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$catproblem.md
The Delivery Billing System is a standard system design interview question, often categorized under "Payment Pipeline" or "Subscription Billing" at companies like Netflix. It focuses on designing a robust, scalable architecture to handle recurring payments and financial data with zero tolerance for loss. Interviewing.io +1 41
Core Problem Statement
Design a system that manages high-volume billing for a global subscriber base. The system must reliably handle:
Recurring Payments: Execute millions of renewals daily without overwhelming payment providers (the "midnight spike").
Data Integrity: Ensure ACID compliance for critical operations to prevent overcharging or under-collection.
Lifecycle Management: Support starting, upgrading, downgrading, or canceling subscriptions with accurate proration.
Financial Reporting: Generate daily roll-ups for accounting and provide a full billing history for customer support. Hello Interview +2
Key Functional Requirements
Flexible Payment Methods: Users must be able to add, remove, and update various payment methods (credit cards, gift cards, etc.) across different regions.
Notification & Retries: Automatic handling of failed payments with transparent grace periods and retries.
Invoicing: Allow users to view and print detailed invoices and receipts. Hello Interview +2
Non-Functional Constraints (Netflix Scale)
Massive Scale: Manage billing for approximately 270M+ subscribers globally.
High Availability & Low Latency: Payment checks must not slow down the user's ability to stream video.
Idempotency: All billing updates must be idempotent to prevent duplicate charges during system retries or network failures.
Fault Tolerance: The system must use patterns like Saga or distributed workflows to handle failures across multiple microservices. Hello Interview +2
Common "Deep Dive" Interview Questions
Thundering Herd: How do you avoid crashing your payment processors when everyone's subscription renews on the first of the month? (Answer: Use deterministic smearing to spread loads).
Entitlement Enforcement: How do you verify a user's paid status at the edge (CDN) without adding significant latency to the video playback start?.
Cross-Region Consistency: How do you maintain a single source of truth for a user's balance in a multi-region deployment?. Hello Interview +2
Would you like to walk through a high-level architectural diagram or focus on a specific component like idempotency?