Design a basic bank system API

Q: Design a basic bank system API

This question evaluates a candidate's ability to design and implement an in-memory banking API, focusing on data modeling, API semantics, invariants (such as monotonic timestamps and non-negative balances), idempotency, and basic concurrency control.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Take-home Project rounds at Meta.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Question

In-Memory Bank System: API, Data Model, Invariants, Semantics, and Edge Cases

Goal

Design and implement an in-memory bank system that supports accounts, deposits, and payments/transfers. The system must be hash-map-based for O(1) average-time lookups and must define behavior for edge cases (timestamps, idempotency, unknown accounts, insufficient funds) and basic concurrency.

API

Interface/class BankSystem with these methods:

bool createAccount(long timestamp, String customerId)
int deposit(long timestamp, String customerId, int amount)
int pay(long timestamp, String sourceAccountId, String targetAccountId, int amount)

Requirements

Data model
- Define account and transaction structures suitable for O(1) lookups via hash maps.
Invariants
- No negative balances.
- Per-account timestamps are monotonic (reject stale/out-of-order ops).
Return semantics
- Specify what the methods return on success and on failure (e.g., new balance vs error codes).
Complexity
- Time and space complexity for each operation.
Edge cases to handle
- Duplicate or out-of-order timestamps.
- Insufficient funds.
- Unknown accounts.
- Idempotency under retries.
- Basic concurrency assumptions.

Constraints

Use hash maps (not lists) for constant-time account and transaction lookups.
The system is in-memory (single process), but may be accessed concurrently by multiple threads.

Design a basic bank system API

Quick Overview

In-Memory Bank System: API, Data Model, Invariants, Semantics, and Edge Cases

Goal

API

Requirements

Constraints

Solution

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