Going from CRUD backend to payments: things I wish someone told me

4 years back I was writing CRUD APIs for a food delivery startup. Then I joined a lending company, and in the first month I broke a reconciliation job, learned what a UTR is, and realised I did not know what a debit was. Writing this for anyone thinking about moving into fintech backend.

Learn basic accounting before anything else. Not the full course. Just double entry, what assets and liabilities are, and why a customer's wallet balance is a liability for you. Two weekends with a beginner bookkeeping book will make every conversation with finance 10x easier. Most engineers skip this and then spend months confused why finance asks such weird questions.

Every operation must be safe to run twice. In CRUD world a duplicate request creates a duplicate row and somebody deletes it later. In payments a duplicate request moves money twice. Idempotency is not an advanced topic here, it is day one.

"Unknown" is a real state. Networks fail in the middle. You will spend more time on what happens when you dont know the result than on the happy path.

Read the contract. Rounding rules, settlement timelines, fee structure, refund windows. They are in the agreement with the bank or gateway and they are the actual spec. I am not joking, ask your manager for the PDF.

Your logs are now a compliance problem. Card numbers, account numbers, PAN, Aadhaar. Mask before logging, not after. One careless console.log(req.body) can become a reportable incident.

Slow is fine, wrong is not. Coming from consumer apps where we shipped 5 times a day, this was the biggest culture shock. Reviews are slower, deploys have more checks, and somebody always asks "what happens if this runs twice". It feels slow for 3 months and then you start asking it yourself.

Was it worth it? For me yes. The problems are harder in a good way and the pay is better, I will not pretend that didnt matter. But it is a different kind of engineering, go in knowing that.