Where to start developing a simple WAF?

Question

linux

Hello!
Experience in developing web applications (python, php, node.js). Now it's interesting for yourself to try to write a simple Web Application Firewall. Unfortunately, no articles on this subject could be found. I know that there are Open Source solutions, but they are huge, and it is very difficult to understand the code.
In this regard, questions arise:
1. What to write on?
2. Where can I find examples of intercepting http requests from Apache (nginx)?
3. What literature to read?
PS. Perhaps there are very small open source WAF projects. I would like to take a look at them, and perhaps participate in the development.