Hi, I am building an app with React Native. While the user is creating a new account, they can select their username. I have set up an event that when the user has stopped typing there should be a network request to check if the username is busy or not. I use cognito to store my users, I am more of a frontend developer. What would be the best way to create an endpoint that checks if the username is busy without creating a way to DDOS myself?

first line of defense is to implement request throttling in the client (if you are using Axios, use something like

axios-request-throttle

). After that best to implement some WAF rules in front of your API via AWS WAF or Cloudflare

Yup, agreed with @Michael Clifford.