[appmesh] VirtualNode in separate stack from Mesh creates circular dependency

Adding a VirtualNode to a Mesh (where the Mesh is in one stack and the VirtualNode is in another) creates an circular reference, but shouldn’t.

Reproduction Steps

Sample code available at buzzsurfr/appmesh-cross-stack-bug

Sample code in master branch works, but uncommenting the following lines…

https://github.com/buzzsurfr/appmesh-cross-stack-bug/blob/e374561e516c328ed90de2add287b2d81153a642/lib/meshed-service-stack.ts#L103-L112

…then run:

npm run build
cdk ls

What did you expect to happen?

This shouldn’t be a circular reference and should work.

What actually happened?

/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/stack.ts:688
      throw new Error(`'${target.node.path}' depends on '${this.node.path}' (${cycle.join(', ')}). Adding this dependency (${reason}) would create a cyclic reference.`);
            ^
Error: 'InfrastructureStack' depends on 'MeshedServiceStack' (InfrastructureStack -> MeshedServiceStack/service/CloudmapService/Resource.Name). Adding this dependency (MeshedServiceStack -> InfrastructureStack/Mesh/Resource.MeshName) would create a cyclic reference.
    at MeshedServiceStack._addAssemblyDependency (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/stack.ts:688:13)
    at Object.addDependency (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/deps.ts:52:20)
    at MeshedServiceStack.addDependency (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/stack.ts:445:5)
    at resolveValue (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/private/refs.ts:101:12)
    at Object.resolveReferences (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/private/refs.ts:31:24)
    at Object.prepareApp (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/private/prepare-app.ts:36:5)
    at Object.synthesize (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/private/synthesis.ts:21:3)
    at App.synth (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/stage.ts:175:23)
    at process.<anonymous> (/Users/username/Sandbox/cdk/appmesh-cross-stack-bug/node_modules/@aws-cdk/core/lib/app.ts:112:45)
    at Object.onceWrapper (events.js:422:26)
Subprocess exited with error 1

Environment

• CLI Version : 1.61.1 (build 347918f)
• Framework Version: ^1.61.1
• Node.js Version: v13.12.0
• OS : macOS Catalina 10.15.6
• Language (Version): TypeScript (3.9.7)

Other

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This is 🐛 Bug Report

[Seiya6329]

I think the only possible action from this might be adding some clarification to the documentation of the various add*() methods in AppMesh that they create a resource in the same Stack as the object they are called on (the Mesh in case of VirtualNode), which might be different than the current Stack.

This is really great point and has been suggested by @dfezzie as well.

Conclusion:

• No actions to change the code
• Update the CDK documentation to clarify the difference of using .add() method and the constructor to create a resource.

[skinny85]

I think I agree with your conclusions @Seiya6329. Looks like I was worried about that new VirtualNode(... case for nothing 🙂.

This looks to be just a matter of various references between CDK constructs causing dependencies between Stacks. I don’t think there’s anything unusual here for AppMesh.

For the problem posted originally in the issue, it’s just a matter of changing from:

        const virtualNode = props.mesh.addVirtualNode('vn', {
            serviceDiscovery: appmesh.ServiceDiscovery.cloudMap({
                service: service.cloudMapService!,
            }),
            listeners: [appmesh.VirtualNodeListener.http({
                port: appContainer.containerPort,
            })],
        });

to:

        const virtualNode = new appmesh.VirtualNode(this, 'vn', {
            mesh: props.mesh,
            serviceDiscovery: appmesh.ServiceDiscovery.cloudMap({
                service: service.cloudMapService!,
            }),
            listeners: [appmesh.VirtualNodeListener.http({
                port: appContainer.containerPort,
            })],
        });

to solve the cycle.

I think the only possible action from this might be adding some clarification to the documentation of the various add*() methods in AppMesh that they create a resource in the same Stack as the object they are called on (the Mesh in case of VirtualNode), which might be different than the current Stack. But I think that 's as far as any code changes are needed here.