Maybe I was not clear. Whatever is the technology that you use to register the active backends in the DNS, rather than doing name => ip address lookup per requ

Maybe I was not clear. Whatever is the technology that you use to register the active backends in the DNS, rather than doing name => ip address lookup per request, you can resolve all those names => ip address maps upon the service being brought up/taken down and push the resolved map as a set of backends into nginx config, thus removing the need to query DNS per request.