If I give some specific label to pods and define replicaset saying to include pods with same labels, it includes that pod in it. That is all fine and good..
( i know pods are not to be created separately, but are supposed to be created with deployments or replicaset.. but still how deployments/replicasets include pods whose label match in the defination, if they are already there for some reason)
BUT, how does this work behind the scene ? How replicaset knows that pod is to be included as it has the same label ? Lets say, I already have a pod with those labels, how does newly created replica set know that pod is to be included if it has pods less than desired number of pods ?
Does it get that information from etcd ? Or pods expose labels somehow ? How does this thing work really behind the scene ?
As stated in the Kubernetes documentation regarding ReplicaSet.
A ReplicaSet is defined with fields, including a selector that specifies how to identify Pods it can acquire, a number of replicas indicating how many Pods it should be maintaining, and a pod template specifying the data of new Pods it should create to meet the number of replicas criteria. A ReplicaSet then fulfills its purpose by creating and deleting Pods as needed to reach the desired number. When a ReplicaSet needs to create new Pods, it uses its Pod template.
It's recommended to use Deployments instead of ReplicaSets.
Deploymentis an object which can own ReplicaSets and update them and their Pods via declarative, server-side rolling updates. While ReplicaSets can be used independently, today they’re mainly used by Deployments as a mechanism to orchestrate Pod creation, deletion and updates. When you use Deployments you don’t have to worry about managing the ReplicaSets that they create. Deployments own and manage their ReplicaSets. As such, it is recommended to use Deployments when you want ReplicaSets.
Like you mentioned if you have a Pod with label matching the ReplicaSet label, ReplicaSet will take control over the pod. If you deploy ReplicaSet with 3 replicas and Pod was deployed before that, then RS will spawn only 2 Pods with the matching label. It's explained with details and examples on Non-Template Pod acquisitions.
As to how it works behind the scenes, you can have a look at slides #47-56 of Kubernetes Architecture - beyond a black box - Part 1