Nginx proxy_cache_key $request_body is ignored for large request body
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 == "".
See also http://mailman.nginx.org/pipermail/nginx/2013-September/040442.html
Rather than using the $request_body within the proxy_cache_key, you may more simply use $content_length.Of course, it comes with its own limitation, but if you know which query you will receive, it can be also a very interesting workaround.
proxy_cache_key "$scheme$request_method$host$request_uri$content_length";You may alternatively use $request_body as well to keep the desired behavior for smaller request payload:
proxy_cache_key "$scheme$request_method$host$request_uri$request_body$content_length";