements
• Build a highly parameterised application to cope with this  anticipated level of change

This is all well and good: difficulties arise when (not "if"):

• Unanticipated change occurs (often driven by regulatory or market change)
• Anticipated change does NOT occur (the system is left with costly, unused and confusing configuration mechanisms)
• There is an attempt to extend the scope of the existing system to areas not covered by initial analysis (new lines of business, new territories)

Very often, despite the best efforts of talented developers, systems will not have adequately been prepared for the change that comes. Chaos ensues.

How can we do things differently?  
For one thing, we can take a very wide definition of which requirements should be regarded as volatile, and plan to support this in a structured way

Volatile requirements

• Anything driven by regulation is volatile
• Anything driven by “products” is volatile
• Anything driven by markets and competition is volatile
• Anything unique to the line of business, market sector, or territory should be regarded as volatile.

We can then analyse these volatile requirements and capture their variation in open-ended and structurally sophisticated configuration.  We call this process modelling, and the configuration definitions thus obtained are models.  Every effort goes into making modelling easy, so that as far as is possible, model change becomes a non-programming skill.

Enduring requirements are those aspects which are unlikely to change over a period of decades. For example:
“In supporting any contract, we need to know who the contracting parties are”; 
“We will need to keep track of financial movements”;
"We need to track changes in contract terms: when, and what changed".
These are all fair candidates for enduring requirements: characteristics of the way in which business is done.

Enduring requirements should be supported by software applications that we call engines. 

Engines provide raw, generic capability.

Models shape that capability into purpose-specific services.

When volatile requirements change, as we expect them to do, these changes are managed without change to the engines, purely as model change.

This is not to say that engines do not change: they do. But engines change only to extend generic capability.  This means that engine change is exclusively a positive thing. This is hugely important. It means that as time progresses, we expect the engines to improve: extended capabilities, better performance, more flexibility in deployment.

Instead of systems succumbing to the ravages of time, they become more resilient. The effect of changing requirements is to strengthen the engine, not to weaken it.

Engines + Models = Services