(assertions): Can't do snapshot testing of Stacks using the same App

[ncaq]

What is the problem?

I migrate AWS CDK v1 to v2. And I use assertion from assert because assert is deprecated.

My project have snapshot test. When I rewrote this into assertions, I received an error that was difficult to resolve. I thought that the complicated configuration I had done so far was bad, but it was reproduced when I created a new AWS CDK project and wrote test code.

Reproduction Steps

mkdir foo
cd foo
cdk init app --language typescript
emacsclient test/snapshot.test.ts

paste.

import { App, Stack } from "aws-cdk-lib";
import { Template } from "aws-cdk-lib/assertions";

test("snapshot", () => {
  const app = new App();

  const fooStack = new Stack(app, "FooStack");
  const fooTemplate = Template.fromStack(fooStack);
  expect(fooTemplate.toJSON()).toMatchSnapshot("FooStack");

  const barStack = new Stack(app, "BarStack");
  const barTemplate = Template.fromStack(barStack);
  expect(barTemplate.toJSON()).toMatchSnapshot("BarStack");
});

npm run test

What did you expect to happen?

This is a snapshot test, so we expect it to succeed unconditionally the first time.

What actually happened?

> foo@0.1.0 test
> jest

 PASS  test/foo.test.ts
 FAIL  test/snapshot.test.ts (7.841 s)
  ● snapshot

    Unable to find artifact with id "BarStack"

      10 |
      11 |   const barStack = new Stack(app, "BarStack");
    > 12 |   const barTemplate = Template.fromStack(barStack);
         |                                ^
      13 |   expect(barTemplate.toJSON()).toMatchSnapshot("BarStack");
      14 | });
      15 |

      at CloudAssembly.getStackArtifact (node_modules/aws-cdk-lib/cx-api/lib/cloud-assembly.ts:79:31)
      at toTemplate (node_modules/aws-cdk-lib/assertions/lib/template.ts:150:19)
      at Function.fromStack (node_modules/aws-cdk-lib/assertions/lib/template.ts:28:17)
      at Object.<anonymous> (test/snapshot.test.ts:12:32)

 › 1 snapshot written.
Snapshot Summary
 › 1 snapshot written from 1 test suite.

Test Suites: 1 failed, 1 passed, 2 total
Tests:       1 failed, 1 passed, 2 total
Snapshots:   1 written, 1 total
Time:        8.175 s
Ran all test suites.

CDK CLI Version

2.10.0 (build e5b301f)

Framework Version

No response

Node.js Version

v16.13.1

OS

Linux strawberry 5.15.19-gentoo #1 SMP Sat Feb 5 10:10:29 JST 2022 x86_64 AMD Ryzen Threadripper 1950X 16-Core Processor AuthenticAMD GNU/Linux

Language

Typescript

Language Version

3.9.7

Other information

relation?

• (assertions): support snapshot testing of the cloud assembly · Issue #16343 · aws/aws-cdk
• cdk 2.1.0 synth, diff subcommands fail with "AssertDescription: CDK bootstrap stack version 6 required...". cdk 2.1.0 deploy subcommand succeeds. · Issue #17942 · aws/aws-cdk

[ncaq]

The reason why I use the same App all the time is that in the actual production code, I create a stack of ECR repositories and reference the fields to the stack of ECS repositories. When I did this with a new App, I got a different error.

[amirmishani]

@ncaq did you ever figure this out? I've run into the same issue.

[ncaq]

@amirmishani It has not been resolved. I am using the previous deprecated package for now.

[anvuori]

Try creating both stacks first, and only after that create the Template objects and assertions. Somehow fixed things for me, still no idea why 🤷

[jamestelfer]

When Template.toStack() is called, it calls synth() on the underlying App instance.

synth() caches the result of the synthesis (note the if (!this.assembly on L182. (Note that App extends Stage.)

https://github.com/aws/aws-cdk/blob/bd056d1d38a2d3f43efe4f857c4d38b30fb9b681/packages/%40aws-cdk/core/lib/stage.ts#L181-L190

So when you reuse the App instance and call Template.toStack() multiple times, you'll only get the first result, which will not have any stacks that you've subsequently added. This behaviour could possibly change, but it's likely to regress test performance negatively when fromStack() is called repeatedly.

The answer to this is:

1. Don't share App instances across tests. This probably applies in the current case: there are two Stacks, use separate tests with different App instances.
2. OR if they're in the same test, arrange your tests in the arrange/act/assert style, setting up the app and both stack instances first, then running assertions later.

[jamestelfer]

So in the current example, I suggest doing something like (comments optional):

import { App, Stack } from "aws-cdk-lib";
import { Template } from "aws-cdk-lib/assertions";

test("snapshot foo", () => {
  // arrange
  const app = new App();

  // act
  const fooStack = new Stack(app, "FooStack");
  const fooTemplate = Template.fromStack(fooStack);

  // assert
  expect(fooTemplate.toJSON()).toMatchSnapshot("FooStack");
});

test("snapshot bar", () => {
  const app = new App();

  const barStack = new Stack(app, "BarStack");
  const barTemplate = Template.fromStack(barStack);

  expect(barTemplate.toJSON()).toMatchSnapshot("BarStack");
});