As stated in http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive that "It should be particularly noted that the keepalive directive does not limit the total number of connections to upstream servers that an nginx worker process can open. The connections parameter should be set to a number small enough to let upstream servers process new incoming connections as well." I want to understand if a new client comes, why can't they use existing keep-alive connections? Do they need to create a new connection with upstream?