Use the CLI to configure MCS to access Services across Kubernetes clusters - Container Service for Kubernetes

The multi-cluster Services (MCS) feature allows you to access Services across Kubernetes clusters without the need to create load balancers. This topic describes how to create a ServiceExport and ServiceImport on a Fleet instance to configure MCS in order to access Services across Kubernetes clusters.

Prerequisites

The Fleet management feature is enabled. For more information, see Enable multi-cluster management.
Two clusters (the service provider cluster and service consumer cluster) are associated with the Fleet instance. For more information, see Associate clusters with a Fleet instance.
Kubernetes versions of the associated clusters are 1.22 or later.
The kubeconfig files of the provider and consumer clusters are obtained and kubectl is used to connect to the clusters. For more information, see Obtain the kubeconfig file of a cluster and use kubectl to connect to the cluster.

Overview

The MCS feature allows you to access Services across Kubernetes clusters. The following figure shows the architecture of the MCS feature.

多集群服务

The administrator deployed application resources, such as namespaces, Deployments, and Services, in a Service provider cluster (ACK Cluster 1) and a Service consumer cluster (ACK Cluster 2). The administrator also created MCS resources, including a ServiceExport and a ServiceImport.
The Fleet instance listens on the ServiceExport and ServiceImport in the associated ACK clusters, and synchronizes the MCS endpoint information.
The client pod in ACK Cluster 2 can access Service 1 in ACK Cluster 1.

Step 1: Create resources for the Service provider

Skip this step if Kubernetes Services and resources are already deployed in ACK Cluster 1 where the Service provider resides.

Run the following command in ACK Cluster 1 to create a namespace for the Service provider.
In this example, the provider-ns namespace is created.
```
kubectl create ns provider-ns
```

Create a Kubernetes Service and a Deployment for the Service provider in ACK Cluster 1.

Create a file named app-meta.yaml and add the following content to the file:

apiVersion: v1       # The Service provider. 
kind: Service
metadata:
  name: service1
  namespace: provider-ns
spec:
  ports:
  - name: http
    port: 80
    protocol: TCP
    targetPort: 8080
  selector:
    app: web-demo
  sessionAffinity: None
  type: ClusterIP
---
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: web-demo
  name: web-demo
  namespace: provider-ns
spec:
  replicas: 2
  selector:
    matchLabels:
      app: web-demo
  template:
    metadata:
      labels:
        app: web-demo
    spec:
      containers:
      - image: acr-multiple-clusters-registry.cn-hangzhou.cr.aliyuncs.com/ack-multiple-clusters/web-demo:0.4.0
        name: web-demo
        env:
        - name: ENV_NAME
          value: cluster1-beijing

Run the following command to create resources in ACK Cluster 1:
```
kubectl apply -f app-meta.yaml
```
Run the following command to create a ServiceExport in ACK Cluster 1 and specify the Service to be exported through a multi-cluster Service:
```
apiVersion: multicluster.x-k8s.io/v1alpha1
kind: ServiceExport
metadata:
  name: service1           # The value must be the same as the name of the Service that you want to export. 
  namespace: provider-ns   # The value must be the same as the namespace of the Service that you want to export.
```
The following table describes the parameters.
Parameter
Description
metadata.name
The name of the Service that you want to export.
metadata.namespace
The namespace of the Service that you want to export.

Step 2: Configure the Service consumer cluster

Run the following command in ACK Cluster 2 to create a namespace for the Service consumer.
In this example, the provider-ns namespace is created.
```
kubectl create ns provider-ns
```

Create a file and add the following content to the file to create a ServiceImport in ACK Cluster 2:

apiVersion: multicluster.x-k8s.io/v1alpha1
kind: ServiceImport
metadata:
  name: service1           
  namespace: provider-ns   
spec:
  ports:                   # The values in this field must be the same as the ports used by the Service that you want to export. 
  - name: http
    port: 80
    protocol: TCP
  type: ClusterSetIP

The spec section of the ServiceImport displays only partial parameters. The following table describes all parameters in the spec section.

Parameter	Description
metadata.name	The Service name. The name must be the same as that of the Service you want to export.
metadata.namespace	The namespace of the Service. The namespace must be the same as that of the Service you want to export.
spec. ports. name	The name of the port. The value must be the same as that of the Service you want to export.
spec. ports. protocol	The protocol. The value must be the same as that of the Service you want to export.
spec. ports. appProtocol	The application protocol. The value must be the same as that of the Service you want to export.
spec. ports. port	The port. The value must be the same as that of the Service you want to export.
spec. ips	The virtual IP address of the Service. This parameter is specified by the Fleet instance. You can ignore this parameter.
spec. type	Valid values: `ClusterSetIP` and `Headless`. When the `ClusterIP` parameter of the Service is set to `None`, the Service is a headless Service. In this scenario, set `type` to `Headless`. In other scenarios, set `type` to `ClusterSetIP`.
spec. sessionAffinity	Session affinity. Valid values: `ClientIP` and `None`. The value must be the same as that of the Service you want to export.
spec. sessionAffinityConfig	The session affinity configuration. The value must be the same as that of the Service you want to export.

Step 3: Access the Service across clusters

The client pod in ACK Cluster 2 can use one of the following methods to access Service 1 in ACK Cluster 1.

Method 1: Use the name of the Service

After you create a ServiceImport in ACK Cluster 2, the Fleet instance creates a multi-cluster Service prefixed with amcs- in ACK Cluster 2.

The client pod in ACK Cluster 2 can use amcs-service1.provider-ns to access Service 1 in ACK Cluster 1.

Run the following command in ACK Cluster 2 to query the Service information:

kubectl get service -n provider-ns

Expected output:

NAME            TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)   AGE
amcs-service1   ClusterIP   172.xx.xx.xx   <none>        80/TCP    26m

Run the following command in the client pod in ACK Cluster 2 to access Service 1 in ACK Cluster 1.
```
curl amcs-service1.provider-ns
```

Method 2: Use the domain name of the Service

Install or update CoreDNS in ACK Cluster 2. The version of CoreDNS must be 1.9.3 or later. For more information, see CoreDNS and Manage components.

Modify the Corefile configurations of CoreDNS.

Run the following command to modify the ConfigMap of CoreDNS:
```
kubectl edit configmap coredns -n kube-system
```

In the Corefile field, add multicluster clusterset.local to enable domain name resolution for multi-cluster Services.

apiVersion: v1
data:
  Corefile: |
    .:53 {
        errors
        health {
           lameduck 15s
        }
        ready
        multicluster clusterset.local    # Add this configuration to enable domain name resolution for multi-cluster Services. 
        kubernetes cluster.local in-addr.arpa ip6.arpa {

          pods verified
          ttl 30
          fallthrough in-addr.arpa ip6.arpa
        }
        ...
    }
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system

Run the following command in the client pod in ACK Cluster 2 to access Service 1 in ACK Cluster 1:
```
curl service1.provider-ns.svc.clusterset.local
```

(Optional) Modify the dnsConfig.searches field of the client pod in ACK Cluster 2 to access the multi-cluster Service.

Add clusterset.local to the dnsConfig.searches field of the client pod.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: client-pod
  namespace: consumer-ns
spec:
  ...
  template:
    ...
    spec:
      dnsPolicy: "ClusterFirst"    
      dnsConfig:                   
        searches:                        #dnsConfig. Add clusterset.local. 
          - svc.clusterset.local
          - clusterset.local
          - consumer-ns.svc.cluster.local
          - svc.cluster.local
          - cluster.local
      containers:
      - name: client-pod
        ...

Run the following command to access the multi-cluster Service:
```
curl service1.provider-ns
```

References

You can also configure MCS in the console. For more information, see Use MCS in the ACK One console.