Doc on multiple gateway(s) and HTTPRoute associate to multiple gateways

docs/multi-sn.md

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+# Multiple Gateways/Service Networks and HTTPRoute Associates to Multiple Gateways
+
+## Multiple Gateways/Service Networks Usecase
+
+Here is one popular multi-cluster architecture:
+
+* config cluster, where is used for configuration management
+* multiple work-load cluster(s), where are used to run application workload(s)
+
+You can see a production usecase at AirBnb [airbnb mullti-cluster](https://www.youtube.com/watch?v=1D8lg36ZNHs)
+
+Here is our example
+
+![Config Cluster and multiple workload cluster](images/multi-sn.png)
+
+* there are 2 gateway(s), gateway-1/lattice-service-network-1 and gateway-2/lattice-service-network-2
+* gateway-1 contains HTTPRoute1 and HTTPRoute2
+* gateway-2 contains HTTPRoute2 and HTTPRoute3
+* blue workload cluster(s) are using gateway-1 to access HTTPRoute1 an HTTPRoute2
+* orange workload cluster(s) are using gateway-2 to access HTTPRoute2 an HTTPRoute3
+
+###  Config Cluster Gateway Configuration
+
+```
+# gateway-1
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: Gateway
+metadata:
+  name: gateway-1
+  annotations:
+    application-networking.k8s.aws/lattice-vpc-association: "false"  <-----> not associate to VPC
+spec:
+  gatewayClassName: amazon-vpc-lattice
+  listeners:
+  ...
+```  
+
+```
+# gateway-2
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: Gateway
+metadata:
+  name: gateway-2
+  annotations:
+    application-networking.k8s.aws/lattice-vpc-association: "false"  <-----> not associate to VPC
+spec:
+  gatewayClassName: amazon-vpc-lattice
+  listeners:
+  ...
+```  
+
+```
+# httproute-1
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: HTTPRoute
+metadata:
+  name: httproute-1
+spec:
+  parentRefs:
+  - name: gateway-1  <---> part of gateway-1/service-network-1
+  ...
+```  
+
+```
+# httproute-2
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: HTTPRoute
+metadata:
+  name: httproute-2
+spec:
+  parentRefs:
+  - name: gateway-1     <------> part of both gateway-1 and gateway-2 
+    sectionName: http
+  - name: gateway-2
+    sectionName: http
+  ...    
+  ```
+
+  ```
+# httproute-3
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: HTTPRoute
+metadata:
+  name: httproute-3
+spec:
+  parentRefs:
+  - name: gateway-2  <---> part of gateway-2/service-network-2
+  ...
+```
+
+### blue workload cluster(s)
+Associate cluster's VPC to gateway-1/service-network-1 so that all Pod(s) in blue workload clusters can access HTTPRoute(s)of gateway-1, HTTPRoute-1 and HTTPRoute-2
+
+```
+# gateway-1
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: Gateway
+metadata:
+  name: gateway-1
+  annotations:
+    application-networking.k8s.aws/lattice-vpc-association: "true"  <----->  associate to VPC
+spec:
+  gatewayClassName: amazon-vpc-lattice
+  listeners:
+  ...
+```  
+
+### orange workload cluster(s)
+Associate cluster's VPC to gateway-2/service-network-2, so that all Pod(s) in orange workload clusters can access HTTPRoute(s) of gateway-2, HTTPRoute-2 an HTTPRoute-3
+
+```
+# gateway-2
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: Gateway
+metadata:
+  name: gateway-2
+  annotations:
+    application-networking.k8s.aws/lattice-vpc-association: "true"  <----->  associate to VPC
+spec:
+  gatewayClassName: amazon-vpc-lattice
+  listeners:
+  ...
+```  
+
+## Defining HTTPRoute in Config Cluster
+
+![ServiceImport](images/serviceimport.png)
+
+### Exporting Kubernetes Service to AWS Lattice Service
+
+In workload cluster, exports k8s service to AWS VPC lattice
+
+```
+# in workload cluster(s)
+apiVersion: multicluster.x-k8s.io/v1alpha1
+kind: ServiceExport
+metadata:
+  name: service-1
+  annotations:
+          multicluster.x-k8s.io/federation: "amazon-vpc-lattice"  <----------->  AWS VPC Lattice
+``` 
+
+### Configure HTTPRoute in config cluster to reference K8S service(s) in worload cluster(s)
+
+```
+# in config cluster
+apiVersion: multicluster.x-k8s.io/v1alpha1
+kind: ServiceImport
+metadata:
+  name: service-1
+spec:
+  type: ClusterSetIP
+  ports:
+  - port: 80
+    protocol: TCP
+```
+
+```
+# httproute 
+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: HTTPRoute
+metadata:
+  name: httproute-1
+spec:
+  parentRefs:
+  - name: gateway-1
+    sectionName: http 
+  rules:
+  - backendRefs:  
+    - name: service-1
+      kind: ServiceImport
+      weight: 25
+    - name: service-2
+      kind: ServiceImport
+      weight: 25  
+    - name: service-3
+      kind: ServiceImport
+      weight: 25
+    - name: service-4
+      kind: ServiceImport
+      weight: 25    
+```      
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