[lambda] 'valueAsString' works well with lazy resolution with lambda environment variables but the type 'Number' does not.

[knihit]

I am using CfnParameter to read various parameters. One of the parameter types is a 'Number' with min value 1 and max value as 100. This value of this parameter is then passed as the lambda environment key value pairs. When I use 'valueAsNumber' method in conjunction with the lambda environment variables, cdk synth does not synthesize this code for lazy resolution through 'Ref'. It rather assigns a value to the lambda environment variable during synth process. 'valueAsString' works well with lazy resolution with lambda environment variables but the type 'Number' does not.

Reproduction Steps

    const numberOfTopics = new CfnParameter(this, 'NumberOfTopics', {
        type: 'Number',
        default: 10,
        minValue: 1,
        maxValue: 100,
        description: 'The number of topics to be discovered by Topic analysis. The min value is 1 and maximum value is 100'
    });

In another section of the code where I read the CfnParameter values

        lambdaFunctionProps: {
            runtime: Runtime.NODEJS_12_X,
            handler: 'index.handler',
            code: Code.fromAsset(`${__dirname}/../../lambda/wf-submit-topic-model`),
            environment: {
                INGESTION_WINDOW: props.ingestionWindow,
                RAW_BUCKET_FEED: props.rawBucket.bucketName,
                **NUMBER_OF_TOPICS: props.numberofTopics,**
                STACK_NAME: props.stackName
            },
            timeout: Duration.minutes(5),
            memorySize: 512
        },

What did you expect to happen?

    "NUMBER_OF_TOPICS": {
      "Ref": "NumberOfTopics"
    },

What actually happened?

Environment

• CLI Version: aws-cli/2.0.44 Python/3.8.5 Darwin/19.6.0 source/x86_64
• Framework Version: 1.62.0
• Node.js Version: 14.9.0
• OS: MacOS
• Language (Version): 4.0.2

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This is 🐛 Bug Report

[rix0rrr]

It's because stringification happens in TypeScript without regard to whether it's a Token or not.

Should have been Tokenization.stringify().

[knihit]

@rix0rrr, can you please help me understand how should I be reading CfnParameter Number types for lambda functions, is there another way to read them. As a workaround, I am using the 'valueAsString' to get the parameters.

[rix0rrr]

Yes that will do it.

[nija-at]

What language are you using here? typescript or javascript?

The type for the environment property is { [key: string]: string }, however numberOfTopics is of type CfnParameter. So typescript should fail at the compile step if you do the assignment you've claimed as problematic.

If you're on javascript, I believe it's up to you to ensure that you're performing the correct assignments.

As noted, using valueAsString() will get you the string representation of the token

[knihit]

The language for the runtime is typescript.

@nija-at agree that the lambda environment variable is a string. Traditionally if I were to use CFN directly, I can define the parameter as a Number and have it Ref into the Lambda environment variables. This way I get appropriate parameter validation. In CDK if I can still use it as 'Number' and then be able to Ref the variable when assigning to Lambda still works fine. So there are two problems that I see

1. Having CfnParameter as a Number does not synthesize into the json template. So I cannot do valueAsNumber().toString() when assigning it to lambda environment variables
2. If I use CfnParameter as a String, it does synthesize correctly and I can use it as valueAsString() but I need to add a regex instead of having to provide a min/ max value. So I lose the capability to perform appropriate validation. The code where I have the problem is here. https://github.com/awslabs/discovering-hot-topics-using-machine-learning/blob/master/source/lib/discovering-hot-topics-stack.ts (refer line number: 63).

[nija-at]

Thanks for the explanation @knihit. I see the problem.

[nija-at]

You can workaround this using Token.asString(numberOfTopics.value)