# Production-Grade Fintech Payment Gateway & Immutable Double-Entry Ledger Architecture ================================================================================ SYSTEM OVERVIEW ================================================================================ Client Applications (Web / Mobile / POS / API Clients) │ ▼ API Gateway (Authentication, Rate Limiting, WAF, OAuth2/JWT) │ ▼ Payment Orchestrator Service │ ┌───────────────┬────────────────┐ │ │ │ ▼ ▼ ▼ Stripe PSP Adyen PSP Internal Retry Queue (Primary) (Fallback) (Kafka/RabbitMQ) │ ▼ Payment Event Bus (Kafka) │ ▼ Ledger Engine (PostgreSQL Immutable Double Entry) │ ▼ Settlement Engine │ ▼ Bank / Treasury Accounts Supporting Infrastructure • Redis Cluster • PostgreSQL HA Cluster • HashiCorp Vault • HSM • Kafka • Prometheus • Grafana • ELK/OpenSearch • SIEM • Fraud Detection Engine • Audit Service ================================================================================ 1. PAYMENT PROCESSING & GATEWAY ORCHESTRATION ================================================================================ ### Payment Intent Lifecycle 1. Client Creates Payment Intent ↓ 2. API Gateway • JWT Validation • Rate Limiting • Request Signature Validation ↓ 3. Payment Orchestrator • Validate Request • Validate Merchant • Generate Global Transaction ID • Generate Idempotency Key • Acquire Distributed Lock ↓ 4. Gateway Selection Engine Primary: Stripe Fallback: Adyen Routing Decision Factors • PSP Availability • Success Rate • Region • Currency • Merchant Preference • Cost Optimization • Latency • Historical Reliability ↓ 5. Payment Authorization ↓ 6. Customer Authentication 3DS2 SCA OTP Risk Checks ↓ 7. Authorization Response Approved ↓ Ledger Reservation Entry ↓ Capture ↓ Ledger Final Entry ↓ Settlement ↓ Completed -------------------------------------------------------------------------------- ### Smart PSP Routing Routing Score PSP Score = Availability + Success Rate + Latency Score + Regional Preference + Currency Support + Merchant Priority − Processing Cost Example Stripe Available? YES → Stripe NO ↓ Adyen Healthy? YES → Adyen NO ↓ Retry Queue -------------------------------------------------------------------------------- ### Retry Strategy Soft Failures • Timeout • Gateway Busy • HTTP 500 • Network Failure Retry 1 sec ↓ 5 sec ↓ 15 sec ↓ 60 sec ↓ Move to Dead Letter Queue Hard Failures • Card Declined • Invalid PAN • Fraud Block No Retry -------------------------------------------------------------------------------- ### Webhook Handling Webhook ↓ Verify Signature ↓ Validate Timestamp ↓ Replay Protection ↓ Idempotency Check ↓ Publish Event ↓ Ledger Update ↓ Notification Service Supported Events • Payment Authorized • Payment Captured • Payment Failed • Refund • Chargeback • Dispute • Settlement ================================================================================ 2. IMMUTABLE DOUBLE-ENTRY LEDGER ENGINE ================================================================================ ### Core Accounting Rule Every transaction MUST satisfy Total Debits = Total Credits Otherwise Reject Transaction -------------------------------------------------------------------------------- ### Ledger Account Topology Assets • Cash • Bank Accounts • Settlement Accounts Liabilities • Merchant Payables • Customer Wallets Revenue • Processing Fees • FX Fees Expenses • Chargebacks • Operational Costs Equity • Retained Earnings -------------------------------------------------------------------------------- ### Example Card Payment Customer Pays ₹1000 Entries Debit Cash Clearing Account ₹1000 Credit Merchant Payable ₹970 Credit Processing Revenue ₹30 Balanced 1000 Debit 1000 Credit -------------------------------------------------------------------------------- ### Immutable Ledger Rules Never UPDATE Never DELETE Only INSERT Corrections Performed Using Reversal Entries Example Wrong Entry Debit A Credit B Correction Debit B Credit A Historical Record Remains Forever -------------------------------------------------------------------------------- ### PostgreSQL ACID Compliance Serializable Isolation MVCC Atomic Commit Foreign Keys CHECK Constraints Append-Only Ledger Table WAL Replication Point-In-Time Recovery -------------------------------------------------------------------------------- ### Multi-Currency Transaction Currency ↓ FX Engine ↓ Daily FX Rate Snapshot ↓ Ledger Stores Original Amount Base Currency FX Rate Example 100 USD FX 1 USD = ₹85 Ledger 100 USD 8500 INR FX Rate 85 Historical FX Never Changes ================================================================================ 3. IDEMPOTENCY, DISTRIBUTED LOCKS & CONSENSUS ================================================================================ ### API Idempotency Client Sends Idempotency-Key ↓ Redis Lookup Exists? YES Return Previous Response NO Continue Processing ↓ Store Response TTL 24 Hours -------------------------------------------------------------------------------- ### Distributed Lock Redis SETNX TransactionID ↓ Lock Acquired? YES Proceed NO Return 409 Conflict Lock Released Only After Commit -------------------------------------------------------------------------------- ### Race Condition Protection Used For • Duplicate Payments • Concurrent Refunds • Multiple Captures • Wallet Transfers Techniques • Redis Lock • PostgreSQL Row Lock • Serializable Transactions • Optimistic Versioning -------------------------------------------------------------------------------- ### Two Phase Commit Phase 1 Prepare ↓ Reserve Funds ↓ Create Ledger Draft ↓ Gateway Authorization ↓ All Success? YES ↓ Phase 2 Commit ↓ Ledger Commit ↓ Capture ↓ Settlement Queue Failure ↓ Rollback Reservation ================================================================================ 4. PCI-DSS COMPLIANCE, SECURITY & TOKENIZATION ================================================================================ ### PAN Tokenization Customer Card ↓ TLS 1.3 ↓ Vault API ↓ HashiCorp Vault ↓ Token Generated ↓ Store Token Only Never Store PAN -------------------------------------------------------------------------------- ### Vault Architecture Application ↓ Vault Authentication ↓ HSM Protected Master Keys ↓ Dynamic Encryption Keys ↓ Encrypted Secrets ↓ Token Returned -------------------------------------------------------------------------------- ### Encryption In Transit TLS 1.3 AES-GCM Session Encryption Mutual TLS At Rest AES-256 Envelope Encryption Vault Managed Keys Database Encryption -------------------------------------------------------------------------------- ### PCI-DSS Controls • No PAN Storage • Tokenization • Quarterly ASV Scan • Annual Penetration Test • Network Segmentation • Least Privilege IAM • MFA Everywhere • Security Logging • Continuous Monitoring -------------------------------------------------------------------------------- ### Tamper-Proof Audit Trail Every Event ↓ SHA-256 Hash ↓ Hash Chain ↓ Signed Log ↓ Immutable Storage ↓ SIEM Audited Events • Login • Payment • Refund • Settlement • Configuration Change • Key Rotation ================================================================================ 5. RECONCILIATION, SETTLEMENT & FRAUD DETECTION ================================================================================ ### End-of-Day Reconciliation Bank Statement ↓ Import ↓ Settlement Engine ↓ Compare Ledger vs Bank ↓ Matched? YES Close Batch NO Exception Queue ↓ Manual Review -------------------------------------------------------------------------------- ### Settlement Workflow Captured Payments ↓ Settlement Batch ↓ PSP Settlement ↓ Bank Credit ↓ Ledger Settlement Entry ↓ Merchant Payout ↓ Settlement Complete -------------------------------------------------------------------------------- ### Chargeback Workflow Chargeback Received ↓ Freeze Merchant Balance ↓ Create Ledger Entries ↓ Notify Merchant ↓ Evidence Collection ↓ Submit Dispute ↓ Issuer Decision Won ↓ Reverse Liability Lost ↓ Finalize Chargeback -------------------------------------------------------------------------------- ### Real-Time Fraud Detection Input Signals • Device Fingerprint • Velocity Checks • BIN Country • IP Reputation • Geo Location • Behavioral Biometrics • Previous Fraud History • Chargeback Rate • Merchant Risk Score • Card Testing Pattern Machine Learning Risk Score 0–30 Approve 31–70 Manual Review 71–100 Decline -------------------------------------------------------------------------------- ### Fraud Engine Architecture Payment Request ↓ Feature Extraction ↓ Rules Engine ↓ Machine Learning Model ↓ Risk Score ↓ Decision Engine ↓ Approve Review Decline ================================================================================ DATABASE COMPONENTS ================================================================================ Core Tables • accounts • ledger_entries • journal_entries • payment_intents • payment_attempts • gateway_transactions • settlements • reconciliation_batches • fx_rates • merchants • customers • wallets • chargebacks • disputes • audit_logs • idempotency_keys • distributed_locks • webhook_events ================================================================================ MICROSERVICES ================================================================================ • API Gateway • Authentication Service • Merchant Service • Customer Service • Payment Orchestrator • Gateway Router • Stripe Connector • Adyen Connector • Ledger Service • Settlement Service • Reconciliation Service • Wallet Service • FX Service • Fraud Detection Service • Notification Service • Audit Service • Vault Integration Service • Reporting Service ================================================================================ TECHNOLOGY STACK ================================================================================ Backend • Java Spring Boot / Go / Rust API • REST + gRPC Database • PostgreSQL HA Cluster Cache • Redis Cluster Message Broker • Apache Kafka Workflow • Temporal / Camunda Secrets • HashiCorp Vault HSM • AWS CloudHSM / Azure Dedicated HSM / Thales Luna HSM Observability • Prometheus • Grafana • OpenTelemetry Logging • ELK / OpenSearch Tracing • Jaeger Security • OAuth2 • JWT • mTLS • TLS 1.3 • AES-256 • PCI-DSS Level 1 Deployment • Kubernetes • Docker • Helm • ArgoCD • Istio Service Mesh ================================================================================ HIGH AVAILABILITY & SCALABILITY ================================================================================ • Multi-AZ Deployment • Active-Active API Layer • PostgreSQL Streaming Replication • Redis Sentinel / Cluster • Kafka Replication Factor ≥ 3 • Automatic Failover • Horizontal Pod Autoscaling • Circuit Breaker Pattern • Bulkhead Isolation • Retry with Exponential Backoff • Dead Letter Queue (DLQ) • Zero-Downtime Deployment • Disaster Recovery with Point-in-Time Recovery (PITR) • Immutable Audit Logs • RPO < 5 Minutes • RTO < 15 Minutes • 99.99% Availability Target • PCI-DSS Level 1 Compliance • SOC 2 Type II Ready • End-to-End Observability & Continuous Monitoring ================================================================================ END-TO-END PAYMENT FLOW ================================================================================ Client → API Gateway → Authentication → Idempotency Validation → Redis Distributed Lock → Payment Orchestrator → Smart PSP Routing → Stripe (Primary) → Adyen (Fallback if Needed) → Authorization → 3DS Authentication → Capture → Kafka Event → Immutable PostgreSQL Double-Entry Ledger → Settlement Engine → Bank Reconciliation → Merchant Payout → Audit Logging → Monitoring & Fraud Analytics → Transaction Successfully Completed