It is fun to write a library or a framework. It allows us to play with many interesting ideas which was not possible before due to the constraints in others' work. However, utmost care must be taken to build it great. In this session, Trustin Lee, the founder of Netty project and Armeria, shares you the opinionated key practices from his recent works which might be useful when you build your own library or framework, or even designing an API for your project.