nginx_cache.conf

proxy_cache_path /var/nginx/cache/site_cache levels=1:2 keys_zone=site:256m max_size=4096m inactive=10m;

location {
    proxy_cache_use_stale updating timeout http_500 http_502 http_504;
    proxy_cache site;
    proxy_cache_lock on;
    proxy_cache_key $common_cache_key;
    proxy_cache_valid 1m;
    proxy_cache_lock_timeout 60s;
    ssi on;
    proxy_cache_methods GET HEAD;
    proxy_pass http://$combiner_backends;
    proxy_intercept_errors on;
}

$ nginx -V
nginx version: nginx/1.6.3
TLS SNI support enabled
configure arguments: --prefix=/usr/local/etc/nginx
--with-cc-opt='-I /usr/local/include'
--with-ld-opt='-L /usr/local/lib'
--conf-path=/usr/local/etc/nginx/nginx.conf
--sbin-path=/usr/local/sbin/nginx
--pid-path=/var/run/nginx.pid
--error-log-path=/var/log/nginx-error.log
--user=www --group=www
--with-file-aio
--http-client-body-temp-path=/var/tmp/nginx/client_body_temp
--http-fastcgi-temp-path=/var/tmp/nginx/fastcgi_temp
--http-proxy-temp-path=/var/tmp/nginx/proxy_temp
--http-scgi-temp-path=/var/tmp/nginx/scgi_temp
--http-uwsgi-temp-path=/var/tmp/nginx/uwsgi_temp
--http-log-path=/var/log/nginx-access.log
--with-http_dav_module
--with-http_geoip_module
--with-http_gzip_static_module
--with-http_realip_module
--with-http_stub_status_module
--with-pcre
--with-http_ssl_module