Start by clarifying requirements and scale, then design the data model and API for creating/canceling schedules. Describe the worker architecture for triggering charges, focusing on idempotency and failure handling, and finally cover history viewing and scaling considerations.
Pro tip: Emphasize idempotency and exactly-once processing by using idempotency keys and transactional outbox patterns; this shows you understand real-world payment system challenges.
Ask about expected user base, donation volume, payment processor SLAs, and whether schedules can be modified. Establish assumptions for scale (e.g., millions of users, thousands of charges per second).
Define entities: User, Charity, DonationSchedule, Charge, PaymentMethod. Design RESTful APIs for creating and canceling schedules, including request/response formats and error codes.
Describe a distributed scheduler (e.g., cron-based or queue-based) that polls due schedules and enqueues charge jobs. Ensure idempotency by using unique job IDs and idempotency keys with the payment processor.
Outline retry logic with exponential backoff, dead-letter queues, and reconciliation with the payment processor. Discuss how to handle partial failures and notify users.
Design an API for users to view donation history, possibly with pagination and filtering. Discuss scaling strategies: sharding by user ID, read replicas, caching, and partitioning schedules by time buckets.
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