Lagos fintech technical assessments typically include: 1) Build a Paystack/Flutterwave webhook handler with HMAC validation and idempotency. 2) System design: design a transfer/payout system or a wallet balance service. 3) SQL: reconcile a transactions table with a settlements table. 4) Debug: given a webhook handler, find the security vulnerabilities. 5) CBN regulation knowledge: what is BVN, what is the Tier 1/2/3 KYC framework, what is the NGN single transfer limit.
| Assessment Type | What You Build | What They Look For |
|---|---|---|
| Webhook handler challenge | Express API endpoint that receives Paystack charge.success, validates HMAC, updates DB idempotently | Signature validation correctness, idempotency, no double-fulfillment |
| System design: payout system | Design a disbursement system for 10,000 vendors | Approval workflow, idempotent transfers, retry logic, audit trail |
| SQL reconciliation | Query to find transactions in DB but not in settlements, or settlements > transactions | JOIN logic, GROUP BY, handling NULLs, understanding of settlement vs. transaction |
| Debug session | Given broken webhook handler code, find and fix 3 security/logic bugs | Security awareness: HMAC bypass, missing idempotency, raw body parsing error |
| API design | Design a REST API for a wallet service (deposit, withdraw, balance, history) | Idempotency keys, concurrency handling, rate limiting, audit log |
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See system design: payout system to prepare for the most common Lagos fintech design question.
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