How can one go about modelling highly specific business requirements, which have no precedent in the system? Take for example the following requirement: When a purchase order contains N lines, is over X value in total and is being recorded against project Y, an email needs to be sent to persons A and B with the details

This requirement supplements other requirements surrounding purchase orders, but comes in at a much later date in response to some ongoing problem elsewhere in the business.

Persons A and B are not part of any role or group in the system, and don't hold any specific responsibility; they are simply the two people the business has appointed to receive these emails in this very specific case. Projects are also data driven, so project Y has no special properties to distinguish it from any other project. The only way to identify it is to compare its identifier to a magic number.

How can one go about modelling this kind of case without introducing too much additional complexity? That I can think of right now, there are a couple of options.

1. Perform the checks and actions inline with the existing code. Here we find the correct spot in the code, check the conditions in the requirement and send the emails to hardcoded addresses. Of course this is fraught with issues. At the very least it stops working if one of these people leaves or changes their email address. At worst you have to ensure that any tests and test data are aware that additional actions are taken for a specific set of criteria.

2. Introduce some form of events system. Here we introduce an eventing system, so that we might react to some event, and fulfil the requirement outside of the usual path of execution. This sounds like a cleaner solution than option 1, but the work involved is ultimately probably slightly overkill for this one small requirement. That said, having it in place does allow the system to handle these kinds of specific requirements consistently and easily in the future.

Are there any other (good/better) ways of handling highly specific requirements? I mean other than telling the other parts of the business no!