Design and implement an in-memory bank system. Provide an interface/class BankSystem with 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). Define the data model, invariants (e.g., no negative balances), return semantics (e.g., new balance vs. error codes), and time/space complexity. Use hash maps (not lists) for constant-time account and transaction lookups. Address edge cases: duplicate or out-of-order timestamps, insufficient funds, unknown accounts, idempotency of retries, and basic concurrency assumptions.

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.

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Design a basic bank system API | Meta Interview 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.

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.

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

Submit Your Answer

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

Submit Your Answer