11 min read
Every Payment Has a Liability Owner
EMV authorization is the easy part. The hard problems are timeouts, retries, SoftPOS lifecycle gaps, store-and-forward declines, and the financial question that remains when two …
38 pages
POS
Point of Sale architecture, terminals, EMV flows, SoftPOS, and payment system design
11 min read
The €400 Wallet Problem: Concurrency, Lost Updates, and Where to Serialize
Two POS terminals charging the same benefit wallet at the same instant expose the classic lost-update problem. Four serialization strategies — pessimistic locks, optimistic …
11 min read
PCI Is Not Just PCI DSS: A Layered Map of Payment Security Standards
The PCI Security Standards Council publishes multiple standards — DSS, PIN, PTS, MPoC, P2PE, and more. Each maps to a different layer of the payment stack, and conflating them …
7 min read
ARQC, TC, and AAC: A Field Guide to EMV Application Cryptograms
In EMV chip transactions, the card does not simply return “approved” or “declined” as plain text. It returns an application cryptogram: an 8-byte MAC bound to that transaction’s …
8 min read
The Future of AI in Payments: From Observer to Participant
The next phase of AI in payments is not faster fraud scoring. It is AI moving from an advisory layer that observes the transaction to an actor that initiates payments and operates …
10 min read
Manual Capture: The Two-Step Logic Behind Tap to Pay and Modern Card Payments
Manual capture is not about typing a card number by hand. It is the deliberate split of a card transaction into authorization and capture, managed as a backend state machine. Why …
8 min read
Double Charges: The Hardest Problem in Payments Is Remembering You Already Did
A duplicate charge is rarely a coding bug. It is a distributed system losing certainty about whether an authorization already happened — and then guessing wrong. Why blind retries …
5 min read
Where AI Really Fits in POS and E-Commerce Payments
This is part 3 of a series on building grounded AI for payment systems. Part 1 made the case that payments need grounded AI, and part 2 Part 2covered the retrieval pattern that …
10 min read
What Makes a Payment Terminal Secure: SmartPOS vs SoftPOS
In the POS systems I have worked on, “is this terminal secure?” almost never has a hardware-only answer. The card read is one boundary. The PIN entry is another. So are …
5 min read
From Wireless Awareness to Payment Awareness: The RAG Pattern
This is part 2 of a series on building grounded AI for payment systems. Part 1 made the case that payments need grounded AI, not a generic LLM guessing from training data. This …