Create a recurring billing platform for Duolingo.

Question

Anonymous · Accepted Answer

A subscription management system for Raisin should enable users to seamlessly
browse, subscribe to, and manage financial products such as savings accounts or
investment plans offered through partner banks. The system must support secure
user authentication, KYC verification, and real-time integration with
third-party banks for initiating and renewing subscriptions. It should track
each subscription’s lifecycle—including status, term dates, and transaction
history—while also handling automatic renewals, cancellations, and
user-triggered changes. Notifications (via email, SMS, or in-app) should alert
users about upcoming renewals, expirations, or product updates. The backend
should be built on a scalable microservices architecture using secure APIs,
event-driven messaging (e.g., Kafka), and a relational database for strong
consistency. Compliance with financial regulations (e.g., GDPR, MiFID II),
encrypted data handling, and audit logging are essential. An admin dashboard
should provide internal teams with tools to monitor subscriptions, handle
exceptions, and support customer inquiries efficiently

Anonymous · Answer

High-Level Blueprint

I propose a Microservices Architecture driven by an Event-Driven backbone. This
decouples the "decision to bill" from the "act of billing."

Core Components:

* API Gateway: Entry point for front-end clients and internal services.

* Subscription Service: The Source of Truth for what plans a user has.

* Billing Scheduler: Determines when a bill is due.

* Payment Processor: Interfaces with Nubank's internal ledger or external
   gateways.

* Invoice Service: Generates documents asynchronously.

* Notification Service: Sends emails/push notifications.

Anonymous · Answer

A Flexible Payment Planner for Figma lets workspace admins simulate and
customize billing schedules for seats, add-ons, and usage across various
cadences (monthly, annual, installments, deferred, or hybrid). It consists of
modular services — a Planner for simulations, a Pricing Engine for proration and
tax calculations, and Subscription, Invoice, and Payment services for execution.
Admins can preview “what-if” cost scenarios, apply discounts, credits, and
regional taxes, then commit a plan that generates invoices and processes
payments through integrated providers like Stripe. The system ensures
auditability, idempotency, and compliance while scaling globally with
multi-currency support and clear state management for subscriptions, invoices,
and entitlements.

Anonymous · Answer

Functional Requirements:
Users should be able to create rules for when payments are made
Rules should be set for when payments are due

Payments executed once

Non-functional requirements:

Correctness > Availability (we really want to avoid charging users for outdated
rules)

Highly reliable (partition fault tolerant with retries)

Auditable and traceable

Secure and compliant (PCI, data encryption)

Core Entities
Users

Payments

Schedules/Rules

Create a recurring billing platform for Duolingo.

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