Add human-in-the-loop-cdk: CDK version of one-click email approval wo… by YogeshNain2015 · Pull Request #417 · aws-samples/step-functions-workflows-collection

YogeshNain2015 · 2026-06-02T15:58:29Z

Summary

CDK (Python) implementation of the existing human-in-the-loop pattern with one-click email approval.

AWS CDK stack (Python) that creates all resources: Lambda functions, SNS topic, API Gateway, and Step Functions state machine
Reuses the same Lambda code and state machine definition from the SAM version (merged in PR Improve human-in-the-loop with one-click email approval #416)
All Lambda functions configured with 512 MB memory and 30s timeout
Follows the same CDK pattern structure as other -cdk workflows in this repo

Resources created

SendApprovalEmailFunction (Python 3.13): URL-encodes task token, sends email via SNS
HandleApprovalFunction (Python 3.13): API Gateway callback handler, calls SendTaskSuccess
ProcessingLambda (Node.js 24.x): Placeholder for post-approval business logic
Amazon SNS Topic: Email subscription for approval notifications
Amazon API Gateway: REST API with GET /respond endpoint
AWS Step Functions: State machine with waitForTaskToken integration

Both "result": "failure" and "result": "rejected" are passed as payload to ProcessingLambda (defined in cdk/cdk_stack.py). The ProcessingLambda is a simple lambda that returns the event as-is i.e currently its not doing anything however it's meant to be replaced by users with their own business logic to handle each outcome differently (e.g., send a rejection notification, trigger a rollback, log the failure, etc.). The result field allows the downstream function to differentiate which path was taken.

bfreiberg · 2026-06-08T18:44:45Z

+        "FunctionName": "${ProcessingLambda}",
+        "Payload": {
+          "input.$": "$",
+          "result": "failure"


Where is this processed?

Both "result": "failure" and "result": "rejected" are passed as payload to ProcessingLambda (defined in cdk/cdk_stack.py). The ProcessingLambda is a simple lambda that returns the event as-is i.e currently its not doing anything however it's meant to be replaced by users with their own business logic to handle each outcome differently (e.g., send a rejection notification, trigger a rollback, log the failure, etc.). The result field allows the downstream function to differentiate which path was taken.

bfreiberg · 2026-06-08T18:46:25Z

+            )
+        )
+
+        state_machine = sfn.CfnStateMachine(self, "HumanInTheLoopStateMachine",


Why not use sfn.StateMachine?

bfreiberg · 2026-06-08T18:47:17Z

+            handler="send_approval_email.lambda_handler",
+            code=_lambda.Code.from_asset("lambda"),
+            timeout=Duration.seconds(30),
+            memory_size=512,


The memory is much higher than in the SAM version for all functions. Any particular reason? Same for the timeout

- Use latest Python runtime (3.14) for all Lambda functions - Remove Node.js ProcessingLambda, rewrite in Python (single language) - Use L2 sfn.StateMachine construct instead of CfnStateMachine - Set Lambda timeout to 10s and updated 128MB memory use - Fix title to "Human in the Loop (AWS CDK)" - Remove numbered prefix from cleanup text - Add copyright year 2026 - Add Comment fields to rejection/failure states for clarity - In normal scenrio the failure processing lambda is unlikely to be invoked but if someone chages the code handleapprovalfunction specially sendtasktoken in that case it would go to hadlefailure flow, also it can be removed as well. feel free to suggest on the same...

YogeshNain2015 · 2026-06-09T09:56:54Z

Updated below changes:

Use latest Python runtime (3.14) for all Lambda functions
Remove Node.js ProcessingLambda, rewrite in Python (single language)
Use L2 sfn.StateMachine construct instead of CfnStateMachine
Set Lambda timeout to 10s and updated 128MB memory use
Fix title to "Human in the Loop (AWS CDK)"
Remove numbered prefix from cleanup text
Add copyright year 2026
Add Comment fields to rejection/failure states for clarity
In normal scenerio the failure processing lambda is unlikely to be invoked but if someone changes the code handleapprovalfunction specially sendtasktoken in that case it would go to hadlefailure flow, also it can be removed as well. feel free to suggest on the same...

bfreiberg · 2026-06-09T11:48:23Z

+    ```bash
+    cdk deploy --parameters ModeratorEmailAddress=your-email@example.com
+    ```
+1. Note the outputs from the CDK deployment. These contain the resource names and/or ARNs which are used for testing.


Let's be more precise. It's the link to the state machine in the Step Functions console

- Rename human_in_the_loop_cdk/ to cdk/ to avoid name duplication - Clarify deployment output as link to state machine in Step Functions console

Co-authored-by: Ben <9841563+bfreiberg@users.noreply.github.com>

bfreiberg

Looks good, thanks for your contribution. Your workflow will be merged soon

bfreiberg suggested changes Jun 8, 2026

View reviewed changes

Clarify CDK outputs in README

6fa4e78

bfreiberg suggested changes Jun 9, 2026

View reviewed changes

Yogesh Nain and others added 4 commits June 9, 2026 18:40

Rename module folder and clarify README outputs

208204d

- Rename human_in_the_loop_cdk/ to cdk/ to avoid name duplication - Clarify deployment output as link to state machine in Step Functions console

Rename stack file to cdk_stack.py for brevity

28fb62c

Apply suggestion from @bfreiberg

5402afc

Co-authored-by: Ben <9841563+bfreiberg@users.noreply.github.com>

Apply suggestion from @bfreiberg

57616bc

Co-authored-by: Ben <9841563+bfreiberg@users.noreply.github.com>

bfreiberg approved these changes Jun 9, 2026

View reviewed changes

+                  "cleanup": {
+                    "headline": "Cleanup",
+                    "text": [
+                      "1. Delete the stack: <code>cdk destroy</code>."

+                      api_endpoint = f"https://{approval_api.rest_api_id}.execute-api.{self.region}.amazonaws.com/prod/respond"
+                      send_approval_email_fn = _lambda.Function(self, "SendApprovalEmailFunction",
+                          runtime=_lambda.Runtime.PYTHON_3_13,

+                      notification_topic.grant_publish(send_approval_email_fn)
+                      handle_approval_fn = _lambda.Function(self, "HandleApprovalFunction",
+                          runtime=_lambda.Runtime.PYTHON_3_13,

+                      )
+                      processing_fn = _lambda.Function(self, "ProcessingLambda",
+                          runtime=_lambda.Runtime.NODEJS_24_X,

	"title": "Human in the Loop (CDK)",
	"title": "Human in the Loop (AWS CDK)",

	"1. Delete the stack: <code>cdk destroy</code>."
	"Delete the stack: <code>cdk destroy</code>."

+              ```
+              ----
+              Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.

	Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
	Copyright 2026 Amazon.com, Inc. or its affiliates. All Rights Reserved.

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YogeshNain2015 commented Jun 2, 2026

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YogeshNain2015 commented Jun 9, 2026

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