You can't really change the upstream list on the fly. However you
could do something like this (psuedo code)

location / {
set $backend "default.backend.server";

rewrite_by_lua '
res = ngx.location.capture("/redis", { method = ngx.HTTP_PUT, body =
"srandmember setwithbackends\r\n" }
-- parse res.body to get the value
ngx.var.backend = backend_that_I_parsed
'
proxy_pass $backend$uri;

}

from http://wiki.nginx.org/HttpRedis2Module#Lua_Interoperability
location /redis {
internal;
redis2_raw_query $echo_request_body;
redis2_pass 127.0.0.1:6379;
}

If redis set has hostnames in it, you'll need to have resolver
enabled. If you wanted, you could wrap the proxy_pass in a capture
and try the list in order until you got a good response.

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