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Table of Contents generated with DocToc

User Guide

Please refer to KubeFed Concepts first before you go through this user guide.

This user guide contains concepts and procedures to help you get started with KubeFed.

For information about installing KubeFed, see the installation documentation.

Deployment Image

If you follow this user guide without any changes you will be using the latest stable released version of the KubeFed image tagged as latest. Alternatively, we support the ability to deploy the latest master image tagged as canary or your own custom image.

Create Clusters

The KubeFed control plane can run on any v1.13 or greater Kubernetes clusters. The following is a list of Kubernetes environments that have been tested and are supported by the KubeFed community:

After completing the steps in one of the above guides, return here to continue the KubeFed deployment.

NOTE: You must set the correct context using the command below as this guide depends on it.

kubectl config use-context cluster1

Helm Chart Deployment

You can refer to helm chart installation guide to install and uninstall a KubeFed control plane.

Cluster Registration

You can join, unjoin and check the status of clusters using the kubefedctl command. See the Cluster Registration documentation for more information.

Federated API types

Enabling federation of an API type

You can enable federation of any Kubernetes API type (including CRDs) by using the kubefedctl command as follows.

NOTE: Federation of a CRD requires that the CRD be installed on all member clusters. If the CRD is not installed on a member cluster, propagation to that cluster will fail.

kubefedctl enable <target kubernetes API type>

The <target kubernetes API type> can be any of the following

  • the Kind (e.g. Deployment)
  • plural name (e.g. deployments)
  • group-qualified plural name (e.g deployment.apps), or
  • short name (e.g. deploy)

for the intended target API type.

The kubefedctl command will create

  • a CRD for the federated type named Federated<Kind>
  • a FederatedTypeConfig in the KubeFed system namespace with the group-qualified plural name of the target type.

A FederatedTypeConfig associates the federated type CRD with the target kubernetes type, enabling propagation of federated resources of the given type to the member clusters.

The format used to name the FederatedTypeConfig is <target kubernetes API type name>.<group name> except kubernetes core group types where the name format used is <target kubernetes API type name>.

You can also output the yaml to stdout instead of applying it to the API Server, using the following command.

kubefedctl enable <target API type> --output=yaml

NOTE: Federation of an API type requires that the API type be installed on all member clusters. If the API type is not installed on a member cluster, propagation to that cluster will fail. See issue 314 for more details.

Verifying API type is installed on all member clusters

If the API type is not installed on one of your member clusters, you will see a repeated controller-manager log error similar to the one reported in issue 314. At this time, you must manually verify that the API type is installed on each of your clusters as the controller-manager log error is the only indication.

For an example API type bars.example.com, you can verify that the API type is installed on each of your clusters by running:

CLUSTER_CONTEXTS="cluster1 cluster2"
for c in ${CLUSTER_CONTEXTS}; do
    echo ----- ${c} -----
    kubectl --context=${c} api-resources --api-group=example.com
done

The output should look like the following:

----- cluster1 -----
NAME   SHORTNAMES   APIGROUP      NAMESPACED   KIND
bars                example.com   true         Bar
----- cluster2 -----
NAME   SHORTNAMES   APIGROUP      NAMESPACED   KIND
bars                example.com   true         Bar

The output shown below is an example if you do not have the API type installed on cluster2. Note that cluster2 did not return any resources:

----- cluster1 -----
NAME   SHORTNAMES   APIGROUP      NAMESPACED   KIND
bars                example.com   true         Bar
----- cluster2 -----
NAME   SHORTNAMES   APIGROUP   NAMESPACED   KIND

Verifying the API type exists on all member clusters will ensure successful propagation to that cluster.

Enabling an API type with a non-default API group

When kubefedctl enable is used to enable types whose plural names (e.g. deployments.example.com and deployments.apps) match, the crd name of the generated federated type would also match (e.g. deployments.types.kubefed.io).

kubefedctl enable --federated-group string specifies the name of the API group to use for the generated federated type. It is types.kubefed.io by default. If a non-default group is used to enable federation of a type, the RBAC permissions for the KubeFed controller manager will need to be updated to include permissions for the new group.

For example, as part of deployment of a KubeFed control plane, deployments.apps is enabled by default. To enable deployments.example.com, you should:

kubefedctl enable deployments.example.com --federated-group kubefed.example.com
kubectl patch clusterrole kubefed-role --type='json' -p='[{"op": "add", "path": "/rules/1", "value": {
            "apiGroups": [
                "kubefed.example.com"
            ],
            "resources": [
                "*"
            ],
            "verbs": [
                "get",
                "watch",
                "list",
                "update"
            ]
        }
}]'

This example is for a cluster-scoped KubeFed control plane. For a namespaced KubeFed control plane, patch role kubefed-role in the KubeFed system namespace instead.

Disabling propagation of an API type

You can disable propagation of an API type by editing its FederatedTypeConfig resource:

kubectl patch --namespace <KUBEFED_SYSTEM_NAMESPACE> federatedtypeconfigs <NAME> \
    --type=merge -p '{"spec": {"propagation": "Disabled"}}'

This patch command sets the propagation field in the FederatedTypeConfig associated with this target API type to Disabled, which will prompt the sync controller for the target API type to be stopped.

If you want to permanently disable federation of the target API type, use:

kubefedctl disable <FederatedTypeConfig Name>

This will remove the FederatedTypeConfig that configures federation of the type. If supplied with the optional --delete-crd flag, the command will also remove the federated type CRD if none of its instances exist.

Federating a target resource

Apart from enabling and disabling a type for propagation as specified in the previous section, kubefedctl can also be used to federate a target resource of an API type. We define the term federate here and use the command keyword federate in kudefedctl with similar meaning.

kubefedctl federate creates a federated resource from a kubernetes resource. The federated resource will embed the kubernetes resource as its template and its placement will select all clusters.

Syntax

kubefedctl federate <target kubernetes API type> <target resource> [flags]

If the flag --namespace is additionally not specified, the <target resource> will be searched for in the namespace according to the client kubeconfig context. Please take note that --namespace flag is of no meaning when federating a namespace itself and is discarded even if specified. Please check the next section for more details about federating a namespace.

Example: Federate a resource named "my-configmap" in namespace "my-namespace" of kubernetes type "configmaps"

kubefedctl federate configmaps my-configmap -n my-namespace

By default, kubefedctl federate creates a federated resource in the same namespace as the target resource. This requires that the target type already be enabled for federation (i.e. via kubefedctl enable).

If --output=yaml is specified, and the target type is not yet enabled for federation, kubefedctl federate will assume the default form of the federated type in generating the federated resource. This may not be compatible with a kubefed control plane that has enabled a federated type in a non-default way (e.g. the group of the federated type has been set to something other than types.kubefed.io).

Federate a namespace with contents

kubefedctl federate can also be used to federate a target namespace and its contained resources with a single invocation. This can be achieved using the flag --contents which is valid only when the <target kubernetes API type> is a namespace. kubefedctl federate with --contents looks up all the existing resources in the target namespace and federates them one by one. It will skip the resources created by controllers (e.g. endpoints and events). It is also possible to explicitly skip resource types with the --skip-api-resources argument.

Example: Federate a namespace named "my-namespace" skipping API Resource "configmaps" and API Resource group "apps"

kubefedctl federate namespace my-namespace --contents --skip-api-resources "configmaps,apps"

Optionally enable type while federating a resource

kubefedctl federate allows optionally enabling the given <target kubernetes API type> before federating the resource by supplying the --enable-type flag. This will enable federation of the target type if it is not already enabled. It's recommended to use kubefedctl enable beforehand if the intention is to specify non default type configuration values.

Example: Federate a configmap named "my-configmap" while also enabling type configmaps for propagation

kubefedctl federate configmap my-configmap --enable-type

Federate resources from input file and stdin

In addition to supporting conversion of resources in a Kubernetes API, kubefedctl federeate supports converting resources to stdout from resources read from a local file. API resources can be read in yaml format via the --filename argument. The command currently does not look up for an already enabled type to use the type configuration values while translating yaml resources and uses default values for the same. The command in this mode can also take input from stdin in place of an actual file. The output could be piped to kubectl create -f - if the intention is to create the federate resource in the federated API surface. No other arguments or flag options are needed in this mode.

Example: Get federated resources for the target resources listed in a yaml file "my-file"

kubefedctl federate --filename ./my-file

Propagation status

When the sync controller reconciles a federated resource with member clusters, propagation status will be written to the resource as per the following example:

apiVersion: types.kubefed.io/v1beta1
kind: FederatedNamespace
metadata:
  name: myns
  namespace: myns
spec:
  placement:
    clusterSelector: {}
status:
  # The status True of the condition of type Propagation
  # indicates that the state of all member clusters is as
  # intended as of the last probe time.
  conditions:
  - type: Propagation
    status: True
    lastTransitionTime: "2019-05-08T01:23:20Z"
    lastUpdateTime: "2019-05-08T01:23:20Z"
  # The namespace 'myns' has been verified to exist in the
  # following clusters as of the lastUpdateTime recorded
  # in the 'Propagation' condition. Since that time, no
  # change has been detected to this resource or the
  # resources it manages.
  clusters:
  - name: cluster1
  - name: cluster2

Troubleshooting condition status

If the sync controller encounters an error in creating, updating or deleting managed resources in member clusters, the Propagation condition will have a status of False and the reason field will be one of the following values:

Reason Description
CheckClusters One or more clusters is not in the desired state.
ClusterRetrievalFailed An error prevented retrieval of member clusters.
ComputePlacementFailed An error prevented computation of placement.
NamespaceNotFederated The containing namespace is not federated.

For reasons other than CheckClusters, an event will be logged with the same reason and can be examined for more detail:

kubectl describe federatednamespace myns -n myns | grep ComputePlacementFailed

Warning  ComputePlacementFailed  5m   federatednamespace-controller  Invalid selector <nil>

If the reason is NamespaceNotFederated, the containing namespace can be federated by invoking kubefedctl federate namespace <namespace name>.

Troubleshooting CheckClusters

If the Propagation condition has status False and reason CheckClusters, the cluster status can be examined to determine the clusters for which reconciliation was not successful. In the following example, namespace myns has been verified to exist in cluster1. The namespace should not exist in cluster2, but deletion has failed.

apiVersion: types.kubefed.io/v1beta1
kind: FederatedNamespace
metadata:
  name: myns
  namespace: myns
spec:
  placement:
    clusters:
    - name: cluster1
status:
  conditions:
  - type: Propagation
    status: False
    reason: CheckClusters
    lastTransitionTime: "2019-05-08T01:23:20Z"
    lastUpdateTime: "2019-05-08T01:23:20Z"
  clusters:
  - name: cluster1
  - name: cluster2
    status: DeletionFailed

When a cluster has a populated status, as in the example above, the sync controller will have written an event with a matching Reason that may provide more detail as to the nature of the problem.

kubectl describe federatednamespace myns -n myns | grep cluster2 | grep DeletionFailed

Warning  DeletionFailed  5m   federatednamespace-controller  Failed to delete Namespace "myns" in cluster "cluster2"...

The following table enumerates the possible values for cluster status:

Status Description
AlreadyExists The target resource already exists in the cluster, and cannot be adopted due to adoptResources being disabled.
ApplyOverridesFailed An error occurred while attempting to apply overrides to the computed form of the target resource.
CachedRetrievalFailed An error occurred when retrieving the cached target resource.
ClientRetrievalFailed An error occurred while attempting to create an API client for the member cluster.
ClusterNotReady The latest health check for the cluster did not succeed.
ComputeResourceFailed An error occurred when determining the form of the target resource that should exist in the cluster.
CreationFailed Creation of the target resource failed.
CreationTimedOut Creation of the target resource timed out.
DeletionFailed Deletion of the target resource failed.
DeletionTimedOut Deletion of the target resource timed out.
FieldRetentionFailed An error occurred while attempting to retain the value of one or more fields in the target resource (e.g. clusterIP for a service)
LabelRemovalFailed Removal of the KubeFed label from the target resource failed.
LabelRemovalTimedOut Removal of the KubeFed label from the target resource timed out.
ManagedLabelFalse Unable to manage the object which has label kubefed.io/managed: false
RetrievalFailed Retrievel of the target resource from the cluster failed.
UpdateFailed Update of the target resource failed.
UpdateTimedOut Update of the target resource timed out.
VersionRetrievalFailed An error occurred while attempting to retrieve the last recorded version of the target resource.
WaitingForRemoval The target resource has been marked for deletion and is awaiting garbage collection.

Deletion policy

All federated resources reconciled by the sync controller have a finalizer (kubefed.io/sync-controller) added to their metadata. This finalizer will prevent deletion of a federated resource until the sync controller has a chance to perform pre-deletion cleanup.

Pre-deletion cleanup of a federated resource includes removal of resources managed by the federated resource from member clusters. To ensure retention of managed resources, add kubefed.io/orphan: true as an annotation to the federated resource prior to deletion:

Pre-deletion cleanup includes removal of resources managed by the federated resource from member clusters.

To prevent removal of these managed resources, add kubefed.io/orphan: true as an annotation to the federated resource prior to deletion, as follows.

You can do it by

kubefedctl orphaning-deletion enable <federated type> <name>

You can also check the current orphaning-deletion status by:

kubefedctl orphaning-deletion status <federated type> <name>

And finally, if you want to return to the default deletion behavior, you can disable the orphaning-deletion by:

 kubefedctl orphaning-deletion disable <federated type> <name>

If the flag --namespace is additionally not specified, the federated resource will be searched for in the namespace according to the client kubeconfig context. If the sync controller for a given federated type is not able to reconcile a federated resource slated for deletion, a federated resource that still has the KubeFed finalizer will linger rather than being garbage collected. If necessary, the KubeFed finalizer can be manually removed to ensure garbage collection.

Verify your deployment is working

You can verify that your deployment is working properly by completing the following example.

The example creates a test namespace with a federatednamespace resource, as well as a federated resource for the following k8s resources.

  • configmap
  • secret
  • deployment
  • service, and
  • serviceaccount

It will then show how to update the federatednamespace resource to move resources.

Creating the test namespace

Create the test-namespace for the test resources.

kubectl apply -f example/sample1/namespace.yaml \
    -f example/sample1/federatednamespace.yaml

Creating test resources

Create test resources.

kubectl apply -R -f example/sample1

NOTE: If you get the following error while creating a test resource

unable to recognize "example/sample1/federated<type>.yaml": no matches for kind "Federated<type>" in version "types.kubefed.io/v1beta1",

then it indicates that a given type may need to be enabled with kubefedctl enable <type>

Checking resources status

Check the status of all the resources in each cluster.

for r in configmaps secrets service deployment serviceaccount job; do
    for c in cluster1 cluster2; do
        echo; echo ------------ ${c} resource: ${r} ------------; echo
        kubectl --context=${c} -n test-namespace get ${r}
        echo; echo
    done
done

The status of propagation is also recorded on each federated resource:

for r in federatedconfigmaps federatedsecrets federatedservice federateddeployment federatedserviceaccount federatedjob; do
    echo; echo ------------ resource: ${r} ------------; echo
    kubectl -n test-namespace get ${r} -o yaml
    echo; echo
done

Ensure nginx is running properly in each cluster:

for c in cluster1 cluster2; do
    NODE_PORT=$(kubectl --context=${c} -n test-namespace get service \
        test-service -o jsonpath='{.spec.ports[0].nodePort}')
    echo; echo ------------ ${c} ------------; echo
    NODE_IP=$(kubectl get node --context=${c} \
        -o jsonpath='{.items[].status.addresses[*].address}'|sed 's/\S*cluster1\S*//'|tr -d " ")
    curl ${NODE_IP}:${NODE_PORT}
    echo; echo
done

Updating FederatedNamespace placement

Remove cluster2 via a patch command or manually.

kubectl -n test-namespace patch federatednamespace test-namespace \
    --type=merge -p '{"spec": {"placement": {"clusters": [{"name": "cluster1"}]}}}'

kubectl -n test-namespace edit federatednamespace test-namespace

Then wait to verify all resources are removed from cluster2:

for r in configmaps secrets service deployment serviceaccount job; do
    for c in cluster1 cluster2; do
        echo; echo ------------ ${c} resource: ${r} ------------; echo
        kubectl --context=${c} -n test-namespace get ${r}
        echo; echo
    done
done

You can quickly add back all the resources by simply updating the FederatedNamespace to add cluster2 again via a patch command or manually:

kubectl -n test-namespace patch federatednamespace test-namespace \
    --type=merge -p '{"spec": {"placement": {"clusters": [{"name": "cluster1"}, {"name": "cluster2"}]}}}'

kubectl -n test-namespace edit federatednamespace test-namespace

Wait and verify all resources are added back to cluster2:

for r in configmaps secrets service deployment serviceaccount job; do
    for c in cluster1 cluster2; do
        echo; echo ------------ ${c} resource: ${r} ------------; echo
        kubectl --context=${c} -n test-namespace get ${r}
        echo; echo
    done
done

Lastly, make sure nginx is running properly in each cluster:

for c in cluster1 cluster2; do
    NODE_PORT=$(kubectl --context=${c} -n test-namespace get service \
        test-service -o jsonpath='{.spec.ports[0].nodePort}')
    echo; echo ------------ ${c} ------------; echo
    curl $(echo -n $(minikube ip -p ${c})):${NODE_PORT}
    echo; echo
done

If you were able to verify the resources removed and added back then you have successfully verified a working KubeFed deployment.

Cleaning up

To cleanup the example simply delete the namespace:

kubectl delete ns test-namespace

NOTE: Deleting the test namespace requires that the KubeFed controllers first perform the removal of managed resources from member clusters. This may take a few moments.

Overrides

Overrides can be specified for any federated resource and allow varying resource content from the template on a per-cluster basis. Overrides are implemented via a subset of jsonpatch, as follows:

  • op defines the operation to perform (add, remove or replace are supported)
    • replace replaces a value
      • if not specified, op will default to replace
    • add adds a value to an object or array
    • remove removes a value from an object or array
  • path specifies a valid location in the managed resource to target for modification
    • path must start with a leading / and entries must be separated by /
      • e.g. /spec/replicas
    • indexed paths start at zero
      • e.g. /spec/template/spec/containers/0/image
  • value specifies the value to add or replace.
    • value is ignored for remove

For example:

kind: FederatedDeployment
...
spec:
  ...
  overrides:
  # Apply overrides to cluster1
    - clusterName: cluster1
      clusterOverrides:
        # Set the replicas field to 5
        - path: "/spec/replicas"
          value: 5
        # Set the image of the first container
        - path: "/spec/template/spec/containers/0/image"
          value: "nginx:1.17.0-alpine"
        # Ensure the annotation "foo: bar" exists
        - path: "/metadata/annotations"
          op: "add"
          value:
            foo: bar
        # Ensure an annotation with key "foo" does not exist
        - path: "/metadata/annotations/foo"
          op: "remove"

Overriding retained fields

When computing the form of a managed resource that should appear in a cluster registered with KubeFed, the following operations are executed in sequence:

  • A new resource is computed from the template of the federated resource
  • If an existing resource is present, the contents of fields subject to retention are preserved
  • Overrides are applied
  • The managed label is set

This order of operations ensures that fields subject to retention can still be overridden (e.g. adding an entry to metadata.annotations). Care should be taken in applying overrides to retained fields that may be modified by controllers in member clusters or a managed resource may end up being continuously updated first by the controller in the member cluster and then by KubeFed.

Using Cluster Selector

In addition to specifying an explicit list of clusters that a resource should be propagated to via the spec.placement.clusters field of a federated resource, it is possible to use the spec.placement.clusterSelector field to provide a label selector that determines a list of clusters at runtime.

If the goal is to select a subset of member clusters, make sure that the KubeFedCluster binaries from pre-reqs [now covered by Helm installation] resources that are intended to be selected have the appropriate labels applied.

The following command is an example to label a KubeFedCluster:

kubectl label kubefedclusters -n kube-federation-system cluster1 foo=bar

Please refer to Kubernetes label command for more information on how kubectl label works.

The following sections detail how spec.placement.clusters and spec.placement.clusterSelector are used in determining the clusters that a federated resource should be propagated to.

Neither spec.placement.clusters nor spec.placement.clusterSelector is provided

spec:
  placement: {}

In this case, you can either set spec: {} as above or remove spec field from your placement policy. The resource will not be propagated to member clusters.

Both spec.placement.clusters and spec.placement.clusterSelector are provided

spec:
  placement:
    clusters:
      - name: cluster2
      - name: cluster1
    clusterSelector:
      matchLabels:
        foo: bar

For this case, spec.placement.clusterSelector will be ignored as spec.placement.clusters is provided. This ensures that the results of runtime scheduling have priority over manual definition of a cluster selector.

spec.placement.clusterSelector will also be ignored if spec.placement.clusters is provided but empty. In the following example, no clusters will be selected:

spec:
  placement:
    clusters: []
    clusterSelector:
      matchLabels:
        foo: bar

spec.placement.clusters is not provided, spec.placement.clusterSelector is provided but empty

In this case, spec.placement.clusterSelector will be ignored, since spec.placement.clusters is provided. This ensures that the results of runtime scheduling have priority over manual definition of a cluster selector.

spec:
  placement:
    clusterSelector: {}

In this case, the resource will be propagated to all member clusters.

spec.placement.clusters is not provided, spec.placement.clusterSelector is provided and not empty

spec:
  placement:
    clusterSelector:
      matchLabels:
        foo: bar

In this case, the resource will only be propagated to member clusters that are labeled with foo: bar.

Troubleshooting

If federated resources are not propagated as expected to the member clusters, you can use the following command to view Events, which may help you to diagnose the problem.

kubectl describe <federated CRD> <CR name> -n test-namespace

An example for CRD of federatedserviceaccounts is as follows:

kubectl describe federatedserviceaccounts test-serviceaccount -n test-namespace

It may also be useful to inspect the KubeFed controller log as follows:

kubectl logs deployment/kubefed-controller-manager -n kube-federation-system

Cleanup

Deployment Cleanup

Resources such as namespaces associated with a FederatedNamespace or FederatedClusterRoles should be deleted before cleaning up the deployment, otherwise, the process will fail.

Run the following command to perform a cleanup of the cluster registry and KubeFed deployments:

./scripts/delete-kubefed.sh

The above script unjoins the all of the clusters from the KubeFed control plane it deploys, by default.

On successful completion of the script, both cluster1 and cluster2 will be unjoined from the deployed KubeFed control plane.

Namespace-scoped control plane

All prior instructions referred to the deployment and use of a cluster-scoped KubeFed control plane. It is also possible to deploy a namespace-scoped control plane. In this mode of operation, KubeFed controllers will target resources in a single namespace on both host and member clusters. This may be desirable when experimenting with KubeFed on a production cluster.

Helm Configuration

To deploy KubeFed in a namespaced configuration, set global.scope to Namespaced as per the Helm chart install instructions.

Cluster Registration

You can join, unjoin and check the status of clusters using the kubefedctl command. See the Cluster Registration documentation for more information.

Local Value Retention

In most cases, the KubeFed sync controller will overwrite any changes made to resources it manages in member clusters. The exceptions appear in the following table. Where retention is conditional, an explanation will be provided in a subsequent section.

Resource Type Fields Retention Requirement
All metadata.annotations Always The annotations field is intended to be managed by controllers in member clusters.
All metadata.finalizers Always The finalizers field is intended to be managed by controllers in member clusters.
All metadata.resourceVersion Always Updates require the most recent resourceVersion for concurrency control.
Scalable spec.replicas Conditional The HPA controller may be managing the replica count of a scalable resource.
Service spec.clusterIP,spec.ports Always A controller may be managing these fields.
ServiceAccount secrets Conditional A controller may be managing this field.

Scalable

For scalable resources (those that have a scale subtype e.g. ReplicaSet and Deployment), retention of the spec.replicas field is controlled by the retainReplicas boolean field of the federated resource. retainReplicas defaults to false, and should be set to true only if the resource will be managed by HPA in member clusters.

Retention of the replicas field is possible for all clusters or no clusters. If a resource will be managed by HPA in some clusters but not others, it will be necessary to create a separate federated resource for each retention strategy (i.e. one with retainReplicas: true and one with retainReplicas: false).

ServiceAccount

A populated secrets field of a ServiceAccount resource managed by KubeFed will be retained if the managing federated resource does not specify a value for the field. This avoids the possibility of the sync controller attempting to repeatedly clear the field while a local serviceaccounts controller attempts to repeatedly set it to a generated value.

Higher order behaviour

The architecture of KubeFed API allows higher level APIs to be constructed using the mechanics provided by the standard form of the federated API types (containing fields for template, placement and override) and associated controllers for a given resource. Further sections describe few of higher level APIs implemented as part of KubeFed.

Multi-Cluster Ingress DNS

Multi-Cluster Ingress DNS provides the ability to programmatically manage DNS resource records of Ingress objects through ExternalDNS integration. Review the guides below for different DNS provider to learn more.

Multi-Cluster Service DNS

Multi-Cluster Service DNS provides the ability to programmatically manage DNS resource records of Service objects through ExternalDNS integration. Review the guides below for different DNS provider to learn more.

ReplicaSchedulingPreference

ReplicaSchedulingPreference provides an automated mechanism of distributing and maintaining total number of replicas for deployment or replicaset based federated workloads into federated clusters. This is based on high level user preferences given by the user. These preferences include the semantics of weighted distribution and limits (min and max) for distributing the replicas. These also include semantics to allow redistribution of replicas dynamically in case some replica pods remain unscheduled in some clusters, for example due to insufficient resources in that cluster.

RSP is used in place of ReplicaSchedulingPreference for brevity in text further on.

The RSP controller works in a sync loop observing the RSP resource and the matching namespace/name pair FederatedDeployment or FederatedReplicaset resource.

If it finds that both RSP and its associated federated resource, the type of which is specified using spec.targetKind, exists, it goes ahead to list currently healthy clusters and distributes the spec.totalReplicas using the associated per cluster user preferences. If the per cluster preferences are absent, it distributes the spec.totalReplicas evenly among all clusters. It updates (or creates if missing) the same namespace/name for the targetKind with the replica values calculated, leveraging the sync controller to actually propagate the k8s resource to federated clusters. It's noteworthy that if an RSP is present, the spec.replicas from the federated resource are unused. RSP also provides a further more useful feature using spec.rebalance. If this is set to true, the RSP controller monitors the replica pods for target replica workload from each federated cluster and if it finds that some clusters are not able to schedule those pods for long, it moves (rebalances) the replicas to clusters where all the pods are running and healthy. This in other words helps moving the replica workloads to those clusters where there is enough capacity and away from those clusters which are currently running out of capacity. The rebalance feature might cause initial shuffle of replicas to reach an eventually balanced state of distribution. The controller might further keep trying to move few replicas back into the cluster(s) which ran out of capacity, to check if it can be scheduled again to reach the normalised state (even distribution or the state desired by user preferences), which apparently is the only mechanism to check if this cluster has capacity now. The spec.rebalance should not be used if this behaviour is unacceptable.

The RSP can be considered as more user friendly mechanism to distribute the replicas, where the inputs needed from the user at federated control plane are reduced. The user only needs to create the RSP resource and associated federated resource (with only spec.template populated) to distribute the replicas. It can also be considered as a more automated approach at distribution and further reconciliation of the workload replicas.

The usage of the RSP semantics is illustrated using some examples below. The examples considers 3 federated clusters A, B and C.

Distribute total replicas evenly in all available clusters

apiVersion: scheduling.kubefed.io/v1alpha1
kind: ReplicaSchedulingPreference
metadata:
  name: test-deployment
  namespace: test-ns
spec:
  targetKind: FederatedDeployment
  totalReplicas: 9

or

apiVersion: scheduling.kubefed.io/v1alpha1
kind: ReplicaSchedulingPreference
metadata:
  name: test-deployment
  namespace: test-ns
spec:
  targetKind: FederatedDeployment
  totalReplicas: 9
  clusters:
    "*":
      weight: 1

A, B and C get 3 replicas each.

Distribute total replicas in weighted proportions

apiVersion: scheduling.kubefed.io/v1alpha1
kind: ReplicaSchedulingPreference
metadata:
  name: test-deployment
  namespace: test-ns
spec:
  targetKind: FederatedDeployment
  totalReplicas: 9
  clusters:
    A:
      weight: 1
    B:
      weight: 2

A gets 3 and B gets 6 replicas in the proportion of 1:2. C does not get any replica as missing weight preference is considered as weight=0.

Distribute replicas in weighted proportions, also enforcing replica limits per cluster

apiVersion: scheduling.kubefed.io/v1alpha1
kind: ReplicaSchedulingPreference
metadata:
  name: test-deployment
  namespace: test-ns
spec:
  targetKind: FederatedDeployment
  totalReplicas: 9
  clusters:
    A:
      minReplicas: 4
      maxReplicas: 6
      weight: 1
    B:
      minReplicas: 4
      maxReplicas: 8
      weight: 2

A gets 4 and B get 5 as weighted distribution is capped by cluster A minReplicas=4.

Distribute replicas evenly in all clusters, however not more than 20 in C

apiVersion: scheduling.kubefed.io/v1alpha1
kind: ReplicaSchedulingPreference
metadata:
  name: test-deployment
  namespace: test-ns
spec:
  targetKind: FederatedDeployment
  totalReplicas: 50
  clusters:
    "*":
      weight: 1
    "C":
      maxReplicas: 20
      weight: 1

Possible scenarios

All have capacity.

Replica layout: A=16 B=17 C=17.

B is offline/has no capacity

Replica layout: A=30 B=0 C=20

A and B are offline:

Replica layout: C=20

Controller-Manager Leader Election

The KubeFed controller manager is always deployed with leader election feature to ensure high availability of the control plane. Leader election module ensures there is always a leader elected among multiple instances which takes care of running the controllers. In case the active instance goes down, one of the standby instances gets elected as leader to ensure minimum downtime. Leader election ensures that only one instance is responsible for reconciliation. You can refer to the helm chart configuration to configure parameters for leader election to tune for your environment (the defaults should be sane for most environments).

Limitations

Immutable Fields

KubeFed API does not implement immutable fields in the federated resource yet.

A kubernetes resource field can be modified at runtime to change the resource specification. An immutable field cannot be modified after the resource is created.

For a federated resource, spec.template defines the resource specification common to all clusters. Though it is possible to modify any template field of a federated resource (or set an override for the field), changing the value of an immutable field will prevent all subsequent updates from completing successfully. This will be indicated by a propagation status of UpdateFailed for affected clusters. These errors can only be resolved by reverting the template field back to the value set at creation.

For example, spec.completions is an immutable field of a job resource. You cannot change it after a job has been created. Changing spec.template.spec.completions of the federated job resource will prevent all subsequent updates to jobs managed by the federated job. The changed value does not propagate to member clusters.

Support for validation of immutable fields in federated resources is intended to be implemented before KubeFed is GA.