We have a requirement on a project that certain cases need to implement a functionality (handling documents notifications).
In Java this would be handled via an interface (and a method "handeDocument(...)" ).
I Pega we could have an activity but it's hard to enforce a contract just as an interface does. One solution is to have a common abstract class with an abstract method but when not all of the cases have to implement the method this would not work.
To answer your question, we don't directly expose our methods as in case of the interface in JAVA, though Pega has its own mechanism to implement the same.
The Pega API enables you to quickly implement client and mobile applications by using built-in Pega REST services. It includes case, assignment, and data APIs that let you leverage your Pega 7 Platform applications.
This quick start tutorial provides a hands-on experience with the Pega API through testing some of the built-in Pega API REST services for the Pega 7 Platform. An example is provided by a Python script to access the GET /case types request URL that you obtain from the Pega API.
You can create a new Service Package and refer your access group and do a save as of Service-REST(or other services available) to the new service package.