So I have this 6 years old rails application monolith that contains all these things:

• user interface
• database
• additional tables for statistics (snowflake schema)
• API for android app
• workflow engine
• batch workers (sidekiq)

Currently I'm running 4 instances of this application, each is configured especially for the customer who's using the instance. Yes, that makes 4 customers, each has ~300 users, so we're talking about 1200 users. A virtual server with 2 cores and 8 GB RAM handles it easily. The web server is nginx and the ruby application server is passenger.

In 2020 a new UI (SPA) will be written, so I can get rid of the UI in rails. However I will have to provide an API for the SPA and it will be totally different than the API for the android app.

What a great opportunity to split the monolith into microservices!

I might split the microservices like this:

• API for new UI
• API for android app
• batch workers
• statistics
• what's left in the old rails app (business logic, workflow engine)

So far I pretty much know what I'm doing. However I don't have much experience in the microservices world, so I have a bunch of questions for you:

• What about user authentication at the API for the new UI? Are JSON web tokens the way to go?
• What about the database? Should I split it along the microservices?
• How do the microservices communicate with each other? I prefer a message bus over direct REST calls, so...?
  • RabbitMQ?
  • Kafka?
  • Nats?
• Is it worth learning the whole Kubernetes stack? Or is docker plus docker-compose (or helm???) enough? Any other alternatives?

Actually my monolith isn't even dockerized, yet. Currently I feel overwhelmed by all the technologies I can chose from. But what's the right technology stack for rails? And for me? This rails application is only a hobby project. Any recommendations?