nteg-tests-alpha: assert response from invokeFunction call

[shanemclawslabgroup]

Describe the bug

I am trying to write some CDK integration tests, making use of the @aws-cdk/integ-tests-alpha package. In these tests, I have created a custom lambda that allows us to perform HTTP calls to health check endpoints for some resources that are only accessible within the VPC.

However, I am having issues getting the integ-tests-alpha framework to parse the response from the lambda correctly. Note that each of the below examples successfully run the cloudformtion deployment/tests, but the test stack output always shows a failed test, seemingly due to how the result from the lambda is passed back to cloudformation.

I have confirmed that the lambda is deployed and is returning a response along the lines of what I'm expecting, which looks like:

{
  "statusCode": 200,
  "body": {
    "message": "Success",
    "data": {
      ...
    }
  }
}

I have tried a number of ways to parse this response using the provided mothods that are available to the invokeFunction() command in the integ-tests-alpha framework, but they all return some sort of error. Below is what I've tried and the return results:

Attempt 1

const lambdaInvoke = integ.assertions.invokeFunction(...)
lambdaInvoke.expect(ExpectedResult.objectLike({
    Payload: {
        statusCode: 200,
        body: {
            message: "success"
        }
    }
}))

results from cloudformation output:
{"status":"fail","message":"{\n  \"ExecutedVersion\": \"$LATEST\",\n!! Expected type object but received string\n  \"Payload\": \"{\\\"statusCode\\\": 200, \\\"body\\\": {\\\"message\\\": \\\"Success\\\", \\\"data\\\": {...}}"

Attempt 2:

const lambdaInvoke = integ.assertions.invokeFunction(...)
lambdaInvoke.expect(ExpectedResult.stringLikeRegexp('"statusCode":200'))

results from cloudformation output:
{"status":"fail","message":"!! Expected a string, but got 'object'\n!! String '[object Object]' did not match pattern '\"statusCode\":200'\n{\n  \"ExecutedVersion\": \"$LATEST\",\n  \"Payload\": \"{\\\"statusCode\\\": 200, \\\"body\\\": {\\\"message\\\": \\\"Success\\\", \\\"data\\\": {...}}"

I have also tried both:

const lambdaInvoke = integ.assertions.invokeFunction(...)
.assertAtPath('message', ExpectedResult.objectLike({
    Payload: {
        statusCode: 200,
        body: {
            message: "success"
        }
    }
}))

returns: "message":"!! Expected type object but received undefined\nundefined"

and

const lambdaInvoke = integ.assertions.invokeFunction(...)
.assertAtPath('message', ExpectedResult.stringLikeRegexp('"statusCode":200'))

 returns  "message":"!! Expected a string, but got 'undefined'\n!! String 'undefined' did not match pattern '\"statusCode\":200'\nundefined"

I have also tried both of the above .assertAtPath() commands with the 'message.Payload' path, but I get the same results.

It appears that there is not a method in the ExpectedResult class that can parse the payload that is returned by the .invokeFunction() call.

The documentation does not seem to elaborate in any way on how to parse the returned result from the lambdaInvoke function. There seems to be missing examples on the lambdaInvoke assertAtPath documentation which looks like it should be giving an example

Regression Issue

• Select this option if this issue appears to be a regression.

Last Known Working CDK Library Version

No response

Expected Behavior

Methods implemented by the ExpectedResult class should be able to parse the payload returned by the .invokeFunction() assertion.

Current Behavior

Methods implemented by the ExpectedResult class cannot parse the payload returned by the .invokeFunction() assertion. The methods appear to require a strict format to be returned by the underlying API call that the .invokeFunction() call does not adhere to

Reproduction Steps

Create a Lambda function that returns an arbitrary response object.

Create a new CDK integration test stack using the @aws-cdk/integ-tests-alpha package

Call the Lambda function using the invokeFunction() method:

const lambdaInvoke = integ.assertions.invokeFunction({
    functionName: <<your-function-name>>,
    invocationType: InvocationType.REQUEST_RESPONSE
})

Expect some result back:

lambdaInvoke.expect(ExpectedResult.objectLike({
    <<your-lambda-return-object>>
}))

Possible Solution

No response

Additional Information/Context

No response

AWS CDK Library version (aws-cdk-lib)

aws-cdk-lib@2.200.1

AWS CDK CLI version

2.1019.1 (build d758c9f)

Node.js Version

v23.10.0

OS

Windows 10 22H2

Language

TypeScript

Language Version

No response

Other information

No response

[pahud]

Hi @shanemclawslabgroup,

Thank you for reporting this issue with the @aws-cdk/integ-tests-alpha package. You've identified a legitimate problem with how Lambda invocation responses are handled in the assertion framework.

The root cause is that when you invoke a Lambda function using the AWS SDK, the response Payload field is returned as a JSON string, but the ExpectedResult methods expect it to be a parsed object. The current implementation doesn't automatically parse Lambda payloads, leading to the type mismatches you're experiencing.

Workaround for now:
You might try using assertAtPath with the path 'Payload' and ExpectedResult.stringLikeRegexp() to match against the JSON string directly, though this isn't ideal.

Proper Solution Needed:
This issue requires enhancing the AwsApiCallHandler class to detect Lambda invocation responses and automatically parse the Payload field from JSON string to object, making it compatible with ExpectedResult.objectLike() and other assertion methods.

This is a valid enhancement request that would improve the developer experience for integration testing with Lambda functions. Would you be interested in contributing a PR to address this? The fix would involve:

1. Detecting when request.service === 'Lambda' and request.api === 'invoke'
2. Parsing the response.Payload field if it's a string
3. Adding proper test coverage for Lambda invocation assertions

The implementation would be in the processEvent method of the AwsApiCallHandler class.

[shanemclawslabgroup]

@pahud Thanks for your input and triaging

I did manage to get the workaround using ExpectedResult.stringLikeRegexp() working with this:

lambdaInvoke.expect(ExpectedResult.objectLike({
    Payload: Match.stringLikeRegexp('"statusCode":\\s*200')
}));

It turned out that I had imported Match from the aws-cdk-lib/assertions package, and not from aws-cdk-lib/integ-test-alpha, and that was resulting in the parsing to fail.