How to get "zero downtime" on HAProxy with a load balancer configuration?

The scenario is, I have two web servers both are running Nginx on it. Also I have configured HAProxy after the LB (load balancer) for both the servers. I've checked with Heartbeat, but no luck to make downtime count "zero". I want some application which will switch to secondary server instantly when primary server goes down, in other words it should not give any downtime.