operators: In, NotIn, Exists, DoesNotExist, Gt, Lt.
-> In - The label value must be in the values list.
-> NotIn - The label value is not in a list.
-> Exists - A specific label exists.
-> DoesNotExist - A specific label does not exist.
-> Gt - The label value is greater than a specified value (string comparison).
-> Lt - The label value is less than a specified value (string comparison).

Create own custom node labels 'KEY=VALUE' pairs.
The some default built-in node labels such as
-> beta.kubernetes.io/arch=amd64
-> beta.kubernetes.io/os=linux
-> kubernetes.io/arch=amd64
-> kubernetes.io/hostname=HOSTNAME
-> kubernetes.io/os=linux
-> node-role.kubernetes.io/control-plane=
The values of these labels may vary depending on the environment (eg: OS), Cloud Provider, etc.

There are two types of affinity.
● requiredDuringSchedulingIgnoredDuringExecution
It is an 'Hard' rule.
Selector has to match a Node label to an schedule.
The existing Pods running before affinity is applied will not be evicted if no affinity.
Pods will be in pending state when no node with matching label.
Pod cannot be scheduled on a node unless it matches the affinity configuration, even if there are no nodes that match.
It is used for a case that the label must exist on the node during scheduling of the k8s object to a node.
* requiredDuringScheduling - The pod must be scheduled to node(s) that match the expressions listed under matchExpressions.
                             The rule must be met for a pod to be scheduled onto a node.
* IgnoredDuringExecution - It indicates that the node affinity only applies during pod scheduling, it doesn’t apply to already running pods.
                           If labels on a node change at runtime such that the node affinity rules on a pod are no longer met, the pod will still continue to run on the node.
● preferredDuringSchedulingIgnoredDuringExecution
It is an 'Soft' rule.
Pods will be scheduled to any eligible node if a node with a matching label is not found.
The existing Pods running before the affinity is applied will not be evicted if no affinity.
The scheduler will attempt to schedule the pod on a node matching the affinity configuration, but if it is unable to do so the pod will still be scheduled on another node.
The 'weight' field in above is in the range 1-100. This value is used to calculate the priority of nodes to take Pods for scheduling.

[Node]
The Node Affinity rules work with the node labels.
The Node Affinity allows scheduling Pods to specific nodes.
The Nodes that can receive a Pod by matching labels of those Nodes.
It supports only Node Affinity and no nothing Node Anti-Affinity. 
* Node Affinity - It similar to a nodeSelector as it allows which nodes your pod is eligible to be scheduled on based on node labels.
                  The pods to be scheduled on a specific node using node labels.

[Pod]
The Nodes that can receive a Pod by matching labels of the existing Pods already running on each of those Nodes.
It define whether a given Pod should or should not be scheduled onto a particular node based on labels of other Pods already running on that node.
The 'topologyKey' field specifies Node Labels Key (topologyKey: NODE_LABELS_KEY) will match with any of the value.
The Inter-Pod supports both Affinity and Anti-Affinity.
* Pod Affinity - It specify that a pod should be scheduled on the same node as another pod.
                 The 'topologyKey' match with these node already running with existing pod labels (KEY & VALUE) are matched with Affinity OR Anti-Affinity new pod labels (KEY & VALUE) it will scheduled on the same node.
* Pod Anti-Affinity - It allows you to specify that a set of pods should be scheduled away from one another on different nodes (one replica per node).
                      It works with 'topologyKey: NODE_LABELS_KEY', the no of Cluster Nodes make in same node label (KEY & VALUE).    
                      The 'topologyKey' match with these node already running with existing pod labels (KEY & VALUE) are matched with Anti-Affinity new pod labels (KEY & VALUE) it won't be schedule on that same node, so the 'topologyKey' match with another node the some other existing pod labels (KEY & VALUE) are not matched with Anti-Affinity new pod labels (KEY & VALUE) it will scheduled on that node.
