I walked into a crypto project thinking I'd just pull data from a blockchain API. That's not how it went.

I join the project. Stack: FastAPI, PostgreSQL, Redis as Celery broker, Celery workers, Docker, Web3. Startup on a hype wave, real money, architecture built on the fly. I look at the payment processing architecture and my first thought: guys, are you serious? Financial operations with real money, zero idempotency, Redis as broker with no persistence, Web3.py synchronous calls inside Celery tasks.

The conversation was short: here's the task, fix what's there. Deadlines were burning.

What Was Broken

First month of prod. A user writes to support: credited twice, withdrew double. I open the logs, clean. Two identical events, both 200, four seconds apart. Both processed. The user got double the balance.

Daily reconciliation with on-chain data showed a discrepancy: several accounts with a balance higher than confirmed transactions should allow.…