Yaroslav K

Crypto Custody: Exchange and Trading Web and Mobile Application

Devima Solutions • Vollzeit

Feb 2021 - Present • 5 yrs 8 mos

The client ran a crypto exchange for Kosovo on a white-label platform and wanted its own product: a web app in English and Albanian where people buy and sell crypto with euros, with Fireblocks custody, OKX trading and SumSub identity checks. My role: backend architect on a team of 4. I designed and built the NestJS, PostgreSQL and Redis backend, plus sign-in and onboarding in the Next.js app. Challenges and how I solved them: - Balances can never be rounded. Amounts are stored as integers in each asset's smallest unit, fees in basis points, on a double-entry ledger. - Providers go down and webhooks arrive twice. Every trade is a sequence of saved states, with idempotent requests, duplicate-safe webhooks, circuit breakers and retries. - A failed transfer to the main vault could leave a sell stuck with no error. I made it one dispatch per deposit and saved failures separately, so they show up and can be retried. - Google sign-in skipped the country check. I added a single-use ticket so the country step runs before sign-in completes, on both backend and frontend. Result: buy and sell flows ran end to end on development and staging, connected to Fireblocks, OKX and SumSub. Operators confirmed payments and issued refunds through the APIs I built.