The Desi Architect
Notes / Docker Sandboxes / UPI Payment System Capstone: Idempotency, Outbox, Sagas & Reconciliations
Docker Sandboxes · Tier 4: Architect & Chaos · Lab 58

UPI Payment System Capstone: Idempotency, Outbox, Sagas & Reconciliations

Build an end-to-end UPI payment infrastructure combining Redis edge idempotency tokens, PostgreSQL double-entry bookkeeping journals, transactional outbox event streaming, distributed saga compensation rollbacks, and midnight bank reconciliation.

Time35 minutes
StackPostgreSQL 16Ports 18120, 15495, 16415, 15675
MemoryAbout 250 MB
Before thisLab 42API Idempotency Key Engine & Distr
Download lab (.zip) View steps
Verified runWindows, Mac, LinuxBash and PowerShell
  1. 1PrepareWhat you will learn
  2. 2PredictGuess before you run
  3. 3RunStep by step
  4. 4ObserveWhat you should see
  5. 5Break itChange one thing
  6. 6Clean upLeave nothing running
On this page
  1. 00 / What You Will Learn
  2. 01 / Why It Matters
  3. 02 / What We Are Building
  4. 03 / Words You Need
  5. 04 / Predict First
  6. 05 / Step by Step
  7. 06 / What You Should See
  8. 07 / Look Inside
  9. 08 / Break It, Fix It
  10. 09 / In Real Projects
  11. 10 / Interview Corner
  12. 11 / Clean Up

00 /What You Will Learn

By the end you will be able to
  • Design production-ready payment architectures with zero double-charge risk and absolute financial balance.
  • Reproduce the negative control: customer double-debit bugs caused by client retries during dropped network connections.
  • Implement edge idempotency using Redis keys and unique transaction identifiers to deduplicate retried payments.
  • Structure double-entry accounting journals where every transaction creates matching debits and credits.
  • Enforce the transactional outbox pattern to publish domain events reliably without distributed two-phase commits.
  • Execute distributed saga compensating transactions to reverse ledger entries when downstream bank gateways time out.
  • Run automated midnight reconciliation audits to verify 100% balance parity across payment receipts and ledger entries.

Before you start

Finish Lab 33 (Transactional Outbox), Lab 34 (API Idempotency Key), and Lab 40 (Saga Pattern with Compensating Transactions) first. You should understand SQL transactions, cache TTLs, message queues, and compensating rollbacks. Docker Desktop must be running.