Availability Location Block

site.conf

server {

...

location ~* ^/api/product/check_availability/.+ {
    # The key zone to use for cache lookups.
    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache
    proxy_cache                   availability;

    # Ignore Cache Control headers to allow proxy cache to work
    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ignore_headers
    proxy_ignore_headers X-Accel-Expires Expires Cache-Control;

    # Only one request at a time will be allowed to populate a new cache element identified.
    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_lock
    proxy_cache_lock on;

    # Allow background updates, serve stale during update.
    # NOTE: requires enabling serve stale on update.
    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_background_update
    proxy_cache_background_update on;

    # Conditions allowing stale responses.
    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_use_stale
    proxy_cache_use_stale         updating;

    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ignore_headers
    # By default, proxy_cache lookups only occur on GET or HEAD methods. Uncomment
    # if this is a POST request.
    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_methods
    #proxy_cache_methods           POST;
    
    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_key
    proxy_cache_key               $product_number;

    # http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_valid
    proxy_cache_valid             200 500 5s; # normal inventory or an error
    proxy_cache_valid             201     1s; # low inventory
    proxy_cache_valid             400     1m; # out of stock
    
    proxy_pass http://availability_service$request_uri;
}