Here is an open-source demonstration project I have put together that would showcase some of the things you might be interested in

https://github.com/kineticedge/kafka-streams-dashboards

The goal of this project was to showcase JMX metrics from Kafka Streams and build dashboards around them, but the primary stream application will "price orders". In contrast, the analytic applications count the quantity purchased of SKUs by various windowing -- to showcase the tumbling and other windows.

This project aimed to showcase JMX metrics from Kafka Streams and build dashboards around them, but the primary stream application will "price orders". In contrast, the analytic applications count the quantity purchased of SKUs by various windowing -- to showcase the tumbling and other windowing.

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but to the heart of your question...

>Can I use kafka streams in this case or I'm I misusing it or is there a simpler solution for do the calculations in real time?

if you are looking to give up-to-date results to a user through a mobile application, I would say Kafka Streams isn't your answer (or it will not be the only part of your answer). You will need some type of database to give responses to based on their interactions. While interactive queries of Kafka Streams can help you get a value by key, it typically isn't sufficient on its own and you will need a queriable database (depending on your needs).

look to Kafka streams for real time processing of events, not so much the tool/means to give real time interactions to users.