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Designing a Scalable Customer Service Voice IVR System How to architect an Interactive Voice Response platform that handles millions of concurrent calls, routes each caller dynamically based on identity, history, and intent, and stays reliable when call volume spikes 10x without warning — where every architectural choice traces back to…
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…