The idea is explained here
In simple words, Nginx Proxy plays the role of load balance and transmits the HTTP/HTTPS requests to servers.
If the number of request times within a certain period from an individual IP exceeds a threshold, it will trigger a captcha for the upcoming requests.
And the end-user must input the correct captcha code before he can continue to access the site.
Do you know any open source / free NGINX module for this usage? I searched on the Internet and here is one of them: https://github.com/snbuback/nginx seems it needs modification.
Any suggestion / experience is welcome, thank you