NGINX proxy to server with cache as backup

Question

We want to set up an NGINX server that will proxy all requests it receives to an upstream server, but when the upstream server fails/becomes unavailable it should fall back to a local cache of the most recently received files. What's the best way to set this up?

The best way I thought of doing it would be setting the 5xx error documents to something like

error_document 500 502 503 504 =200 /cache/;


location /cache/ {
    #Send cached files
}

But I'm not sure how to effectively* get NGINX to cache all files while still proxying to the upstream and then how to pull the files back from the cache via a location.

_{*Without A) filling up the disk very quickly with multiple versions of the same cached file and B) not slowing down requests too much}