Deposit Letter App - DevOps Overview

What it does
Helps landlords and property managers generate security-deposit letters and evidence packets. Users enter case data, pay $2 via Stripe, and download a court-ready packet. Evidence is stored privately in S3.

Who this is for
Business stakeholders, landlords, and security reviewers. Plain language, no AI jargon.

Architecture at a glance

• Frontend: Next.js on AWS Amplify (web/). Talks to the API with x-case-token for case-level access.
• API: API Gateway (REST) with CORS restricted to the configured frontend origin; headers allow x-case-token.
• Compute: Lambda handlers (infra/lambda/*) for cases, payments, PDF generation, evidence uploads, and Stripe events.
• Data: DynamoDB CaseFiles (case metadata, status, case secret); S3 evidence bucket (private, versioned).
• Payments: Stripe PaymentIntent/Checkout. Charge is $2.00 (200 cents). Case must be PAID before packet generation.

Diagram (Mermaid)

flowchart LR
  User[Browser Amplify] -->|HTTPS| APIGW[API Gateway<br/>CORS allowlist]
  APIGW -->|Lambda proxy| Lambdas[Lambda Functions]
  Lambdas --> Dynamo[(CaseFiles DynamoDB)]
  Lambdas --> S3[(Evidence S3 private)]
  Lambdas --> Stripe[Stripe API]
  Stripe -->|events| EB[EventBridge]
  EB --> Lambdas
  User -->|Presigned uploads/downloads| S3

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Security posture

• CORS locked to the frontend origin; x-case-token allowed on preflight.
• Case-level auth via caseSecret sent in x-case-token.
• Encryption at rest: S3 managed keys; DynamoDB default encryption. In transit: HTTPS only.
• S3 Block Public Access enabled; evidence accessed only via short-lived presigned URLs.
• Least-privilege IAM: Lambdas scoped to the DynamoDB table, evidence bucket, and read-only Stripe secret.
• Logging: CloudWatch Logs per function (retention 2 years).

Deploy & configure

Prereqs: Node/npm and AWS credentials for the target account/region.

Backend (CDK, infra/):

1. npm --prefix infra install (first time)
2. npm --prefix infra run cdk deploy -- --parameters FrontendUrl=https://<your-frontend-domain>

Frontend (Amplify, web/):

• Env vars: NEXT_PUBLIC_API_BASE_URL (API Gateway stage URL), NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY.
• Build (see amplify.yml): npm ci && npm run build.

Operations

• Payments are $2.00 (PaymentIntent amount 200 cents).
• Packet generation only runs when paymentStatus = PAID.
• Evidence uploads use presigned URLs; bucket remains private.
• Monitor via CloudWatch Logs (Lambda) and API Gateway 4XX/5XX metrics.
• Tunables without code changes: FrontendUrl CDK parameter; Stripe secret in Secrets Manager.

Key directories

• web/ - Next.js frontend.
• infra/ - CDK infrastructure and Lambda sources under infra/lambda/.