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.
You should read about High Availability and Fault Tolerant systems.
From your description you have a High Availability configured in Active-Passive mode, withing this scenario you will not get zero downtime. As the application has to start on the second server when it fails on the primary server.
Unless you application is Active-Active wise then you might get the zero-downtime or close enough.
