Suppose we are going to develop a system for processing Online payments, which would need to interact with different subsystems like Banking system, CRM, Billing and email system, each of them bringing their own complexities. Since we don't have information about the context of each subsystem, this can be a difficult task related to the interactions with these subsystems like how to interact with which subsystem etc. Which design pattern will be used to deal with these high-level operations that are going to be in-charge of interacting with the subsystems and abstracting the system from dealing with these complex interactions?