Automated Refund Worker

An event-driven, enterprise-grade TypeScript background worker engineered for idempotent full and partial refund pipelines, pre-dispatch fraud velocity auditing, and distributed vendor payout reversal sagas.

---

🗺️ Refund Saga & Fraud Audit Execution Flow

[ Refund Trigger (Full / Partial) ]
                 │
                 ▼
[ Idempotency & Cumulative Cap Guard ] ──(Enforce: Σ Refunds <= Captured BigInt)
                 │
                 ▼
[ Pre-Dispatch Fraud Velocity Auditor ] ─(Score < 40: Approve │ >= 40: Hold/Reject)
                 │
                 ▼
[ Pro-Rata Vendor Payout Reversal ] ─────(Clawback Connected Vendor & Fee Split)
                 │
                 ▼
[ Acquiring PSP Refund Worker Queue ] ───(Full-Jitter Exponential Backoff)
                 │
    ┌────────────┴────────────────────┐
    ▼ (2xx Refund Settled)            ▼ (Terminal Reject / Timeout)
[ Finalize Ledger Audit Trail ]   [ Compensating Saga Rollback / DLQ Quarantine ]

---

🏛️ Architectural Overview & Saga Flow

Designed as the asynchronous reversal and risk-control bridge between checkout-flow-sdk and payout-gateway-service, this worker enforces strict financial invariants across every refund lifecycle stage:

1. Cumulative Partial Refund Cap Invariant: Synchronously locks and validates that settled plus in-flight partial refunds never exceed the original captured payment amount using 64-bit BigInt minor units (src/guards/cumulative-refund-cap-guard.ts).
2. Pre-Dispatch Fraud & Velocity Auditing: Evaluates 24-hour refund velocity, high-value thresholds, and account age heuristics prior to balance reversal, routing anomalous requests to manual review holds (src/fraud/refund-fraud-auditor.ts).
3. Pro-Rata Connected Vendor Clawbacks: Computes exact pro-rata vendor balance reversals and platform fee adjustments (src/reversals/vendor-payout-reversal-calculator.ts) with zero floating-point rounding drift.
4. Compensating Saga Orchestration: Coordinates multi-step state transitions between vendor balance clawbacks and acquiring PSP dispatches, automatically executing compensating vendor credits if downstream card networks terminally reject a refund (src/saga/refund-saga-orchestrator.ts).
5. Full-Jitter Backoff & DLQ Quarantine: Mitigates acquirer rate-limiting with AWS-style Full-Jitter exponential backoff and isolates ambiguous timeouts in a Dead-Letter Queue (src/workers/refund-backoff-dlq-policy.ts).

---

✨ Key Architectural Features

• Deterministic Idempotency Guards: Payload fingerprint attestation preventing duplicate customer credits across worker retries (src/guards/refund-idempotency-guard.ts).
• Zero-Trust Webhook Attestation: Constant-time signature verification and 300-second replay window guards (src/security/webhook-signature-verifier.ts).
• Recursive PAN, IBAN & PII Scrubbing: Automated telemetry sanitization masking sensitive payment and banking identifiers (src/telemetry/refund-pii-redactor.ts).
• Formal Verification (TLA+): Dual mathematical specifications proving strict cumulative refund caps and saga rollback consistency (docs/formal-verification/).
• High-Throughput Micro-Benchmarks: Deterministic benchmarks for BigInt pro-rata allocation, fraud velocity scoring, and Full-Jitter backoff (benchmarks/).

---

🔒 Security, Compliance & Vulnerability Disclosure

We treat refund integrity, fraud prevention, and cardholder data protection as non-negotiable priorities:

• Security Policy & Compliance Boundaries: Review our SECURITY.md for cumulative cap locks, fraud gating, and PII redaction standards.
• STRIDE Threat Model: Comprehensive threat analysis and cryptographic mitigations are documented in docs/threat-model.md.
• Card Network & ISO 20022 Compliance Matrix: Alignment with pacs.004 payment returns, Visa/Mastercard fraud controls, and SOC 1 split-settlement accounting is documented in docs/compliance/dispute-and-chargeback-controls.md.
• RFC 9116 Compliance: Standardized vulnerability disclosure contact policy is published at .well-known/security.txt.

---

📚 Documentation, Specifications & Runbooks

Architecture Decision Records (ADRs)

• ADR 0001: Cumulative Partial Refund Cap and BigInt Minor-Unit Invariant
• ADR 0002: Distributed Saga Orchestration for Connected Vendor Payout Reversals
• ADR 0003: Deterministic Pre-Dispatch Fraud and Velocity Scoring Gates
• ADR 0004: Idempotent Queue Workers, Full-Jitter Backoff, and DLQ Quarantine

Formal Verification (TLA+), API Contracts & Runbooks

• TLA+ Invariant 1: Cumulative Refund Cap Specification
• TLA+ Invariant 2: Vendor Reversal Saga State Machine Specification
• OpenAPI 3.1 Specification: Refund Worker Orchestration Contract
• Operational Runbook: DLQ Refund Saga Quarantine & Recovery SOP

---

🤝 Contributing

We welcome contributions from distributed systems engineers and security researchers. Please review our CONTRIBUTING.md and adhere strictly to Conventional Commits standards.

---

📄 License

Distributed under the Apache-2.0 License. See LICENSE for details.