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Federated ReplicaSets

Note: Federation V1, the current Kubernetes federation API which reuses the Kubernetes API resources ‘as is’, is currently considered alpha for many of its features, and there is no clear path to evolve the API to GA. However, there is a Federation V2 effort in progress to implement a dedicated federation API apart from the Kubernetes API. The details can be found at sig-multicluster community page.

This guide explains how to use ReplicaSets in the Federation control plane.

ReplicaSets in the federation control plane (referred to as “federated ReplicaSets” in this guide) are very similar to the traditional Kubernetes ReplicaSets, and provide the same functionality. Creating them in the federation control plane ensures that the desired number of replicas exist across the registered clusters.

Before you begin

Creating a Federated ReplicaSet

The API for Federated ReplicaSet is 100% compatible with the API for traditional Kubernetes ReplicaSet. You can create a ReplicaSet by sending a request to the federation apiserver.

You can do that using kubectl by running:

kubectl --context=federation-cluster create -f myrs.yaml

The --context=federation-cluster flag tells kubectl to submit the request to the Federation apiserver instead of sending it to a Kubernetes cluster.

Once a federated ReplicaSet is created, the federation control plane will create a ReplicaSet in all underlying Kubernetes clusters. You can verify this by checking each of the underlying clusters, for example:

kubectl --context=gce-asia-east1a get rs myrs

The above assumes that you have a context named ‘gce-asia-east1a’ configured in your client for your cluster in that zone.

The ReplicaSets in the underlying clusters will match the federation ReplicaSet except in the number of replicas. The federation control plane will ensure that the sum of the replicas in each cluster match the desired number of replicas in the federation ReplicaSet.

Spreading Replicas in Underlying Clusters

By default, replicas are spread equally in all the underlying clusters. For example: if you have 3 registered clusters and you create a federated ReplicaSet with spec.replicas = 9, then each ReplicaSet in the 3 clusters will have spec.replicas=3. To modify the number of replicas in each cluster, you can add an annotation with key federation.kubernetes.io/replica-set-preferences to the federated ReplicaSet. The value of the annoation is a serialized JSON that contains fields shown in the following example:

{
  "rebalance": true,
  "clusters": {
    "foo": {
      "minReplicas": 10,
      "maxReplicas": 50,
      "weight": 100
    },
    "bar": {
      "minReplicas": 10,
      "maxReplicas": 100,
      "weight": 200
    }
  }
}

The rebalance boolean field specifies whether replicas already scheduled and running may be moved in order to match current state to the specified preferences. The clusters object field contains a map where users can specify the constraints for replica placement across the clusters (foo and bar in the example). For each cluster, you can specify the minimum number of replicas that should be assigned to it (default is zero), the maximum number of replicas the cluster can accept (default is unbounded) and a number expressing the relative weight of preferences to place additional replicas to that cluster.

Updating a Federated ReplicaSet

You can update a federated ReplicaSet as you would update a Kubernetes ReplicaSet; however, for a federated ReplicaSet, you must send the request to the federation apiserver instead of sending it to a specific Kubernetes cluster. The Federation control plane ensures that whenever the federated ReplicaSet is updated, it updates the corresponding ReplicaSet in all underlying clusters to match it. If your update includes a change in number of replicas, the federation control plane will change the number of replicas in underlying clusters to ensure that their sum remains equal to the number of desired replicas in federated ReplicaSet.

Deleting a Federated ReplicaSet

You can delete a federated ReplicaSet as you would delete a Kubernetes ReplicaSet; however, for a federated ReplicaSet, you must send the request to the federation apiserver instead of sending it to a specific Kubernetes cluster.

For example, you can do that using kubectl by running:

kubectl --context=federation-cluster delete rs myrs

Note that at this point, deleting a federated ReplicaSet will not delete the corresponding ReplicaSets from underlying clusters. You must delete the underlying ReplicaSets manually. We intend to fix this in the future.