Interesting. I would like to learn more when this approach is a good or better solution. Especially how to exclude ETL, and using OLTP and OLAP. However, I would not agree with "bad" examples of API. Introduced example of cc transaction is just example of not good choice of API point rather than wrong user case. For tasks using couple business domain resources and because of good user experience designer can choose coarse grained API. Note Restful API is not Agile. API requires a thoughtful design and collaboration with users / clients.
Have you considered using Onyx instead of Samza to write your business logic in pure Clojure and benefit from all the extra features Onyx offers over Samza ?
While I like the CQRS/ES architecture from an operational standpoint, I do not see where you stopped the proliferation of APIs. You semantically rich domain message "customer pays to account" that you write out to Kafka is nothing but an api call, the way you describe it. And you even state "here you can extend the functionality of the system" by using *more* of these commands, and there you got your API proliferation back into your system. Every command is an api call. So you moved your api from http to some unstructured message. Not really progress in my eyes. I think the constraints of http verbs are a good thing, even if I'm looking into speech acts inspired by agent communication languages right now.
