feat(cloudformation): add option to restrict data returned by AwsCustomResource

packages/@aws-cdk/aws-cloudformation/lib/aws-custom-resource-provider/index.ts

@@ -33,12 +33,26 @@ function fixBooleans(object: object) {
       : v);
 }
 
+/**
+ * Filters the keys of an object.
+ */
+function filterKeys(object: object, pred: (key: string) => boolean) {
+  return Object.entries(object)
+    .reduce(
+      (acc, [k, v]) => pred(k)
+        ? { ...acc, [k]: v }
+        : acc,
+        {}
+    );
+}
+
 export async function handler(event: AWSLambda.CloudFormationCustomResourceEvent, context: AWSLambda.Context) {
   try {
     console.log(JSON.stringify(event));
     console.log('AWS SDK VERSION: ' + (AWS as any).VERSION);
 
     let physicalResourceId = (event as any).PhysicalResourceId;
+    let flatData: { [key: string]: string } = {};
     let data: { [key: string]: string } = {};
     const call: AwsSdkCall | undefined = event.ResourceProperties[event.RequestType];
 
@@ -47,18 +61,19 @@ export async function handler(event: AWSLambda.CloudFormationCustomResourceEvent
 
       try {
         const response = await awsService[call.action](call.parameters && fixBooleans(call.parameters)).promise();
-        data = flatten(response);
+        flatData = flatten(response);
+        data = call.outputPath
+          ? filterKeys(flatData, k => k.startsWith(call.outputPath!))
+          : flatData;
       } catch (e) {
         if (!call.catchErrorPattern || !new RegExp(call.catchErrorPattern).test(e.code)) {
           throw e;
         }
       }
 
-      if (call.physicalResourceIdPath) {
-        physicalResourceId = data[call.physicalResourceIdPath];
-      } else {
-        physicalResourceId = call.physicalResourceId!;
-      }
+      physicalResourceId = call.physicalResourceIdPath
+        ? flatData[call.physicalResourceIdPath]
+        : call.physicalResourceId;
     }
 
     await respond('SUCCESS', 'OK', physicalResourceId, data);

packages/@aws-cdk/aws-cloudformation/lib/aws-custom-resource.ts

@@ -71,6 +71,18 @@ export interface AwsSdkCall {
    * @default use latest available API version
    */
   readonly apiVersion?: string;
+
+  /**
+   * Restrict the data returned by the custom resource to a specific path in
+   * the API response. Use this to limit the data returned by the custom
+   * resource if working with API calls that could potentially result in custom
+   * response objects exceeding the hard limit of 4096 bytes.
+   *
+   * Example for ECS / updateService: 'service.deploymentConfiguration.maximumPercent'
+   *
+   * @default return all data
+   */
+  readonly outputPath?: string;
 }
 
 export interface AwsCustomResourceProps {
@@ -158,6 +170,7 @@ export class AwsCustomResource extends cdk.Construct {
 
   /**
    * Returns response data for the AWS SDK call.
+   *
    * Example for S3 / listBucket : 'Buckets.0.Name'
    *
    * @param dataPath the path to the data

packages/@aws-cdk/aws-cloudformation/test/test.aws-custom-resource-provider.ts

@@ -223,5 +223,52 @@ export = {
     test.equal(request.isDone(), true);
 
     test.done();
-  }
+  },
+
+  async 'restrict output path'(test: Test) {
+    const listObjectsFake = sinon.fake.resolves({
+      Contents: [
+        {
+          Key: 'first-key',
+          ETag: 'first-key-etag'
+        },
+        {
+          Key: 'second-key',
+          ETag: 'second-key-etag',
+        }
+      ]
+    } as SDK.S3.ListObjectsOutput);
+
+    AWS.mock('S3', 'listObjects', listObjectsFake);
+
+    const event: AWSLambda.CloudFormationCustomResourceCreateEvent = {
+      ...eventCommon,
+      RequestType: 'Create',
+      ResourceProperties: {
+        ServiceToken: 'token',
+        Create: {
+          service: 'S3',
+          action: 'listObjects',
+          parameters: {
+            Bucket: 'my-bucket'
+          },
+          physicalResourceId: 'id',
+          outputPath: 'Contents.0'
+        } as AwsSdkCall
+      }
+    };
+
+    const request = createRequest(body =>
+      body.Status === 'SUCCESS' &&
+      body.PhysicalResourceId === 'id' &&
+      body.Data!['Contents.0.Key'] === 'first-key' &&
+      body.Data!['Contents.1.Key'] === undefined
+    );
+
+    await handler(event, {} as AWSLambda.Context);
+
+    test.equal(request.isDone(), true);
+
+    test.done();
+  },
 };