https://nodejs.org/api/cluster.html
Core reason for using stateless Node.js (youtube clip)
what-is-serverless.pdf
https://medium.com/@ObjectiveCopley/become-a-stateless-pirate-in-a-javascript-world-d9a4eaee4bb2

https://medium.freecodecamp.org/scaling-node-js-applications-8492bd8afadc
Microservices patterns and use of state
Use JWT security because each transaction is self-contained and digitally signed.

Callbacks that write should write just one field, not lock a whole collection. Nested callbacks (Christmas trees) should only contain one write and the end of callback tree. This way the only error that can occur is if the write fails -- everything else is a read.