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Federated Deployment

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 Deployments in the Federation control plane.

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

FEATURE STATE: Kubernetes 1.5 alpha
This feature is currently in a alpha state, meaning:

  • The version names contain alpha (e.g. v1alpha1).
  • Might be buggy. Enabling the feature may expose bugs. Disabled by default.
  • Support for feature may be dropped at any time without notice.
  • The API may change in incompatible ways in a later software release without notice.
  • Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.

Some features (such as full rollout compatibility) are still in development.

Before you begin

Creating a Federated Deployment

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

You can do that using kubectl by running:

kubectl --context=federation-cluster create -f mydeployment.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 Deployment is created, the federation control plane will create a Deployment in all underlying Kubernetes clusters. You can verify this by checking each of the underlying clusters, for example:

kubectl --context=gce-asia-east1a get deployment mydep

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

These Deployments in underlying clusters will match the federation Deployment except in the number of replicas and revision-related annotations. Federation control plane ensures that the sum of replicas in each cluster combined matches the desired number of replicas in the Federated Deployment.

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 Deployment with spec.replicas = 9, then each Deployment 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/deployment-preferences on Federated Deployment.

Updating a Federated Deployment

You can update a Federated Deployment as you would update a Kubernetes Deployment; however, for a Federated Deployment, 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 Deployment is updated, it updates the corresponding Deployments in all underlying clusters to match it. So if the rolling update strategy was chosen then the underlying cluster will do the rolling update independently and maxSurge and maxUnavailable will apply only to individual clusters. This behavior may change in the future.

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 Deployment.

Deleting a Federated Deployment

You can delete a Federated Deployment as you would delete a Kubernetes Deployment; however, for a Federated Deployment, 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 deployment mydep