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.

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🗺️ 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)
                 │
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[ 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)
[ Emit Double-Entry Contra-Journal ]   [ Compensating Saga Rollback / DLQ Quarantine ]

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🏛️ Architectural Overview & Ecosystem Integration

Serving as the asynchronous reversal and risk-control bridge connecting checkout-flow-sdk (Inbound Capture), payout-gateway-service (Vendor Disbursement), and double-entry-ledger-core (Immutable Accounting), this worker enforces strict financial invariants:

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. Double-Entry Contra-Journal Emission: Emits balanced compensating contra-entry journal batches (src/ledger/contra-journal-emitter.ts) linked via reversesJournalId to double-entry-ledger-core.

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✨ Key Architectural Features

• Fencing-Token Distributed Saga Locks: Prevents concurrent workers from mutating the same parent capture state (src/concurrency/saga-distributed-lock.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+): Triple mathematical specifications proving strict cumulative refund caps, saga rollback consistency, and fraud quarantine gate safety (docs/formal-verification/).
• High-Throughput Micro-Benchmarks: Deterministic benchmarks for BigInt pro-rata allocation, fraud velocity scoring, 64-bit fingerprinting, Full-Jitter backoff, and saga compensation (benchmarks/).

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🔒 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.

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📚 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
• ADR 0005: ISO 20022 PACS.004 Payment Return Reason Code Mapping
• ADR 0006: Double-Entry Contra-Entry Journal Emission for Refund Reversals

Formal Verification (TLA+), API Contracts & Operational Runbooks

• TLA+ Invariant 1: Cumulative Refund Cap Specification
• TLA+ Invariant 2: Vendor Reversal Saga State Machine Specification
• TLA+ Invariant 3: Pre-Dispatch Fraud Quarantine Gate Specification
• OpenAPI 3.1 Specification: Refund Worker Orchestration Contract
• Operational Runbook 1: DLQ Refund Saga Quarantine & Recovery SOP
• Operational Runbook 2: Fraud Velocity Hold & Treasury Review SOP

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🤝 Contributing

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

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📄 License

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