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Real-Time Spending Limit Enforcement for Corporate Expense Cards How to approve or decline a card swipe in under 100 milliseconds, correctly, when the limit that applies depends on which employee is swiping, what they’re buying, and what time period you’re currently inside — an interview-ready walkthrough of atomic multi-key check-and-reserve…
Designing a Real-Time Loan Eligibility & Interest Rate Engine A production-grade, interview-focused deep dive into building a system that can look at a borrower, a rapidly changing risk model, and current market conditions — and answer “are you eligible, and at what rate?” in under a second, millions of times…
Designing a Push Notification System at Billion-Device Scale A ground-up, interview-ready architecture for reliably delivering push notifications to a billion iOS and Android devices — covering APNs and FCM internals, fan-out at scale, device-token lifecycle, retries, and the trade-offs real engineering teams live with. 01 · Intro & History 02…
Designing Instant Account Opening for a Neobank How to let a stranger go from “I’ve never heard of this bank” to “I have a fully funded, usable account” in a couple of minutes on their phone — while verifying they are who they claim to be, screening them against fraud…
Designing for a Partial Payment Gateway Outage How to keep a payment system trustworthy when the gateway you depend on breaks for only some transaction types — and stays half-working instead of fully down. A deep, interview-ready walkthrough of instrument-level health signals, fine-grained circuit breakers, bulkhead isolation, idempotency, transactional outbox,…
Guaranteeing Message Delivery Order in Group Chat Systems A production-grade, interview-focused deep dive into how systems like WhatsApp, Slack, Discord and Messenger make sure every participant in a group sees messages in the same, correct order — even when senders are on flaky networks scattered across the globe. 01 ·…
Instant Peer-to-Peer Payment Requests Designing a system where one user can request money from another, with real-time notification and frictionless one-tap payment, for a modern financial platform. 01 · Intro & History 02 · Problem 03 · Architecture 04 · Internal Working 05 · Data Flow 06 · Trade-offs 07…
Designing an Instant Merchant Settlement System A complete, interview-focused walkthrough of building a payment platform capability that moves money to merchants within seconds or minutes of a sale, replacing the traditional multi-day settlement cycle — without exposing the platform to unacceptable financial risk. 01 · Introduction02 · Architecture03 · Internals04…
Designing Real-Time Voice Transcription for Live Phone Call Captions A deep, interview-ready walkthrough of how to build a low-latency speech-to-text pipeline that turns a live phone call into readable captions fast enough to feel like part of the conversation — for accessibility, not as an afterthought. We design the streaming…
Designing Graceful Network Degradation for a Video Calling App How production video platforms keep a participant “in the room” when their network gets bad — instead of dropping them — through adaptive bitrate, layered video, jitter buffering, forward error correction, and state-machine-driven reconnection. 01 · Intro & History 02 ·…