Change in proxy_cache_use_stale updating behavior with proxy_cache_background_update enabled from nginx 1.19.0 onward
When both proxy_cache_use_stale updating and proxy_cache_background_update are enabled, nginx must decide whether to return the existing stale cache entry immediately or to wait for the asynchronous background fetch to finish before responding. The documentation notes that the stale response is served only if the request arrives before the background update
nginx