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Remember that actors interact only using message passing. In order to check actors behavior you can do it through the messages sent to them and back from them. So how do you test actors? You send them messages :) To test actors that communicate only with messages, you need to send it a message and get reply back and check it.
The Actor Model is meant to simplify the notoriously difficult world of concurrent programming.
You may have heard of the Reactive Manifesto – in a nutshell, it’s about building flexible, scalable, fault tolerant systems that are loosely coupled.