We are developing nginx openresty app that will have a high load upstream traffic. ex - https://upstream.com
However are spending around 100 ms every time we connect to upstream.
To leverage connection pool, I started using
http://nginx.org/en/docs/http/ngx_http_upstream_module.html with keep alive directive.
in this directive we have tried to make the server name dynamic using the balancer by lua block.
https://github.com/openresty/lua-nginx-module#balancer_by_lua_block
and using https://github.com/openresty/lua-resty-dns#tcp_query for dns resolution.

Though these things works for me, I am trying to find out if there is anyway we can achieve dns resolution on openresty + nginx opensource and still avoid the hard coupling of not entering the host dns directly in upstream block.
Because if we explicitly mention the server dns for upstream then nginx+openresty app fails to start when it cant resolve the dns.

Sample code

upstream upstream1 {
server 127.0.0.1 fail_timeout=10;

balancer_by_lua_block {
local balancer = require "ngx.balancer"
 -- use Lua to do something interesting here
 -- as a dynamic balancer
 local host, port = ngx.var.upstream:match( "([^://]+):(%d+)" )
 if(host == nil)
 then
    host = string.gsub(ngx.var.upstream, "https://", "")
 end;
 --local maps to cache host and ip
 local targetIp = upstreamSvcHostIpMap[host]
 local port = upstreamSvcHostPortsMap[host]
 if(port == nil) then port = 443 end;
 --ngx.log(ngx.ERR, "Set the current peer: ip -- ", targetIp , " port --", port)
 local ok, err = balancer.set_current_peer(targetIp, port)
 if not ok then
     ngx.log(ngx.ERR, "failed to set the current peer: ", err)
     return ngx.exit(500)
 end

}
keepalive 20;
keepalive_timeout 60s;
keepalive_time 1h;

}

dns query
local resolver = require "resty.dns.resolver"
local _M = {}

function _M.getIpFromDNS(host)
local nameServerIp = "8.8.8.8"
if(nameServer ~= nil) then nameServerIp = nameServer; end
ngx.log(ngx.ERR, "nameServerIp: ", nameServerIp)
local r, err = resolver:new{
nameservers = { nameServerIp}
}
if not r then
ngx.log(ngx.ERR, "failed to instantiate the resolver: ", err)
return
end

local ans, err = r:tcp_query(host, { qtype = r.TYPE_A })
if not ans then
    ngx.log(ngx.ERR, "failed to query: ", err)
    return
end
 for i, ans1 in ipairs(ans) do
    ngx.log(ngx.ERR, ans1.name, " ", ans1.address or ans1.cname,
            " type:", ans1.type, " class:", ans1.class,
            " ttl:", ans1.ttl)
    return ans1.address or ans1.cname
end

return nil

end

function _M.readNameServer()
local nameServerLocal = nil
local fp = assert(io.open("/etc/resolv.conf"))
for line in fp:lines() do
if line.find(line, "nameserver") then
nameServerLocal = line:match(" (.*)")
end

end
fp:close()
ngx.log(ngx.ERR, "nameServerLocal:", nameServerLocal)
return nameServerLocal

end

Any help in this regard will be appreciated.