https://nodejs.org/ko/docs/guides/blocking-vs-non-blocking/
https://velog.io/@codemcd/Sync-VS-Async-Blocking-VS-Non-Blocking-sak6d01fhx
http://homoefficio.github.io/2017/02/19/Blocking-NonBlocking-Synchronous-Asynchronous/
10. Non_blocking_vs_blocking