AWS CDK: run external build command in CDK sequence?

Question

Is it possible to run an external build command as part of a CDK stack sequence? Intention: 1) create a rest API, 2) write rest URL to config file, 3) build and deploy a React app:

import apigateway = require('@aws-cdk/aws-apigateway');
import cdk = require('@aws-cdk/core');
import fs = require('fs')
import s3deployment = require('@aws-cdk/aws-s3-deployment');

export class MyStack extends cdk.Stack {

  const restApi = new apigateway.RestApi(this, ..);
  fs.writeFile('src/app-config.json', 
    JSON.stringify({ "api": restApi.deploymentStage.urlForPath('/myResource') }))

  // TODO locally run 'npm run build', create 'build' folder incl rest api config

  const websiteBucket = new s3.Bucket(this, ..)
  new s3deployment.BucketDeployment(this, .. {
      sources: [s3deployment.Source.asset('build')],
      destinationBucket: websiteBucket
  }) 
}

Answer 1

Unfortunately, it is not possible, as the necessary references are only available after deploy and therefore after you try to write the file (the file will contain cdk tokens).

I personally have solved this problem by telling cdk to output the apigateway URLs to a file and then parse it after the deploy to upload it so a S3 bucket, to do it you need:

• deploy with the output file options, for example: cdk deploy -O ./cdk.out/deploy-output.json
• In ./cdk.out/deploy-output.json you will find a JSON object with a key for each stack that produced an output (e.g. your stack that contains an API gateway)
• manually parse that JSON to get your apigateway url
• create your configuration file and upload it to S3 (you can do it via aws-sdk)

Of course, you have the last steps in a custom script, which means that you have to wrap your cdk deploy. I suggest to do so with a nodejs script, so that you can leverage aws-sdk to upload your file to S3 easily.

Answer 2

Accepting that cdk doesn't support this, I split logic into two cdk scripts, accessed API gateway URL as cdk output via the cli, then wrapped everything in a bash script.

AWS CDK:

// API gateway
const api = new apigateway.RestApi(this, 'my-api', ..)

// output url
const myResourceURL = api.deploymentStage.urlForPath('/myResource');
new cdk.CfnOutput(this, 'MyRestURL', { value: myResourceURL });

Bash:

# deploy api gw
cdk deploy --app (..)  

# read url via cli with --query
export rest_url=`aws cloudformation describe-stacks --stack-name (..) --query "Stacks[0].Outputs[?OutputKey=='MyRestURL'].OutputValue" --output text`

# configure React app
echo "{ \"api\" : { \"invokeUrl\" : \"$rest_url\" } }" > src/app-config.json

# build React app with url
npm run build

# run second cdk app to deploy React built output folder
cdk deploy --app (..)  

Is there a better way?