The ordinal index, also known as ordinal number in set theory, is simply a set of numbers that are used to order a collection of objects, one after the other. Here, Kubernetes is using them to order, as well as identify each Pod. You can think of it akin to an auto-incrementing index in a SQL column.
The "first" Pod in the StatefulSet has an ordinal number of 0, the "second" Pod has the ordinal number of 1, and so on.
Our StatefulSet is named elasticsearch and we indicated 3 replicas, so our Pods will now be named elasticsearch-0, elasticsearch-1, and elasticsearch-2.
Most importantly, a Pod's cardinal index, and thus its identity, is sticky—if the Pod gets rescheduled onto another Node, it will keep this same ordinal and identity. ...
