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Sig·Software Engineer·Onsite - System Design / Architecture·Senior

Senior
May 2026

Summary

System design round at Sig for a software engineer role. The prompt was a supermarket checkout system, which sounds straightforward until you start pulling on the threads around promotions and state management.

Questions Asked (1)

Q1

Design the software for a supermarket checkout system, covering product lookup, pricing, payment, receipt generation, and register session management. Also discuss how you'd extend it later for loyalty programs, returns, and split payments.

System DesignData ModelingTechnical Trade-offs
Author's notes

I started with the session lifecycle because that felt like the spine of the whole thing: open register, scan items, apply discounts, collect payment, close.

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AI HintsAI Generated

Suggested Approach

Start by clarifying functional and non-functional requirements, then design a modular system with clear separation of concerns (e.g., product catalog, pricing engine, payment processing, receipt generation, session management). Walk through the core checkout flow, then discuss data models and trade-offs, and finally explain how to extend the design for loyalty, returns, and split payments using extensible patterns like strategy or plugin architectures.

Pro tip: Emphasize idempotency and consistency in payment and receipt generation to handle retries and failures gracefully, and mention that you'd use feature flags or modular services to roll out extensions without disrupting existing operations.

1. Clarify Requirements and Scope

Ask about expected scale (transactions per second, number of registers), consistency needs (e.g., inventory accuracy), and integration points (payment gateways, loyalty systems). Define core use cases and constraints.

2. Design Core Components and Data Models

Outline modules: Product Catalog (with SKU lookup), Pricing Engine (handles discounts, taxes), Payment Processor (integrates with external gateways), Receipt Generator, and Register Session Manager. Define key entities like Product, Price, Transaction, Receipt, and Session.

3. Define Checkout Flow and Interactions

Describe the sequence: scan item → lookup product → calculate price → add to cart → apply payment → generate receipt → close session. Highlight error handling, concurrency (e.g., multiple registers), and idempotency for payment and receipt.

4. Discuss Trade-offs and Technology Choices

Compare monolithic vs. microservices, SQL vs. NoSQL for product catalog, synchronous vs. asynchronous payment processing, and consistency models. Justify choices based on requirements.

5. Plan Extensibility for Future Features

Explain how to add loyalty programs (e.g., strategy pattern for discount rules), returns (reverse transaction flow with refund processing), and split payments (payment aggregation with multiple payment methods). Use interfaces and modular design to minimize impact.

Key Points to Mention

  • Modular architecture with clear separation of concerns (e.g., product lookup, pricing, payment, receipt, session management).
  • Data modeling for products, pricing rules, transactions, and receipts, including versioning for price changes.
  • Idempotency and consistency in payment processing and receipt generation to handle retries and failures.
  • Use of design patterns (e.g., Strategy for pricing/loyalty, Observer for receipt generation, State for session management).
  • Extensibility via pluggable components or microservices to add loyalty, returns, and split payments without major refactoring.
  • Trade-offs between consistency and availability (CAP theorem), and between monolithic and distributed architectures.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.