Nginx UDP Load balancing

Question

We're currently working on implementing a high-availability solution for our UDP tool, Logstash, by using Nginx as a UDP load balancer. However, I'm facing one problem, In our configuration file, we're trying to route syslog traffic to the backend Logstash servers. However, when one of the Logstash servers goes down, Nginx continues to forward logs to the unavailable server instead of routing all traffic to the available one. We've configured it to use round-robin load balancing, but it doesn't seem to be working as expected.

user  nse;
worker_processes  auto;
#error_log  logs/error.log;
#error_log  logs/error.log  notice;
#error_log  logs/error.log  info;
pid        /home/nse/etc/nginx/nginx.pid;
events {
    worker_connections  1024;
}
http {
    include       mime.types;
    default_type  application/octet-stream;
#log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
#                  '$status $body_bytes_sent "$http_referer" '
#                  '"$http_user_agent" "$http_x_forwarded_for"';
error_log /home/nse/var/log/nginx/error.log info;
access_log /home/nse/var/log/nginx/access.log;
#access_log  logs/access.log  main;
#proxy_timeout 1s;
#proxy_responses 10;
#proxy_connect_timeout 180s;
#proxy_send_timeout 180s;
#proxy_read_timeout 180s;
sendfile        on;
#tcp_nopush     on;

#keepalive_timeout  0;
keepalive_timeout  65;

#gzip  on;

}
stream {
    upstream logstash {
        zone dns_zone 64k;
        server 10.139.168.12:10514 fail_timeout=30s;
        server 10.139.168.11:10544 fail_timeout=30s;
    }
server {

    listen 5144 udp;
    proxy_pass logstash;
    health_check interval=5 passes=2 fails=2 udp;
    #proxy_timeout 1s;
    #proxy_responses 1;
    proxy_buffer_size 16k;
    proxy_responses 60;
}

}