Design core bank operations

Q: Design core bank operations

This question evaluates system design competency with emphasis on data modeling, API and identifier semantics, validation rules, return-value conventions, transaction recording, and complexity analysis for an in-memory banking service.

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 medium 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

Design a BankSystem Class (Take‑home Project)

Goal

Design and document a BankSystem class that supports three operations:

createAccount(timestamp, customerId) -> bool
deposit(timestamp, customerId, amount) -> int
pay(timestamp, sourceAccountId, targetAccountId, amount) -> int

Assume this is an in-memory service intended for correctness and clarity (not persistence or distributed systems).

What to Produce

Provide a complete specification and design that covers:

Data Models

Define the core entities (e.g., Account, Transaction) and their fields.
Specify how balances and transaction records are stored and updated.

Identifier Semantics

Clarify what customerId and accountId represent and how they relate (e.g., 1:1 vs. 1:many).
Reconcile that deposit takes customerId while pay uses accountIds.

Validation Rules

Duplicate vs. nonexistent accounts.
Nonpositive amounts (zero or negative).
Insufficient funds for payments.
Any other constraints you deem important (e.g., overflow, same source/target).

Return Values

Define exactly what the bool and int returns mean in all cases (success and error).
If using error codes, enumerate them.

Data Structures

Describe the primary data structures (e.g., hash maps keyed by accountId) used to implement O(1) operations.

Operational Semantics

For each API, describe step-by-step how state changes and transaction records are appended.
Include how timestamps are used in records.

Complexity

Time and space complexity for each operation and overall storage.

Assumptions

State any reasonable assumptions you make (e.g., single-threaded, amounts in cents, integer timestamps).

You may include concise pseudocode to illustrate the implementation.

Design core bank operations

Quick Overview

Design a BankSystem Class (Take‑home Project)

Goal

What to Produce

Solution

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