Mobile + Hardware POS • Deployed on Commercial Sunmi Terminals

QuickChargingPOS — Sunmi POS Hardware Terminal App with Java HMAC Bridge & Offline Fuel Queue

An offline-capable fuel-station terminal that connects React Native to native payment and printer hardware.

React NativeJavaSunmiHMACOffline queue

Role: Solo mobile and hardware integration engineer
Production status: Deployed on physical Sunmi POS terminals at commercial fuel stations
Core stack: React Native · TypeScript · Java · NamiPayModule · SkyBand payment kernel · Sunmi ESC/POS · AsyncStorage · .NET API
Business outcome: Delivered a fuel-station POS system for Saudi terminals that performs on-device payment handshakes and records dispatches through unreliable network coverage.

The real-world problem

What the system had to solve

Fuel-station attendants operating in remote desert locations face unstable cellular coverage, making standard online-only POS apps lose sales transactions. Additionally, standard React Native plugins cannot interface directly with proprietary Sunmi thermal receipt printers or SkyBand bank payment kernels.

What I built

Architecture shaped by the constraints

I built a commercial Android point-of-sale app in React Native with a custom Java native bridge (NamiPayModule) for SHA-256 HMAC-signed binary payment payloads. The system integrates Sunmi thermal printer hardware and implements an AsyncStorage local queue that caches fuel delivery records offline and dispatches them when connectivity returns.

Production proof

Delivered system

Deployed directly on physical Sunmi POS terminal hardware at commercial fuel stations, processing high-volume fuel dispatch logs and printed receipts under fluctuating network conditions.

System flow

Architecture at a glance

React Native terminal

Fuel workflow and local transaction queue.

Java device modules

Signed payment intent and Sunmi receipt printer integration.

Fuel platform API

Queued transactions synchronize when service is restored.

Engineering decisions

Why the system works this way

Signed native bank-payment bridge

Constraint: Cloud payment requests add several seconds per vehicle, while local EMV transactions require signed IPC with the certified bank application.
What I engineered: Built a Java NamiPayModule that creates an ecr-txn-event Android intent with terminal-specific SHA-256 HMAC payloads and returns the payment result.
Production outcome: Moved the bank handshake onto the device and reduced payment processing to sub-second speeds.

Offline delivery ledger

Constraint: Fuel station connectivity can disappear during a delivery, so cloud-only records risk losing sales and inventory data.
What I engineered: Stored delivery events and before/after tank volumes in an AsyncStorage append-only queue with client UUIDs, then drained the queue when the network returned.
Production outcome: Preserved delivery records through outages and resumed synchronization after reconnection.

Direct thermal receipt output

Constraint: The React Native runtime cannot drive Sunmi's built-in receipt printer at the required byte level, and older printer packages failed on current Android builds.
What I engineered: Patched the Sunmi bridge and generated ESC/POS buffers for aligned bilingual receipts, with a logging fallback for development.
Production outcome: Printed a physical receipt directly from the handheld terminal after payment confirmation.

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