Multicluster-Service-Account
Multicluster-service-account makes it easy for pods in a cluster to call the Kubernetes APIs of other clusters. It imports remote service account tokens into local secrets, as kubeconfig files, and automounts them inside annotated pods.
Multicluster-service-account can be used to run any Kubernetes client from another cluster. It can also be used to build operators that control Kubernetes resources across multiple clusters, e.g., with multicluster-controller.
Why? Check out Admiralty's blog post introducing multicluster-service-account.
How it Works
Multicluster-service-account consists of:
- A binary,
kubemcsa
, to bootstrap clusters, allowing them to import service account secrets from one another;
- a ServiceAccountImport custom resource definition (CRD) and controller to import remote service account secrets as kubeconfig files;
- Here is a sample service account import object:
apiVersion: multicluster.admiralty.io/v1alpha1
kind: ServiceAccountImport
metadata:
name: cluster2-default-pod-lister
spec:
clusterName: cluster2
namespace: default # source and target namespaces can be different
name: pod-lister
- which would generate a secret like this:
apiVersion: v1
kind: Secret
metadata:
name: cluster2-default-pod-lister-token-6456p
... # owner reference, etc.
type: Opaque
data:
config: ... # serialized kubeconfig
- a dynamic admission webhook to automount service account import secrets inside annotated pods, the same way regular service accounts are automounted inside Pods;
- (optional) Go helper functions (in the
pkg/config
package) to list and load mounted service account imports.
Note: Before v0.4.0, service account import secrets used a custom format (like regular service account secrets, with an additional "server" field to locate the remote cluster's Kubernetes API). Clients were required to use custom code, e.g., the provided Go helper functions, to load the secrets as REST configs. v0.4.0 leverages the standard kubeconfig format to make it even easier to use multicluster-service-account, without any code change, with clients written in any language.
Getting Started
We assume that you are a cluster admin on two clusters, associated with, e.g., the contexts "cluster1" and "cluster2" in your kubeconfig. We're going to install multicluster-service-account and run a multi-cluster client example in cluster1, listing pods in cluster2.
CLUSTER1=cluster1 # change me
CLUSTER2=cluster2 # change me
Step 1: Installation
Install multicluster-service-account in cluster1:
RELEASE_URL=https://github.com/admiraltyio/multicluster-service-account/releases/download/v0.4.0
MANIFEST_URL=$RELEASE_URL/install.yaml
kubectl apply -f $MANIFEST_URL --context $CLUSTER1
Cluster1 is now able to import service accounts, but it hasn't been given permission to import them from cluster2 yet. This is a chicken-and-egg problem: cluster1 needs a token from cluster2, before it can import service accounts from it. To solve this problem, download the kubemcsa binary and run the bootstrap command:
OS=linux # or darwin (i.e., OS X) or windows
ARCH=amd64 # if you're on a different platform, you must know how to build from source
BINARY_URL="$RELEASE_URL/kubemcsa-$OS-$ARCH"
curl -Lo kubemcsa $BINARY_URL
chmod +x kubemcsa
sudo mv kubemcsa /usr/local/bin
kubemcsa bootstrap $CLUSTER1 $CLUSTER2
Step 2: Example
The multicluster-client
example includes:
- in cluster2:
- a service account named
pod-lister
in the default namespace, bound to a role that can only list pods in its namespace;
- a dummy NGINX deployment (to have pods to list);
- in cluster1:
- a new label on the
default
namespace, multicluster-service-account=enabled
, to instruct multicluster-service-account to automount service account import secrets inside annotated pods;
- a service account import named
cluster2-default-pod-lister
, importing pod-lister
from the default namespace of cluster2;
- a
multicluster-client
job, whose pod is annotated to automount cluster2-default-pod-lister
's secret—it will list the pods in the default namespace of cluster2, and stop without restarting (we'll check the logs).
kubectl config use-context $CLUSTER2
kubectl create serviceaccount pod-lister
kubectl create role pod-lister --verb=list --resource=pods
kubectl create rolebinding pod-lister --role=pod-lister \
--serviceaccount=default:pod-lister
kubectl run nginx --image nginx
kubectl config use-context $CLUSTER1
kubectl label namespace default multicluster-service-account=enabled
cat <<EOF | kubectl create -f -
apiVersion: multicluster.admiralty.io/v1alpha1
kind: ServiceAccountImport
metadata:
name: $CLUSTER2-default-pod-lister
spec:
clusterName: $CLUSTER2
namespace: default
name: pod-lister
---
apiVersion: batch/v1
kind: Job
metadata:
name: multicluster-client
spec:
template:
metadata:
annotations:
multicluster.admiralty.io/service-account-import.name: $CLUSTER2-default-pod-lister
spec:
restartPolicy: Never
containers:
- name: multicluster-client
image: quay.io/admiralty/multicluster-service-account-example-multicluster-client:latest
EOF
In cluster1, check that:
- The service account import controller created a secret for the
cluster2-default-pod-lister
service account import, containing a kubeconfig file populated with the token and namespace of the remote service account, and the URL and root certificate of the remote Kubernetes API:
kubectl get secret -l multicluster.admiralty.io/service-account-import.name=$CLUSTER2-default-pod-lister -o jsonpath={.items[0].data.config} | base64 -D
# the data is base64-encoded
- The service account import secret was mounted inside the
multicluster-client
pod by the service account import admission controller:
kubectl get pod -l job-name=multicluster-client -o yaml
# look at volumes and volume mounts
- The
multicluster-client
pod was able to list pods in the default namespace of cluster2:
kubectl logs job/multicluster-client
Service Account Imports
Service account imports tell the service account import controller to maintain a secret in the same namespace, containing the remote service account's namespace and token, as well as the URL and root certificate of the remote Kubernetes API, which are all necessary data to configure a Kubernetes client. The secret is formatted as a standard kubeconfig file, which most Kubernetes clients can understand. If a pod needs to call several clusters, it will use several service account imports, e.g.:
apiVersion: multicluster.admiralty.io/v1alpha1
kind: ServiceAccountImport
metadata:
name: cluster2-default-pod-lister
spec:
clusterName: cluster2
namespace: default
name: pod-lister
---
apiVersion: multicluster.admiralty.io/v1alpha1
kind: ServiceAccountImport
metadata:
name: cluster3-default-pod-lister
spec:
clusterName: cluster3
namespace: default
name: pod-lister
Annotations
In namespaces labeled with multicluster-service-account=enabled
, the multicluster.admiralty.io/service-account-import.name
annotation on a pod (or pod template) tells the service account import admission controller to automount the corresponding secrets inside it. If a pod needs several service account imports, separate their names with commas, e.g.:
apiVersion: v1
kind: Pod
metadata:
name: multicluster-client
annotations:
multicluster.admiralty.io/service-account-import.name: cluster2-default-pod-lister,cluster3-default-pod-lister
spec:
# ...
Note: just like with local service accounts, there is a race condition if a service account import and a pod requesting it are created at the same time: the service account import admission controller will likely reject the pod because the secret to automount won't be ready. Luckily, if the pod is controlled by another object, such as a deployment, job, etc., pod creation will be retried.
(Optional) Client Configuration
Multicluster-service-account includes a Go library (cf. pkg/config
) to facilitate the creation of client-go rest.Config
instances from service account imports. From there, you can create kubernetes.Clientset
instances as usual. The namespaces of the remote service accounts are also provided:
cfg, ns, err := NamedServiceAccountImportConfigAndNamespace("cluster2-default-pod-lister")
// ...
clientset, err := kubernetes.NewForConfig(cfg)
// ...
pods, err := clientset.CoreV1().Pods(ns).List(metav1.ListOptions{})
Usually, however, you don't want to hardcode the name of the mounted service account import. If you only expect one, you can get a Config for it and its remote namespace like this:
cfg, ns, err := ServiceAccountImportConfigAndNamespace()
If several service account imports are mounted, you can get Configs and namespaces for all of them by name as a map[string]ConfigAndNamespaceTuple
:
all, err := AllServiceAccountImportConfigsAndNamespaces()
// ...
for name, cfgAndNs := range all {
cfg := cfgAndNs.Config
ns := cfgAndNs.Namespace
// ...
}
Generic Client Configuration
The true power of multicluster-service-account's config
package is in its generic functions, that can fall back to kubeconfig contexts or regular service accounts when no service account import is mounted:
cfg, ns, err := ConfigAndNamespace()
all, err := AllNamedConfigsAndNamespaces()
The service account import controller uses AllNamedConfigsAndNamespaces()
internally. The generic client example uses ConfigAndNamespace()
.
API Reference
For more details on the config
package, or to better understand how the service account import controller and admission control work, please refer to the API documentation:
https://godoc.org/admiralty.io/multicluster-service-account/
or
go get admiralty.io/multicluster-service-account
godoc -http=:6060
then http://localhost:6060/pkg/admiralty.io/multicluster-service-account/