Senior React Native (Expo) + Node/Postgres engineer - defined backlog on two live apps

We build and operate automated self-serve beverage stations. Two applications are already written, tested and running: a consumer iOS app that lets a customer order and pay from their phone, and an internal operations web app that field staff use to manage sites, deliveries, inventory and service tasks. Both have substantial automated test suites that pass today. Neither is finished. We are looking for one engineer to take both across the finish line and then help connect the consumer app to the machine hardware. This is not a greenfield build and it is not a rescue job. The code is in reasonable shape, the remaining work is written down, and most tasks come with a test that tells you when you are done. WHAT THE WORK ACTUALLY IS Phase one, roughly the first two weeks, is remote and well defined. You will work through a written backlog: a payment route that charges before it records a durable claim, a service method that four handlers call but nobody implemented, a scheduled sweep that was written and never wired up, and a CI pipeline that reports success while silently skipping its own test suite. Each item has a file, a line number and an acceptance test. You will also take the operations app through its first production deploy once we supply the outstanding environment configuration. Phase two is publishing. Getting the iOS app through TestFlight and into the App Store: signing, provisioning, store metadata, review. You will need to have shipped an iOS app before. Phase three is the interesting part, and it is why the rate is what it is. The consumer app cannot yet dispense a drink. The machine runs Android 7.1 on an embedded board, with a drink controller on a serial line and a payment peripheral on a separate bus. The protocol has already been reverse engineered and documented. What remains is to build and qualify the on-machine control path so that a paid order produces exactly one drink, and a lost response or a crashed process never produces two. This is done in stages: read-only observation first, then bench actuation, then supervised live testing. You will not travel and you will not touch a machine. The owner is on site with the hardware and works through each session with you on a call while you drive from your keyboard. All physical actions are the owner's. STACK Consumer app: React Native 0.81 on Expo SDK 54, JavaScript rather than TypeScript, EAS build and submit, Apple Pay. Backends: Node 20.19.4 or newer, Express 5, PostgreSQL, Drizzle ORM on one service and raw SQL with row level security on the other, Stripe, deployed on Railway. Operations front end: React 19 as a PWA. Machine side: Java and Kotlin on Android, plus a serial protocol at 38400 baud with checksummed frames. You do not need to be equally strong in all of it. Depth in React Native and Node with real payment experience is the requirement. Android and embedded work is a strong advantage and is what separates a good fit from a great one. HOW WE DEFINE DONE Every task ends with a command that anyone can run and a result anyone can read. Tests pass, or the work is not finished. We do not accept a passing suite that was made to pass by changing an assertion, skipping a test, or loosening a check. If something cannot be made to work, we want to be told plainly, with the evidence, rather than have it papered over. That standard applies to us as much as to you, and you will find the existing code and documentation written the same way. Two rules are absolute, because this equipment dispenses hot liquid in a building we do not own. Nothing is deployed to a machine without an explicit approval step. Nothing that could actuate hardware is enabled until it has been proven safe under failure, including lost responses, crashes mid-action, and restarts. REQUIREMENTS Five or more years building production software. You have shipped at least one React Native app to the App Store yourself, end to end. You have built payment flows that handle real money and can explain, without prompting, how you make a charge idempotent and what happens when the network drops between the charge and the record. You are comfortable in a codebase you did not write, with tests as your guide. You write clearly in English and you are reachable during some overlap with US Eastern hours. Strong advantage: Android development, embedded or serial protocol work, vending or kiosk systems, or hardware integration of any kind where exactly-once behavior mattered. WHAT WE PROVIDE A written backlog with file level detail and acceptance criteria. Extensive existing documentation, including architecture, protocol analysis and prior audit reports. Staged access to private repositories. Direct access to the owner, who is technical, makes decisions quickly and is on the machine end of every hardware session. TO APPLY Skip the template. Three things, briefly: 1. Link one iOS app you shipped, and say what part of it was yours. 2. Answer this: a test suite reports all green, but the feature is broken in production. Describe a real time this happened to you, what actually caused it, and how you found out. 3. Tell us your availability per week and your earliest start date. Applications that do not answer question two will not be read. Shortlisted candidates receive the full backlog and a short paid trial task before any longer engagement.