I design and maintain mission-critical financial infrastructure where idempotency, zero-sum mathematical conservation, cryptographic auditability, and zero-data-loss recovery are non-negotiable.

         ┌────────────────────────────────────────────────┐
         │               checkout-flow-sdk                │
         │   (Client Tokenization, EMV 3DS2 & Failover)   │
         └───────────────────────┬────────────────────────┘
                                 │ 1. Signed BigInt Capture
                                 ▼
         ┌────────────────────────────────────────────────┐
         │           double-entry-ledger-core             │
         │  (Immutable Zero-Sum Journal & Merkle Chain)   │
         └───────────┬────────────────────────┬───────────┘
                     │                        │
   2a. Two-Phase Lock│                        │2b. Contra-Journal
       & Settlement  ▼                        ▼    & Split Clawback
┌─────────────────────────────┐      ┌─────────────────────────────┐
│   payout-gateway-service    │◄────►│   automated-refund-worker   │
│ (Multi-Rail & 3-Way Match)  │      │ (Cap Guard & Refund Saga)   │
└─────────────────────────────┘      └─────────────────────────────┘

• Inbound Payment Orchestration & Tokenization (checkout-flow-sdk): Client-side PCI-DSS v4.0 scope reduction, EMV 3-D Secure 2.2 (3DS2) Strong Customer Authentication guards, deterministic FSM circuit breakers (CLOSED / OPEN / HALF_OPEN), and latency-aware multi-PSP failover routing.
• Immutable Double-Entry Accounting (double-entry-ledger-core): Strict 64-bit BigInt zero-sum conservation ($\sum \text{Debits} = \sum \text{Credits}$), append-only contra-entry journals, sequential cryptographic hash chaining (prevHash $\rightarrow$ entryHash), binary Merkle root sealing, deadlock-free Optimistic Concurrency Control (OCC), and multi-currency FX clearing.
• Outbound Disbursement & Reconciliation (payout-gateway-service): Synchronous two-phase balance reservation (available $\rightarrow$ pending) eliminating TOCTOU overdrafts, deterministic multi-rail routing (INSTANT_RTP, SEPA_CREDIT, ACH_SAME_DAY, SWIFT_WIRE), and automated three-way reconciliation against ISO 20022 camt.053 bank statements.
• Compensating Refund Sagas & Fraud Auditing (automated-refund-worker): Atomic cumulative partial refund cap enforcement ($\sum \text{Refunds} \le \text{Captured}$), pre-dispatch 24h fraud velocity scoring, pro-rata connected vendor balance clawbacks, and fencing-token distributed sagas with Dead-Letter Queue (DLQ) isolation.
• Formal Verification & Regulatory Rigor: Every repository is standardized with 6 Architecture Decision Records (24 ADRs total), 3 TLA+ formal specifications (12 mathematical models total), OpenAPI 3.1 contracts, STRIDE threat models, RFC 9116 disclosures, and 5 deterministic micro-benchmarks (20 suites total).

1. Idempotency & Payload Attestation by Default: Every state-mutating request across checkout, payout, refund, and ledger boundaries requires a deterministic Idempotency-Key bound to a canonical cryptographic payload fingerprint to eliminate duplicate financial execution.
2. Strict Zero-Sum & Two-Phase Balance Integrity: Monetary values are processed exclusively in 64-bit BigInt minor units. Outbound payouts and partial refunds enforce synchronous balance/cap reservations prior to external banking dispatch, backed by append-only double-entry journal batches ($\sum \text{Debits} = \sum \text{Credits}$).
3. Graceful Degradation & Compensating Sagas: External gateway brownouts trigger deterministic FSM circuit breakers, multi-rail failover routing, or Dead-Letter Queue (DLQ) quarantine with AWS-style Full-Jitter exponential backoff—never premature balance rollbacks.
4. Mathematical Formal Verification & Tamper Evidence: Critical concurrency locks, conservation laws, and state machines are proven via TLA+ specifications and sealed into sequential cryptographic hash chains and binary Merkle trees.

• Email: lumilieju@hotmail.com
• Security Policy: Adheres strictly to RFC 9116 (.well-known/security.txt) vulnerability disclosure standards and STRIDE threat modeling across all managed repositories.