What is Clustering? How it works?

What is Clustering?

Clustering is a technology in which one or more computers work together as a single instance by sharing their resources. Clustering is used mainly for redundancy of the services hosted on it. In other words, in a cluster all the hosts are work together, if one of the host fails all the services hosted on it comes online immediately on to another host with out any downtime. Apart from the redundancy a cluster is used for load balancing also, in this case what ever the load comes to the cluster is shared by all the hosts in it.

What are the types of Clusters and How do we configure them ?

Clustering is two types, Active/Active and Active/Passive.

Active/Active : In this scenario all the hosts configured in the cluster takes all the load comes to that cluster, and if any one of the host fails the remaining hosts share the requests to make it online all the time. If you have same copies of application configured on multiple hosts this type of clustering is applicable.

Active/Passive : In this scenario load balancing is not works, as at any point of time only one host receives all the requests, and if any of the host fails the remaining hosts takes those services and make them available online. That means, only one host is active at any point of time and the other one is passive.

What are the requirements to configure a cluster and How it works?

There are so many requirements to configure the cluster, but all those requirements are inter dependable on each other. And these requirements (we call them as resources in clustering), are shared between all the hosts, that means these should be available to all the hosts in the cluster. for example, SAN storage resource, the storage which configured in network and is accessible by all hosts.  In the same way all the resources are accessible by all the hosts in a cluster. If one of the host fails, all these resources are moved to another host. In detail, at a particular time one host is accessing these resources, at that time those are locked by that particular host, whenever this host fails, the lock was released and those resources are locked by another host. But in active/active clusters these resources have multiple copies as multiple instances on all hosts.