docs(appmesh): note that virtual nodes allow at most one listener at this time

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

@@ -208,6 +208,10 @@ export class VirtualNode extends VirtualNodeBase {
    * Utility method to add an inbound listener for this VirtualNode
    */
   public addListener(listener: VirtualNodeListener) {
+    if (this.listeners.length >= 1) {
+      // @see https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-appmesh-virtualnode-virtualnodespec.html#cfn-appmesh-virtualnode-virtualnodespec-listeners
+      throw new Error('Cannot add `VirtualNodeListener`. Virtual Nodes support at most one listener at this time.');
+    }
     this.listeners.push(listener.bind(this));
   }
 

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

@@ -518,6 +518,7 @@ export = {
 
     test.done();
   },
+
   'Can import Virtual Nodes using attributes'(test: Test) {
     // GIVEN
     const stack = new cdk.Stack();
@@ -534,4 +535,42 @@ export = {
 
     test.done();
   },
+
+  'Throws when more than one listener added'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const meshName = 'testMesh';
+
+    // WHEN
+    test.throws(() => {
+      new appmesh.VirtualNode(stack, 'VirtualNode', {
+        mesh: appmesh.Mesh.fromMeshName(stack, 'Mesh', meshName),
+        listeners: [
+          appmesh.VirtualNodeListener.http(),
+          appmesh.VirtualNodeListener.grpc(),
+        ],
+      });
+    }, /at most one listener/i);
+
+    test.done();
+  },
+
+  'Throws when more than one listener added by addListener'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const meshName = 'testMesh';
+    const virtualNode = new appmesh.VirtualNode(stack, 'VirtualNode', {
+      mesh: appmesh.Mesh.fromMeshName(stack, 'Mesh', meshName),
+      listeners: [
+        appmesh.VirtualNodeListener.http(),
+      ],
+    });
+
+    // WHEN
+    test.throws(() => {
+      virtualNode.addListener(appmesh.VirtualNodeListener.grpc());
+    }, /at most one listener/i);
+
+    test.done();
+  },
 };