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Set up placement policies in Federation

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. 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 are available at sig-multicluster community page.

This page shows how to enforce policy-based placement decisions over Federated resources using an external policy engine.

Before you begin

You need to have a running Kubernetes cluster (which is referenced as host cluster). Please see one of the getting started guides for installation instructions for your platform.

Deploying Federation and configuring an external policy engine

The Federation control plane can be deployed using kubefed init.

After deploying the Federation control plane, you must configure an Admission Controller in the Federation API server that enforces placement decisions received from the external policy engine.

kubectl create -f scheduling-policy-admission.yaml

Shown below is an example ConfigMap for the Admission Controller:

federation/scheduling-policy-admission.yaml 
apiVersion: v1
kind: ConfigMap
metadata:
  name: admission
  namespace: federation-system
data:
  config.yml: |
    apiVersion: apiserver.k8s.io/v1alpha1
    kind: AdmissionConfiguration
    plugins:
    - name: SchedulingPolicy
      path: /etc/kubernetes/admission/scheduling-policy-config.yml
  scheduling-policy-config.yml: |
    kubeconfig: /etc/kubernetes/admission/opa-kubeconfig
  opa-kubeconfig: |
    clusters:
      - name: opa-api
        cluster:
          server: http://opa.federation-system.svc.cluster.local:8181/v0/data/kubernetes/placement
    users:
      - name: scheduling-policy
        user:
          token: deadbeefsecret
    contexts:
      - name: default
        context:
          cluster: opa-api
          user: scheduling-policy
    current-context: default

The ConfigMap contains three files:

Edit the Federation API server deployment to enable the SchedulingPolicy Admission Controller.

kubectl -n federation-system edit deployment federation-apiserver

Update the Federation API server command line arguments to enable the Admission Controller and mount the ConfigMap into the container. If there’s an existing --enable-admission-plugins flag, append ,SchedulingPolicy instead of adding another line.

--enable-admission-plugins=SchedulingPolicy
--admission-control-config-file=/etc/kubernetes/admission/config.yml

Add the following volume to the Federation API server pod:

- name: admission-config
  configMap:
    name: admission

Add the following volume mount the Federation API server apiserver container:

volumeMounts:
- name: admission-config
  mountPath: /etc/kubernetes/admission

Deploying an external policy engine

The Open Policy Agent (OPA) is an open source, general-purpose policy engine that you can use to enforce policy-based placement decisions in the Federation control plane.

Create a Service in the host cluster to contact the external policy engine:

kubectl create -f policy-engine-service.yaml

Shown below is an example Service for OPA.

federation/policy-engine-service.yaml 
kind: Service
apiVersion: v1
metadata:
  name: opa
  namespace: federation-system
spec:
  selector:
    app: opa
  ports:
  - name: http
    protocol: TCP
    port: 8181
    targetPort: 8181

Create a Deployment in the host cluster with the Federation control plane:

kubectl create -f policy-engine-deployment.yaml

Shown below is an example Deployment for OPA.

federation/policy-engine-deployment.yaml 
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: opa
  name: opa
  namespace: federation-system
spec:
  replicas: 1
  selector:
    matchLabels:
      app: opa
  template:
    metadata:
      labels:
        app: opa
      name: opa
    spec:
      containers:
        - name: opa
          image: openpolicyagent/opa:0.4.10
          args:
          - "run"
          - "--server"
        - name: kube-mgmt
          image: openpolicyagent/kube-mgmt:0.2
          args:
          - "-kubeconfig=/srv/kubernetes/kubeconfig"
          - "-cluster=federation/v1beta1/clusters"
          volumeMounts:
           - name: federation-kubeconfig
             mountPath: /srv/kubernetes
             readOnly: true
      volumes:
      - name: federation-kubeconfig
        secret:
          secretName: federation-controller-manager-kubeconfig

Configuring placement policies via ConfigMaps

The external policy engine will discover placement policies created in the kube-federation-scheduling-policy namespace in the Federation API server.

Create the namespace if it does not already exist:

kubectl --context=federation create namespace kube-federation-scheduling-policy

Configure a sample policy to test the external policy engine:

policy.rego docs/tasks/federation 
# OPA supports a high-level declarative language named Rego for authoring and
# enforcing policies. For more information on Rego, visit
# http://openpolicyagent.org.

# Rego policies are namespaced by the "package" directive.
package kubernetes.placement

# Imports provide aliases for data inside the policy engine. In this case, the
# policy simply refers to "clusters" below.
import data.kubernetes.clusters

# The "annotations" rule generates a JSON object containing the key
# "federation.kubernetes.io/replica-set-preferences" mapped to <preferences>.
# The preferences values is generated dynamically by OPA when it evaluates the
# rule.
#
# The SchedulingPolicy Admission Controller running inside the Federation API
# server will merge these annotations into incoming Federated resources. By
# setting replica-set-preferences, we can control the placement of Federated
# ReplicaSets.
#
# Rules are defined to generate JSON values (booleans, strings, objects, etc.)
# When OPA evaluates a rule, it generates a value IF all of the expressions in
# the body evaluate successfully. All rules can be understood intuitively as
# <head> if <body> where <body> is true if <expr-1> AND <expr-2> AND ...
# <expr-N> is true (for some set of data.)
annotations["federation.kubernetes.io/replica-set-preferences"] = preferences {
    input.kind = "ReplicaSet"
    value = {"clusters": cluster_map, "rebalance": true}
    json.marshal(value, preferences)
}

# This "annotations" rule generates a value for the "federation.alpha.kubernetes.io/cluster-selector"
# annotation.
#
# In English, the policy asserts that resources in the "production" namespace
# that are not annotated with "criticality=low" MUST be placed on clusters
# labelled with "on-premises=true".
annotations["federation.alpha.kubernetes.io/cluster-selector"] = selector {
    input.metadata.namespace = "production"
    not input.metadata.annotations.criticality = "low"
    json.marshal([{
        "operator": "=",
        "key": "on-premises",
        "values": "[true]",
    }], selector)
}

# Generates a set of cluster names that satisfy the incoming Federated
# ReplicaSet's requirements. In this case, just PCI compliance.
replica_set_clusters[cluster_name] {
    clusters[cluster_name]
    not insufficient_pci[cluster_name]
}

# Generates a set of clusters that must not be used for Federated ReplicaSets
# that request PCI compliance.
insufficient_pci[cluster_name] {
    clusters[cluster_name]
    input.metadata.annotations["requires-pci"] = "true"
    not pci_clusters[cluster_name]
}

# Generates a set of clusters that are PCI certified. In this case, we assume
# clusters are annotated to indicate if they have passed PCI compliance audits.
pci_clusters[cluster_name] {
    clusters[cluster_name].metadata.annotations["pci-certified"] = "true"
}

# Helper rule to generate a mapping of desired clusters to weights. In this
# case, weights are static.
cluster_map[cluster_name] = {"weight": 1} {
    replica_set_clusters[cluster_name]
}

Shown below is the command to create the sample policy:

kubectl --context=federation -n kube-federation-scheduling-policy create configmap scheduling-policy --from-file=policy.rego

This sample policy illustrates a few key ideas:

Testing placement policies

Annotate one of the clusters to indicate that it is PCI certified.

kubectl --context=federation annotate clusters cluster-name-1 pci-certified=true

Deploy a Federated ReplicaSet to test the placement policy.

federation/replicaset-example-policy.yaml 
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  labels:
    app: nginx-pci
  name: nginx-pci
  annotations:
    requires-pci: "true"
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx-pci
  template:
    metadata:
      labels:
        app: nginx-pci
    spec:
      containers:
      - image: nginx
        name: nginx-pci

Shown below is the command to deploy a ReplicaSet that does match the policy.

kubectl --context=federation create -f replicaset-example-policy.yaml

Inspect the ReplicaSet to confirm the appropriate annotations have been applied:

kubectl --context=federation get rs nginx-pci -o jsonpath='{.metadata.annotations}'

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