If a system has an API which allows third party applications to access it, then the consideration needs to be given as to how the third parties can test their applications. This is of major importance if the system provides critical services to the third party applications.
To support third party developers, special test installations can be provided with an instance of the system dedicated to testing of third party applications. These systems need to have their inputs exercised and outputs monitored so that they have sufficient behaviour to enable testing of API applications.
Environmental simulators are ideal for API testing installations. Not only can they provide the necessary data into the system, they can also run scenarios allowing applications to test against real world conditions.