Docs: Add Multi-cluster Service doc
Signed-off-by: changzhen <changzhen5@huawei.com>
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# Multi-cluster Service Discovery
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Users are able to **export** and **import** services between clusters with [Multi-cluster Service APIs](https://github.com/kubernetes-sigs/mcs-api).
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## Prerequisites
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### Karmada has been installed
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We can install Karmada by referring to [quick-start](https://github.com/karmada-io/karmada#quick-start), or directly run `hack/karmada-bootstrap.sh` script which is also used to run our E2E cases.
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### Member Cluster Network
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Ensure that at least two clusters have been added to Karmada, and the container networks between member clusters are connected.
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- If you use the `hack/karmada-bootstrap.sh` script to deploy Karmada, Karmada will have three member clusters, and the container networks of the `member1` and `member2` will be connected.
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- If you use `Kind` tool to create member clusters, you can refer to the [hack/util.sh](https://github.com/karmada-io/karmada/blob/af5f544cbe68d6b37a730b42dcc1ead0fac16915/hack/util.sh#L452-L470) script to enable the container networks.
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### The ServiceExport and ServiceImport CRDs have been installed
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We need to install ServiceExport and ServiceImport in the member clusters.
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After ServiceExport and ServiceImport have been installed on the **karmada control-plane**, we can create `ClusterPropagationPolicy` to propagate those two CRDs to the member clusters.
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```yaml
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# propagate ServiceExport CRD
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apiVersion: policy.karmada.io/v1alpha1
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kind: ClusterPropagationPolicy
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metadata:
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name: serviceexport-policy
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spec:
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resourceSelectors:
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- apiVersion: apiextensions.k8s.io/v1
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kind: CustomResourceDefinition
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name: serviceexports.multicluster.x-k8s.io
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placement:
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clusterAffinity:
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clusterNames:
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- member1
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- member2
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---
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# propagate ServiceImport CRD
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apiVersion: policy.karmada.io/v1alpha1
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kind: ClusterPropagationPolicy
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metadata:
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name: serviceimport-policy
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spec:
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resourceSelectors:
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- apiVersion: apiextensions.k8s.io/v1
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kind: CustomResourceDefinition
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name: serviceimports.multicluster.x-k8s.io
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placement:
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clusterAffinity:
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clusterNames:
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- member1
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- member2
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```
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## Example
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### Step 1: Deploy service on the `member1` cluster
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We need to deploy service on the `member1` cluster for discovery.
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```yaml
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apiVersion: apps/v1
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kind: Deployment
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metadata:
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name: serve
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spec:
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replicas: 1
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selector:
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matchLabels:
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app: serve
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template:
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metadata:
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labels:
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app: serve
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spec:
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containers:
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- name: serve
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image: jeremyot/serve:0a40de8
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args:
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- "--message='hello from cluster 1 (Node: {{env \"NODE_NAME\"}} Pod: {{env \"POD_NAME\"}} Address: {{addr}})'"
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env:
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- name: NODE_NAME
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valueFrom:
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fieldRef:
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fieldPath: spec.nodeName
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- name: POD_NAME
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valueFrom:
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fieldRef:
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fieldPath: metadata.name
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---
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apiVersion: v1
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kind: Service
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metadata:
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name: serve
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spec:
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ports:
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- port: 80
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targetPort: 8080
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selector:
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app: serve
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---
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apiVersion: policy.karmada.io/v1alpha1
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kind: PropagationPolicy
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metadata:
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name: mcs-workload
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spec:
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resourceSelectors:
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- apiVersion: apps/v1
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kind: Deployment
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name: serve
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- apiVersion: v1
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kind: Service
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name: serve
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placement:
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clusterAffinity:
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clusterNames:
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- member1
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```
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### Step 2: Export service to the `member2` cluster
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- Create a `ServiceExport` object on **karmada control-plane**, and then create a `PropagationPolicy` to propagate the `ServiceExport` object to the `member1` cluster.
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```yaml
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apiVersion: multicluster.x-k8s.io/v1alpha1
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kind: ServiceExport
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metadata:
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name: serve
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---
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apiVersion: policy.karmada.io/v1alpha1
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kind: PropagationPolicy
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metadata:
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name: serve-export-policy
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spec:
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resourceSelectors:
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- apiVersion: multicluster.x-k8s.io/v1alpha1
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kind: ServiceExport
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name: serve
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placement:
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clusterAffinity:
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clusterNames:
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- member1
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```
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- Create a `ServiceImport` object on **karmada control-plane**, and then create a `PropagationPlicy` to propagate the `ServiceImport` object to the `member2` cluster.
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```yaml
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apiVersion: multicluster.x-k8s.io/v1alpha1
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kind: ServiceImport
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metadata:
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name: serve
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spec:
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type: ClusterSetIP
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ports:
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- port: 80
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protocol: TCP
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---
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apiVersion: policy.karmada.io/v1alpha1
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kind: PropagationPolicy
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metadata:
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name: serve-import-policy
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spec:
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resourceSelectors:
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- apiVersion: multicluster.x-k8s.io/v1alpha1
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kind: ServiceImport
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name: serve
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placement:
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clusterAffinity:
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clusterNames:
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- member2
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```
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### Step 3: Consume service from `member2` cluster
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After the above steps, we can find the **derived service** which has the prefix `derived-` on the `member2` cluster. Then, we can access the **derived service** to access the service on the `member1` cluster.
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Start a Pod `request` on the `member2` cluster to access the ClusterIP of **derived service**:
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```
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kubectl run -i --rm --restart=Never --image=jeremyot/request:0a40de8 request -- --duration={duration-time} --address={ClusterIP of derived service}
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```
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