Extending application logic with Apex interfaces

An Apex interface can be used to describe a point in your application logic where custom code written by Developer X can be called. For example, in order to provide an alternative means to calculate championship points driven by Developer X, we might expose a global interface describing an application callout that looks like this:

global class ContestantService {
   global interface IAwardChampionshipPoints { 
    void calculate(List<Contestant__c> contestants); 
  } 
} 

By querying Custom Metadata records from the Callouts Custom Metadata type, which has been included in the source code for this chapter, code in the application can determine whether Developer X has provided an implementation of this interface to call instead of the standard calculation code.

Using Custom Metadata is an excellent use case for this sort of requirement...