Engineering Ic interview prep.

Behavioural questions to expect

1. Walk me through your CV.
2. Tell me about your most impactful technical project.
3. Tell me about a weakness, a failure, or feedback you've received and worked on.
4. Why payments / fintech engineering - and why the firm's segment vs general SaaS or pure infra?
5. Which engineering team or technical area would you want to work on, and why?
6. Why the firm?
7. How would you describe the firm's engineering organisation and architecture in your own words?
8. How does engineering actually drive value at a payments firm like the firm?

Technical concepts to master

• Idempotency + exactly-once semantics for money

  Idempotency keys · At-least-once delivery + dedupe · Sagas + compensating transactions · Outbox + transactional event emission

• Double-entry ledger design

  Double-entry basics · Immutability + append-only · Balance computation - rolled-up vs derived · Reconciliation - internal + external

• PCI-DSS scope + tokenization

  PCI-DSS scope - what's in · Tokenization · Network tokens vs vault tokens · Compensating controls + segmentation

• Webhooks + partner-integration patterns

  Webhook delivery - signed + retried · Inbound webhook handling · Circuit breaker + fail-safe partner posture · Replay + reconcile after partner outage

Practical drills

• the firm's auth API receives 20M card transactions / day, average payload 3KB, with a target P99 of 600ms (incl. issuer) and 99.97% availability. Settlement batches run nightly. (a) Average + peak auth TPS. (b) Daily + annual storage growth for the auth log. (c) If reads on the dashboard / reconciliation side are 5x writes, peak read TPS? (d) Settlement batch: 20M transactions to process in a 4-hour window - required throughput? (e) How many app servers (assume ~300 TPS / server for an auth-heavy mixed workload)?
• Design a multi-currency payment authorisation service for the firm that supports card auths in 30+ currencies, with idempotent retries, fraud-scoring in path (target <50ms), ledger writes on success, and webhook notifications to merchants. Walk me through it.
• Customer support escalates: 12 merchants report a total of $480K in payments that were authorised + the customer was charged, but the merchants never received the payout. Ledger shows the debits + credits both posted. The merchants haven't received the settlement file entry. Walk me through how you'd diagnose + fix.

Smart-question anchors

• Team + scope - the team's surface area, the money-movement system the role would own in 6-12 months
• Stack + architecture - the current stack, ledger model, recent architecture changes, where the engineering team thinks it's heading
• Correctness + reliability - on-call rotation, SLO posture on the auth + settlement paths, reconciliation cadence, recent incident learnings
• Partner integration - which issuers / acquirers / networks the team integrates with, partner-engineering posture, fallback + redundancy
• Regulatory + PCI posture - PCI-DSS scope, tokenization strategy, SCA / PSD2 coverage, how engineering partners with compliance + risk

Sourced from

interviewing.io — Senior Engineer's Guide to the System Design Interview · Hello Interview — Payment Systems + Distributed Money Movement Designs · Modern Treasury — Engineering blog (Ledger design, idempotency, double-entry) · PCI Security Standards Council — PCI-DSS v4.0 + Tokenization Guidance · Google SRE Book — The Four Golden Signals + observability canon · Tech Interview Handbook — Behavioral Interview Questions for Software Engineers