Fintech System Design Kit

Reference latency and unit tables, assumptions register, QPS, storage, bandwidth, cache, server and cost formulas, worked example, sensitivity, traceability

UUIDv4 and v7 per RFC 9562, Snowflake 41/10/12 layout, clock skew and backwards clocks, leased worker IDs, index locality, 53-bit JS limit, ticket servers

Event-sourced service design: fit check, event catalogue, versioning and upcasting, store, projections, idempotency, crypto-shredding

Payments and double-entry ledger: idempotency keys, payment state machine, PSPs, outbox, reconciliation, audit, PCI DSS scope

Stored-value wallet: double-entry ledger, cross-shard transfer saga vs 2PC vs event sourcing, idempotency, hot accounts, reconciliation, limits and KYC hooks

Cart, quote boundary for pricing, tax and promotions, stock reservation, oversell prevention, payment handoff, order state machine, flash-sale throttling

Real-time payment fraud scoring: latency budget, streaming features, rules plus ML, review queues, label delay, selection bias, model monitoring

Outbound webhook platform design: at-least-once delivery, HMAC signing, per-endpoint queues, backoff and circuit breaking, ordering, replay, SSRF protection

Seat and room inventory, holds with TTL, overbooking math, conditional writes, idempotency, payment saga, waiting room, search vs booking paths

Design review checklist in 13 areas, evidence-based scoring rubric, weighted score, red flags, findings register, decision record, 45-minute interview rubric

STRIDE threat model: scope, DFD with trust boundaries, assets, threats per element, L x I rating, mitigations, risk acceptance