Why might someone use an N+1 approach for a cluster?
N+1 or N+1 redundancy is a popular concept in network virtualization and the design of IT architectures. Companies generally use this design to provide effective backup or ensure smooth system operation with a single point of failure.
The name “N+1” signifies a process by which engineers include a range of functioning nodes in a cluster, and then add one extra, so that if there is a single point of failure, that one extra unit can stand in the gap. This process can also be called “active/passive” or “standby” redundancy.
Companies use an N+1 design to make sure that if one server or virtual machine fails, the system is not impacted. However, a greater discussion has emerged about whether N+1 redundancy is sufficient for a given system. There is the recommendation against trying to provide a one-size-fits-all approach when providing redundancy for high availability. IT pros also understand that the stricter a client is with high availability requirements, the more redundancy is needed.
In response to this philosophy, engineers have provided things like N+X+Y, in which many more resources are added to the system to make sure that even a multipoint failure doesn't impact operations. Another particular consideration is the size of each virtual machine or node in the cluster – for example, if a single VM is 100 GB and the others are under 50 GB, an N+1 approach would not ensure functionality if that larger VM is compromised.
In general, N+1 is simply a tool and an approach to managing resources like CPU and memory across any shared environment such as a network cluster. It is evaluated for its effectiveness and efficacy in a particular IT system depending on resource allocation and the overall setup.