With the k describe output below, we can see the difference between the two strategies.
First one is a from a deployment that uses Rolling Update strategy. Before updating the image via k set image, the old replica frontend-6765b99794 has 4 pods. After setting the image, you can see that incrementally, 1 by 1, new replica set frontend-854b57fbbf is scaled up while new replica set frontend-6765b99794 is scaled down. Scaling up adds new pod/replica, scaling down reduces pod/replica. The total replicas set in the deployment is 4. The maximum number of replicas that can be added is determined by maxUnavailable, which is set to 25% (i.e., 1 replica since 25% of 4 total replicas is 1). On the other hand, the maximum number of replicas that can be unavailable is also 25% of the total replicas.
This is the default strategy of a deployment.
RollingUpdateStrategy: 25% max unavailable, 25% max surge
...
OldReplicaSets: frontend-6765b99794 (0/0 replicas created)
NewReplicaSet: frontend-854b57fbbf (4/4 replicas created)
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal ScalingReplicaSet 7m41s deployment-controller Scaled up replica set frontend-6765b99794 from 0 to 4
Normal ScalingReplicaSet 88s deployment-controller Scaled up replica set frontend-854b57fbbf from 0 to 1
Normal ScalingReplicaSet 88s deployment-controller Scaled down replica set frontend-6765b99794 from 4 to 3
Normal ScalingReplicaSet 88s deployment-controller Scaled up replica set frontend-854b57fbbf from 1 to 2
Normal ScalingReplicaSet 66s deployment-controller Scaled down replica set frontend-6765b99794 from 3 to 1
Normal ScalingReplicaSet 66s deployment-controller Scaled up replica set frontend-854b57fbbf from 2 to 4
Normal ScalingReplicaSet 44s deployment-controller Scaled down replica set frontend-6765b99794 from 1 to 0Now, for the Recreate strategy, notice that all at once, the old replica set frontend-6765b99794 was scaled down to 0 and subsequently, the new replica set frontend-7c594bbd6d is scaled up to 4.
StrategyType: Recreate
...
OldReplicaSets: frontend-854b57fbbf (0/0 replicas created)
NewReplicaSet: frontend-7c594bbd6d (4/4 replicas created)
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
...
Normal ScalingReplicaSet 52s deployment-controller Scaled down replica set frontend-854b57fbbf from 4 to 0
Normal ScalingReplicaSet 20s deployment-controller Scaled up replica set frontend-7c594bbd6d from 0 to 4