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Federated ReplicaSets

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

Prerequisites

This guide assumes that you have a running Kubernetes Cluster Federation installation. If not, then head over to the federation admin guide to learn how to bring up a cluster federation (or have your cluster administrator do this for you). Other tutorials, for example this one by Kelsey Hightower, are also available to help you.

You are also expected to have a basic working knowledge of Kubernetes in general and ReplicaSets in particular.

Overview

Replica Sets in federation control plane (referred to as “federated replica sets” 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.

Creating a Federated Replica Set

The API for Federated Replica Set is 100% compatible with the API for traditional Kubernetes Replica Set. You can create a replica set 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 replica set is created, the federation control plane will create a replica set 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.

These replica sets in underlying clusters will match the federation replica set except in the number of replicas. Federation control plane will ensure that the sum of replicas in each cluster match the desired number of replicas in the federation replica set.

Spreading Replicas in Underlying Clusters

By default, replicas are spread equally in all the underlying clusters. For ex: if you have 3 registered clusters and you create a federated replica set with spec.replicas = 9, then each replica set in the 3 clusters will have spec.replicas=3. To modify the number of replicas in each cluster, you can specify FederatedReplicaSetPreference as an annotation with key federation.kubernetes.io/replica-set-preferences on federated replica set.

Updating a Federated Replica Set

You can update a federated replica set as you would update a Kubernetes replica set; however, for a federated replica set, you must send the request to the federation apiserver instead of sending it to a specific Kubernetes cluster. The Federation control plan ensures that whenever the federated replica set is updated, it updates the corresponding replica sets 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 replica set.

Deleting a Federated Replica Set

You can delete a federated replica set as you would delete a Kubernetes replica set; however, for a federated replica set, 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 replica set will not delete the corresponding replica sets from underlying clusters. You must delete the underlying Replica Sets manually. We intend to fix this in the future.

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