Sometimes we need to add redundancy to some service or server which happen to be a public-facing entry point of our infrastructure. For example, imagine we want to add a high availability pair for a load balancer which sits on the edge of network and forwards traffic to alive backend servers.
┌─────────────┐
│ │
┌─────►│ Backend 1 │
│ │ │
│ └─────────────┘