Why it is recommended to have more than one master nodes

Is elasticsearch using more than one master nodes at a time or the other one is just a backup so that it can be use in case of if one of the master nodes is not working. Can any guide me this is a very simple question but i'm new to elasticsearch.

The short answer is that if you don't have a master-eligible node, your cluster won't work. You can read a bit more about master nodes and elections here. As in most clustered systems where information needs to be kept in sync, there is likely one or more nodes that are responsible for that role. If you only have one node doing that, you'll have a lot harder time. Imagine, if a hardware failure brings down your only master-eligible node and while the cluster should be able to perform, there aren't any more nodes that would be elected to keep it working.
You need to get more information on how an Elasticsearch cluster works, depending on your use case. There are a ton of blog posts and documentation where you can find information on that and other aspects you'll need to have a stable cluster.

You need at least 3 master eligible nodes for high availability. These nodes will elect one of these as master based on a strict majority decision. If you have 3 or more master eligible nodes a strict majority can be formed even if one of the master eligible nodes is missing. If you only have 2 master eligible nodes both of these are needed to elect a master as the strict majority of 2 is 2.

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