Design a Subscription Management Service

To design a robust subscription management service for a platform like Netflix, I would start by defining the core entities: User, Subscription Plan, and Invoice. Given the financial nature, we must prioritize data consistency and idempotency. I propose a microservices architecture where the Subscription Service acts as the source of truth, communicating asynchronously with a Payment Gateway via a dedicated Adapter pattern. The critical flow involves a scheduled job triggering billing events. When a charge is initiated, we generate a unique idempotency key stored in Redis. If the gateway returns a success but our internal update fails, the retry logic uses this key to detect duplicates rather than charging again. For state management, we maintain a finite state machine for subscriptions (Active, Paused, Delinquent, Cancelled). Transitions are triggered by webhooks from the payment provider; for instance, a 'charge_failed' webhook moves a user to 'Delinquent' after three attempts, automatically pausing access to content streams. Regarding scalability, since billing is write-heavy but read-light for most users, we can shard user data by region. We also need a dead-letter queue for failed webhook deliveries to ensure no payment event is lost. Finally, to align with Netflix's reliability standards, we should implement circuit breakers on the payment gateway calls and run chaos experiments to simulate gateway outages during peak billing times, ensuring the system degrades gracefully without corrupting financial records.