With concurrent mode coming in React 18, let's talk about the complexities behind providing declarative APIs for concurrent rendering. While implementing concurrent mode APIs from scratch for Brahmos.js, I came across lot of use cases and variability which made it one of the most interesting problem to solve, and appreciate more the React's effort on advocating Concurrent UI. In this talk we will see what concurrent mode means for a web app, what are the internal complexities and how I solved it for Brahmos.js.