What is the difference between ReplicaSet and ReplicationController?

Question

From what I can tell in the documentation, a ReplicaSet is created when running a Deployment. It seems to support some of the same features of a ReplicationController - scale up

Answer 1

The functionality of both Replica Controller and Replica Set is quiet the same - they are responsible to make sure that X number of pods with label that is equal to there label selector will be scheduled to different nodes on the cluster.
(Where X is the value that is specified in the spec.replicas field in the Replica Controller / Replica Set yaml).

ReplicaSet is a replacement for the Replica controller and supports richer expressions for the label selector. You can choose between 4 values of operators In, NotIn, Exists, DoesNotExist - see Set-based requirement.

A rule of thumb: When you see Replica Controller is mentioned in one the docs or other tutorials - refer to it as ReplicaSet AND consider using Deployment instead.

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There is also a small difference in the syntax between Replica Controller:

apiVersion: v1
kind: ReplicationController
metadata:
  name: nginx
spec:
  replicas: 3
  selector:
    app: nginx

And the ReplicaSet which contains matchLabels field under the selector:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx
spec:
  replicas: 3
  selector:
    matchLabels: #<-- This was added
      tier: nginx