fix(appmesh): allow a Virtual Node have as a backend a Virtual Service whose provider is that Node (#18265)

Addresses a circular dependency issue between Virtual Nodes and Virtual Services that works for Virtual Services created with a defined `virtualServiceName` and a randomly generated name. 

One such example of this problem was a case where a Virtual Node had a backend that is a Virtual Service whose provider was given as the same Virtual Node. This led to the Virtual Node being dependent on the creation of the Virtual Service, and the Virtual Service being dependent on the creation of the Virtual Node.

Fixes #17322

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: wplucinsky

packages/@aws-cdk-containers/ecs-service-extensions/test/integ.all-service-addons.expected.json

@@ -3185,22 +3185,12 @@
           "Backends": [
             {
               "VirtualService": {
-                "VirtualServiceName": {
-                  "Fn::GetAtt": [
-                    "namevirtualservice3DDDDF1E",
-                    "VirtualServiceName"
-                  ]
-                }
+                "VirtualServiceName": "name.production"
               }
             },
             {
               "VirtualService": {
-                "VirtualServiceName": {
-                  "Fn::GetAtt": [
-                    "greetingvirtualservice60AD3AD9",
-                    "VirtualServiceName"
-                  ]
-                }
+                "VirtualServiceName": "greeting.production"
               }
             }
           ],

packages/@aws-cdk/aws-appmesh/README.md

@@ -236,6 +236,24 @@ The `backends` property can be added with `node.addBackend()`. In the example, w
 
 The `backendDefaults` property is added to the node while creating the virtual node. These are the virtual node's default settings for all backends.
 
+The `VirtualNode.addBackend()` method is especially useful if you want to create a circular traffic flow by having a Virtual Service as a backend whose provider is that same Virtual Node:
+
+```ts
+declare const mesh: appmesh.Mesh;
+
+const node = new appmesh.VirtualNode(this, 'node', {
+  mesh,
+  serviceDiscovery: appmesh.ServiceDiscovery.dns('node'),
+});
+
+const virtualService = new appmesh.VirtualService(this, 'service-1', {
+  virtualServiceProvider: appmesh.VirtualServiceProvider.virtualNode(node),
+  virtualServiceName: 'service1.domain.local',
+});
+
+node.addBackend(appmesh.Backend.virtualService(virtualService));
+```
+
 ### Adding TLS to a listener
 
 The `tls` property specifies TLS configuration when creating a listener for a virtual node or a virtual gateway. 

packages/@aws-cdk/aws-appmesh/lib/shared-interfaces.ts

@@ -238,7 +238,14 @@ class VirtualServiceBackend extends Backend {
     return {
       virtualServiceBackend: {
         virtualService: {
-          virtualServiceName: this.virtualService.virtualServiceName,
+          /**
+           * We want to use the name of the Virtual Service here directly instead of
+           * a `{ 'Fn::GetAtt' }` CFN expression. This avoids a circular dependency in
+           * the case where this Virtual Node is the Virtual Service's provider.
+           */
+          virtualServiceName: cdk.Token.isUnresolved(this.virtualService.virtualServiceName)
+            ? (this.virtualService as any).physicalName
+            : this.virtualService.virtualServiceName,
           clientPolicy: this.tlsClientPolicy
             ? {
               tls: renderTlsClientPolicy(scope, this.tlsClientPolicy),

packages/@aws-cdk/aws-appmesh/test/integ.mesh.expected.json

@@ -1040,22 +1040,12 @@
           "Backends": [
             {
               "VirtualService": {
-                "VirtualServiceName": {
-                  "Fn::GetAtt": [
-                    "service6D174F83",
-                    "VirtualServiceName"
-                  ]
-                }
+                "VirtualServiceName": "service1.domain.local"
               }
             },
             {
               "VirtualService": {
-                "VirtualServiceName": {
-                  "Fn::GetAtt": [
-                    "service27C65CF7D",
-                    "VirtualServiceName"
-                  ]
-                }
+                "VirtualServiceName": "service2.domain.local"
               }
             }
           ],
@@ -1111,12 +1101,7 @@
           "Backends": [
             {
               "VirtualService": {
-                "VirtualServiceName": {
-                  "Fn::GetAtt": [
-                    "service3859EB104",
-                    "VirtualServiceName"
-                  ]
-                }
+                "VirtualServiceName": "service3.domain.local"
               }
             }
           ],
@@ -1241,12 +1226,7 @@
           "Backends": [
             {
               "VirtualService": {
-                "VirtualServiceName": {
-                  "Fn::GetAtt": [
-                    "service4983B61EE",
-                    "VirtualServiceName"
-                  ]
-                }
+                "VirtualServiceName": "service4.domain.local"
               }
             }
           ],

packages/@aws-cdk/aws-appmesh/test/mesh.test.ts

@@ -314,9 +314,7 @@ describe('mesh', () => {
               Backends: [
                 {
                   VirtualService: {
-                    VirtualServiceName: {
-                      'Fn::GetAtt': ['service1A48078CF', 'VirtualServiceName'],
-                    },
+                    VirtualServiceName: 'service1.domain.local',
                   },
                 },
               ],

packages/@aws-cdk/aws-appmesh/test/virtual-node.test.ts

@@ -41,24 +41,18 @@ describe('virtual node', () => {
             Backends: [
               {
                 VirtualService: {
-                  VirtualServiceName: {
-                    'Fn::GetAtt': ['service1A48078CF', 'VirtualServiceName'],
-                  },
+                  VirtualServiceName: 'service1.domain.local',
                 },
               },
               {
                 VirtualService: {
-                  VirtualServiceName: {
-                    'Fn::GetAtt': ['service27C65CF7D', 'VirtualServiceName'],
-                  },
+                  VirtualServiceName: 'service2.domain.local',
                 },
               },
             ],
           },
           MeshOwner: ABSENT,
         });
-
-
       });
     });
 
@@ -458,9 +452,7 @@ describe('virtual node', () => {
             Backends: [
               {
                 VirtualService: {
-                  VirtualServiceName: {
-                    'Fn::GetAtt': ['service1A48078CF', 'VirtualServiceName'],
-                  },
+                  VirtualServiceName: 'service1.domain.local',
                   ClientPolicy: {
                     TLS: {
                       Ports: [8080, 8081],
@@ -478,8 +470,75 @@ describe('virtual node', () => {
             ],
           },
         });
+      });
+
+      test('you can add a Virtual Service as a backend to a Virtual Node which is the provider for that Virtual Service', () => {
+        // GIVEN
+        const stack = new cdk.Stack();
+
+        // WHEN
+        const mesh = new appmesh.Mesh(stack, 'mesh', {
+          meshName: 'test-mesh',
+        });
+
+        const node = new appmesh.VirtualNode(stack, 'test-node', {
+          mesh,
+          serviceDiscovery: appmesh.ServiceDiscovery.dns('test'),
+        });
+
+        const myVirtualService = new appmesh.VirtualService(stack, 'service-1', {
+          virtualServiceProvider: appmesh.VirtualServiceProvider.virtualNode(node),
+          virtualServiceName: 'service1.domain.local',
+        });
+
+        node.addBackend(appmesh.Backend.virtualService(myVirtualService));
+
+        // THEN
+        expect(stack).toHaveResourceLike('AWS::AppMesh::VirtualNode', {
+          Spec: {
+            Backends: [
+              {
+                VirtualService: {
+                  VirtualServiceName: 'service1.domain.local',
+                },
+              },
+            ],
+          },
+        });
+      });
+
+      test('you can add a Virtual Service with an automated name as a backend to a Virtual Node which is the provider for that Virtual Service, ', () => {
+        // GIVEN
+        const stack = new cdk.Stack();
 
+        // WHEN
+        const mesh = new appmesh.Mesh(stack, 'mesh', {
+          meshName: 'test-mesh',
+        });
 
+        const node = new appmesh.VirtualNode(stack, 'test-node', {
+          mesh,
+          serviceDiscovery: appmesh.ServiceDiscovery.dns('test'),
+        });
+
+        const myVirtualService = new appmesh.VirtualService(stack, 'service-1', {
+          virtualServiceProvider: appmesh.VirtualServiceProvider.virtualNode(node),
+        });
+
+        node.addBackend(appmesh.Backend.virtualService(myVirtualService));
+
+        // THEN
+        expect(stack).toHaveResourceLike('AWS::AppMesh::VirtualNode', {
+          Spec: {
+            Backends: [
+              {
+                VirtualService: {
+                  VirtualServiceName: 'service1',
+                },
+              },
+            ],
+          },
+        });
       });
     });