##### Question Design an account management system that supports the following operations with correct return values and constraints: 1) createAccount(int timestamp, String accountId) – create an account, return false if account exists, otherwise true; 2) deposit(int timestamp, String accountId, int amount) – deposit into an existing account, return Optional.of(balance) on success, Optional.empty() if account does not exist; 3) transfer(int timestamp, String sourceAccountId, String targetAccountId, int amount) – transfer between two distinct existing accounts when source has sufficient balance, return Optional.of(newBalance) on success, Optional.empty() otherwise; 4) List topSpenders(int timestamp, int n) – return the n accounts with the highest total outgoing transfer amounts (or all if fewer than n); 5) Optional pay(int timestamp, String accountId, int amount) – deduct amount from an account, return unique paymentId on success, Optional.empty() on failure (account missing or insufficient funds), automatically schedule 2% cashback after 24 hours which does not count toward topSpenders; 6) Optional getPaymentStatus(int timestamp, String accountId, String paymentId) – query payment status, returning IN_PROGRESS or CASHBACK_RECEIVED when applicable, or Optional.empty() for invalid account/paymentId.

This question evaluates concurrent system design competencies including atomic balance updates, delayed event scheduling (cashback), transactional correctness under concurrency, and leaderboard aggregation for transfer spenders.

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Design account system with cashback | Coinbase Interview Question

System Design: Account Management with Transfers, Payments, and Cashback

You are to design and implement an in-memory account management service that supports atomic balance updates, delayed cashback, and a leaderboard of top transfer spenders. Assume integer timestamps (e.g., seconds since epoch) and integer amounts in the smallest currency unit (e.g., cents).

Assumptions to make explicit:

Amounts must be positive integers; operations with non-positive amounts fail.
Timestamps are provided by the caller and should be used to determine due cashback.
2% cashback is computed as floor(0.02 × amount) (integer math: (amount * 2) / 100 ).
Return Optional.empty() for any failure condition (e.g., missing account, insufficient funds, invalid input), unless otherwise stated.
Transfer spending for topSpenders counts only outgoing transfer amounts from the transfer operation and excludes pay and cashback.

Implement the following operations with the specified semantics:

createAccount(int timestamp, String accountId)
- Create a new account with 0 balance.
- Return false if the account already exists; otherwise true.
deposit(int timestamp, String accountId, int amount)
- Deposit amount into an existing account.
- Return Optional.of(newBalance) on success.
- Return Optional.empty() if the account does not exist or amount ≤ 0.
transfer(int timestamp, String sourceAccountId, String targetAccountId, int amount)
- Transfer amount between two distinct existing accounts if the source has sufficient balance.
- On success, return Optional.of(sourceNewBalance).
- Return Optional.empty() if any precondition fails (missing accounts, identical accounts, amount ≤ 0, insufficient funds).
- Increase only the source account's cumulative outgoing transfer total (used for topSpenders).
topSpenders(int timestamp, int n)
- Return a List <String> of up to n accountIds with the highest cumulative outgoing transfer totals.
- If fewer than n accounts exist, return all.
- Use a deterministic tie-break (e.g., lexicographic accountId ascending).
pay(int timestamp, String accountId, int amount)
- Deduct amount from the account if sufficient funds; return Optional.of(uniquePaymentId) on success.
- Return Optional.empty() on failure (missing account, amount ≤ 0, insufficient funds).
- Automatically schedule 2% cashback after 24 hours (86,400 seconds) from the provided timestamp.
- The cashback credit must not affect topSpenders.
getPaymentStatus(int timestamp, String accountId, String paymentId)
- Return Optional.of("IN_PROGRESS") if the cashback has not yet been applied.
- Return Optional.of("CASHBACK_RECEIVED") after the cashback is applied.
- Return Optional.empty() if the account or paymentId is invalid or mismatched.

Additional requirements:

Ensure atomicity and correctness under concurrency (e.g., transfers must not lose or double-count funds).
Ensure each call first processes any due cashbacks up to the provided timestamp.

System Design: Account Management with Transfers, Payments, and Cashback

Assumptions to make explicit:

Amounts must be positive integers; operations with non-positive amounts fail.

Timestamps are provided by the caller and should be used to determine due cashback.

2% cashback is computed as floor(0.02 × amount) (integer math: (amount * 2) / 100 ).

Return Optional.empty() for any failure condition (e.g., missing account, insufficient funds, invalid input), unless otherwise stated.

Transfer spending for topSpenders counts only outgoing transfer amounts from the transfer operation and excludes pay and cashback.

Implement the following operations with the specified semantics:

createAccount(int timestamp, String accountId)

Create a new account with 0 balance.
Return false if the account already exists; otherwise true.

deposit(int timestamp, String accountId, int amount)

Deposit amount into an existing account.
Return Optional.of(newBalance) on success.
Return Optional.empty() if the account does not exist or amount ≤ 0.

transfer(int timestamp, String sourceAccountId, String targetAccountId, int amount)

Transfer amount between two distinct existing accounts if the source has sufficient balance.
On success, return Optional.of(sourceNewBalance).
Return Optional.empty() if any precondition fails (missing accounts, identical accounts, amount ≤ 0, insufficient funds).
Increase only the source account's cumulative outgoing transfer total (used for topSpenders).

topSpenders(int timestamp, int n)

Return a List <String> of up to n accountIds with the highest cumulative outgoing transfer totals.
If fewer than n accounts exist, return all.
Use a deterministic tie-break (e.g., lexicographic accountId ascending).

pay(int timestamp, String accountId, int amount)

Deduct amount from the account if sufficient funds; return Optional.of(uniquePaymentId) on success.
Return Optional.empty() on failure (missing account, amount ≤ 0, insufficient funds).
Automatically schedule 2% cashback after 24 hours (86,400 seconds) from the provided timestamp.
The cashback credit must not affect topSpenders.

getPaymentStatus(int timestamp, String accountId, String paymentId)

Return Optional.of("IN_PROGRESS") if the cashback has not yet been applied.
Return Optional.of("CASHBACK_RECEIVED") after the cashback is applied.
Return Optional.empty() if the account or paymentId is invalid or mismatched.

Additional requirements:

Ensure atomicity and correctness under concurrency (e.g., transfers must not lose or double-count funds).

Ensure each call first processes any due cashbacks up to the provided timestamp.

Design account system with cashback

Quick Overview

System Design: Account Management with Transfers, Payments, and Cashback

Solution

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Design account system with cashback

Quick Overview

System Design: Account Management with Transfers, Payments, and Cashback

Solution

Comments (0)