在 CDK 中跨不同堆栈共享 API 网关端点 URL

Question

I have following AWS CDK backed solution:

1. Static S3 based webpage which communicates with
2. API Gateway which then sends data to
3. AWS lambda.

The problem is that S3 page needs to be aware of API gateway endpoint URL. Obviously this is not achievable within the same CDK stack. So I have defined two stacks:

1. Backend (API gateway + lambda)
2. Frontend (S3 based static webpage)

They are linked as dependant in CDK code:

    const app = new cdk.App();
    const backStack = new BackendStack(app, 'Stack-back', {...});
    new FrontendStack(app, 'Stack-front', {...}).addDependency(backStack, "API URL from backend is needed");

I try to share URL as follows. Code from backend stack definition:

      const api = new apiGW.RestApi(this, 'MyAPI', {
          restApiName: 'My API',
          description: 'This service provides interface towards web app',
          defaultCorsPreflightOptions: {
              allowOrigins: apiGW.Cors.ALL_ORIGINS,
          }
      });
    
      api.root.addMethod("POST", lambdaIntegration); 
    
      new CfnOutput(this, 'ApiUrlRef', {
          value: api.url,
          description: 'API Gateway URL',
          exportName: 'ApiUrl',
      });

Code from frontend stack definition:

    const apiUrl = Fn.importValue('ApiUrl');

Unfortunately, instead of URL I get token (${Token[TOKEN.256]}). At the same time, I see URL is resolved in CDK generated files:

./cdk.out/Stack-back.template.json:

    "ApiUrlRef": {
      "Description": "API Gateway URL",
      "Value": {
        "Fn::Join": [
          "",
          [
            "https://",
            {
              "Ref": "MyAPI7DAA778AA"
            },
            ".execute-api.us-west-1.",
            {
              "Ref": "AWS::URLSuffix"
            },
            "/",
            {
              "Ref": "MyAPIDeploymentStageprodA7777A7A"
            },
            "/"
          ]
        ]
      },
      "Export": {
        "Name": "ApiUrl"
      }
    }
  },

What I'm doing wrong?

UPD:

After advice of fedonev to pass data as props, situation did not changed much. Now url looks like that:

"https://${Token[TOKEN.225]}.execute-api.us-west-1.${Token[AWS.URLSuffix.3]}/${Token[TOKEN.244]}/"

I think important part I missed (which was also pointed by
Milan Gatyas) is how I create HTML with URL of gateway.

In my frontend-stack.ts, I use template file. After template is filled, I store it in S3:

      const filledTemplatePath: string = path.join(processedWebFileDir,'index.html');
      const webTemplate: string = fs.readFileSync(filledTemplatePath, 'utf8')
      const Handlebars = require("handlebars")
      let template = Handlebars.compile(webTemplate)
      const adjustedHtml: string = template({ apiGwEndpoint: apiUrl.toString() })
      fs.writeFileSync(filledTemplatePath, adjustedHtml)

      // bucket
      const bucket: S3.Bucket = new S3.Bucket(this, "WebsiteBucket", 
      {
          bucketName: 'frontend',
          websiteIndexDocument: 'index.html',
          websiteErrorDocument: 'error.html',
          publicReadAccess: true,

      })

      new S3Deploy.BucketDeployment(this, 'DeployWebsite', {
          sources: [S3Deploy.Source.asset(processedWebFileDir)],
          destinationBucket: bucket,
      });

(I'm new to TS and web, please don't judge much:) )

Am I correct that S3 is populated on synth, deploy does not change anything and this is why I get tokens in html? Will be grateful for a link or explanation so that I could understand the process better, there are so much new information to me that some parts are still quite foggy.

Answer 1

[Edit: rewrote the answer to reflect updates to the OP]

Am I correct that S3 is populated on synth, deploy does not change anything and this is why I get tokens in html?

Yes. The API URL will resolve only at deploy-time . You are trying to consume it at synth-time when you write to the template file. At synth-time, CDK represents not-yet-available values as Tokens like ${Token[TOKEN.256]} , the CDK's clever way of handling such deferred values.

What I'm doing wrong?

You need to defer the consumption of API URL until its value is resolved (= until the API is deployed). In most cases, passing constructs as props between stacks is the right approach. But not in your case: you want to inject the URL into the template file. As usual with AWS, you have many options:

1. Split the stacks into separate apps, deployed separately. Deploy BackendStack . Hardcode the url into FrontendStack . Quick and dirty.
2. Instead of S3, use Amplify front-end hosting , which can expose the URL to your template as an environment variable. Beginner friendly, has CDK support .
3. Add a CustomResource construct, which would be backed by a Lambda that writes the URL to the template file as part of the deploy lifecycle. This solution is elegant but not newbie-friendly.
4. Use a Pipeline to inject the URL variable as a build step during deploy. Another advanced approach.

Answer 2

As @fedonev mentioned, the tokens are just placeholder values in the TypeScript application. CDK app replaces tokens with intrinsic functions when the CloudFormation template is produced.

However, your use case is different. You try to know the information inside the CDK app which is available only at synthesis time, and you can't use the intrinsic function to resolve the URL while being in CDK app to write to file.

If possible you can utilize the custom domain for the API Gateway. Then you can work with beforehand known custom domain in your static file and assign the custom domain to the API Gateway in your CDK App.