Design a banking system with payments and merging

Design a banking system (progressive requirements)

Design the backend for a simplified banking system that supports accounts, money movement, scheduled payments, cashback, ranking, and account merging.

Core requirements

1. Accounts & ledger
   • Create accounts and store balances.
   • Record all transactions with an auditable history (do not lose history).
2. Rank accounts by outgoing transactions
   • Support querying a ranking/leaderboard of accounts based on outgoing transaction volume (e.g., total outgoing amount).
   • Clarify whether ranking is over all time or a time window (assume a configurable window such as last day/week/month).
3. Scheduled payments with cashback + status tracking
   • Users can schedule a payment to execute in the future.
   • Each scheduled payment has a status that can be queried.
   • Payments may provide cashback (e.g., a percentage of the outgoing amount) credited after the payment is executed.
4. Merge two accounts
   • Support merging two accounts while retaining both balances and both transaction histories for audit.
   • After merge, define how future transactions behave (assume: future activity goes to a “merged” view, while original accounts remain queryable/read-only for history).

Non-functional requirements (state assumptions)

• Correctness and consistency are critical (no double-spend).
• Idempotency for payment execution.
• Handle concurrent transfers and scheduled executions.
• Scale to millions of accounts and high transaction throughput.

What to produce

• High-level architecture and key services.
• Data model (tables/entities) and critical fields.
• API sketches for: transfers, scheduled payments, status checks, ranking query, merge.
• Consistency approach and failure handling (retries, idempotency, reconciliation).
• How ranking and cashback are computed efficiently at scale.