expense-approval-durable-lambda

A human-in-the-loop expense approval workflow built with AWS Lambda Durable Execution — no Step Functions, no DynamoDB, no SQS. Just TypeScript.

Companion code for the Medium article "Is Step Functions Still Necessary? The Case for Lambda Durable Functions in 2026"

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How It Works

1. POST /expenses → Orchestrator starts a Durable Worker, which pauses at waitForCallback and sends an approval email
2. POST /approve?callbackId=xxx → Orchestrator calls SendDurableExecutionCallbackSuccess → Worker resumes and finalizes

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Project Structure

├── lib/
│   └── durable-approval-stack.ts  # CDK stack
├── lambda/
│   ├── worker.ts                  # Durable workflow (pauses, resumes)
│   └── orchestrator.ts            # API router
└── cdk.json

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Getting Started

Prerequisites: Node.js 20+, AWS CDK v2, AWS CLI configured

npm install
cdk bootstrap   # first time only
cdk deploy

Then submit an expense:

curl -X POST https://<api-url>/expenses \
  -H "Content-Type: application/json" \
  -d '{ "expenseId": "exp-001", "amount": 450, "submittedBy": "alice@example.com" }'

And approve it:

curl -X POST "https://<api-url>/approve?callbackId=<id>"

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References

• AWS Lambda Durable Execution Docs
• Medium Article

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License

MIT