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

SeniorPrefer not to say
May 2026

Summary

TikTok SWE interview with a system design question on currency exchange. Not a lot of context in what I was given so I'm filling in some blanks from memory, but it was a solid design round that pushed me on edge cases I hadn't fully thought through.

Questions Asked (1)

Q1

Design a currency exchange system.

System DesignTechnical Trade-offsAPI & Integrations
Author's notes

I started with the obvious stuff, exchange rates, conversion endpoints, maybe a rate-fetching service pulling from some external provider.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements: scope (e.g., real-time rates, historical data, transaction volume), supported currencies, and integration with external rate providers. Then design a scalable, fault-tolerant system that handles rate fetching, caching, conversion, and transaction processing, while discussing trade-offs like consistency vs. availability and latency vs. accuracy.

Pro tip: Emphasize idempotency and exactly-once processing for financial transactions, and discuss how to handle rate fluctuations and stale rates with appropriate caching and fallback mechanisms.

1. Clarify Requirements and Scope

Ask questions to understand functional and non-functional requirements: expected QPS, supported currencies, rate update frequency, consistency needs, and integration with external providers.

2. High-Level Architecture

Outline core components: API gateway, rate service, conversion service, transaction service, database, and cache. Explain data flow for rate updates and currency conversion requests.

3. Deep Dive into Key Components

Detail the rate service (fetching from providers, caching, fallback), conversion logic (handling precision, rounding), and transaction processing (idempotency, consistency).

4. Scalability and Reliability

Discuss scaling strategies (horizontal scaling, sharding), fault tolerance (redundant providers, retries), and monitoring (rate staleness, error rates).

5. Trade-offs and Extensions

Summarize key trade-offs (e.g., consistency vs. latency, cost vs. accuracy) and suggest potential extensions like multi-currency wallets or historical rate analysis.

Key Points to Mention

  • Idempotency and exactly-once processing for financial transactions to prevent duplicate conversions.
  • Caching strategies for exchange rates with TTL and fallback to stale rates during provider outages.
  • Handling currency precision and rounding rules to avoid financial discrepancies.
  • Integration with external rate providers, including rate limiting, failover, and data normalization.
  • Consistency models: strong consistency for transactions vs. eventual consistency for rate updates.
  • Scalability considerations: partitioning by currency pair, read replicas, and asynchronous processing.

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