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This is a hard difficulty System Design question, commonly asked during Technical Screen rounds at OpenAI.

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

Design credit balance with vector-clock expirations

Quick Overview

This question evaluates understanding of distributed-systems concepts such as vector clocks, partial orders, concurrency control and idempotency, plus practical competencies in data structures and algorithms for efficient per-user state management and correctness under concurrent operations.

Credit Balance Service with Vector-Clock Expiries

You are designing a backend service that maintains per-user promotional credits. Credits are granted as independent buckets, each with an expiry time represented by a vector clock (no wall-clock time exists). Clients issue operations with an attached vector clock.

Implement the service and explain your design.

Operations

add(userId, creditId, amount, expiryVc)
- Adds a credit bucket with a unique creditId, positive amount, and an expiry represented by a vector clock expiryVc.
use(userId, amount, atVc)
- Spends credits at logical time atVc. The service must consume from credits that expire soonest first.
- A credit is unavailable (expired) for this spend if expiryVc <= atVc under vector-clock partial order. Expired credits cannot be used.

Requirements

Specify APIs (request/response, idempotency), core data structures, and algorithms to support efficient inserts and spends across many active credits per user.
Define how you compare vector clocks (<=, <, ==, concurrent) and how you impose a deterministic total order among credits when expiries are partially ordered (concurrent). State tie-break policies.
Describe safeguards against double-spend, against using expired credits, and how you handle identical expiries.
Provide complexity analysis.
Walk through test cases, including edge cases: concurrent expiries, expiry exactly at atVc, identical expiries, zero/negative amounts, insufficient balance.

Assumptions you may make:

Vector clocks are maps from processId -> integer counter; missing components are treated as 0.
The service processes operations per user on a single shard/actor (or uses equivalent transactional concurrency control) so that each user's operations are linearizable.

Quick Overview

Credit Balance Service with Vector-Clock Expiries

Implement the service and explain your design.

Operations

add(userId, creditId, amount, expiryVc)

Adds a credit bucket with a unique creditId, positive amount, and an expiry represented by a vector clock expiryVc.

use(userId, amount, atVc)

Spends credits at logical time atVc. The service must consume from credits that expire soonest first.
A credit is unavailable (expired) for this spend if expiryVc <= atVc under vector-clock partial order. Expired credits cannot be used.

Requirements

Specify APIs (request/response, idempotency), core data structures, and algorithms to support efficient inserts and spends across many active credits per user.

Define how you compare vector clocks (<=, <, ==, concurrent) and how you impose a deterministic total order among credits when expiries are partially ordered (concurrent). State tie-break policies.

Describe safeguards against double-spend, against using expired credits, and how you handle identical expiries.

Provide complexity analysis.

Walk through test cases, including edge cases: concurrent expiries, expiry exactly at atVc, identical expiries, zero/negative amounts, insufficient balance.

Assumptions you may make:

Vector clocks are maps from processId -> integer counter; missing components are treated as 0.

The service processes operations per user on a single shard/actor (or uses equivalent transactional concurrency control) so that each user's operations are linearizable.

Design credit balance with vector-clock expirations

Quick Overview

Credit Balance Service with Vector-Clock Expiries

Operations

Requirements

Solution

Submit Your Answer to Earn 20XP

Design credit balance with vector-clock expirations

Quick Overview

Credit Balance Service with Vector-Clock Expiries

Operations

Requirements

Solution

Submit Your Answer to Earn 20XP