http nginx reverse-proxy multipartform-data cache-control

Nginx proxy_cache_key $request_body is ignored for large request body

I use nginx as a reverse proxy and I would like it to cache POST requests. My back-end is correctly configured to return appropriate cache-control headers for POST requests. In nginx I have configured:

proxy_cache_methods POST;
proxy_cache_key "$request_method$request_uri$request_body";

This works great for small HTTP POST requests. However I started noticing that for large requests (e.g. file uploads) it seems like the $request_body is ignored in the proxy_cache_key. When a form containing a file upload is submitted twice with completely different data, nginx will return the cached result.

What could cause this? How can I configure nginx to use the $request_body (or a hash of $request_body) in the proxy_cache_key even for large POST requests?

Solution

So it turns out that when $content_length > client_body_buffer_size, then the request body is written to a file and the variable $request_body == "".