This post is for Day 15 of Merpay & Mercoin Advent Calendar 2025  , brought to you by  @timo  from the Merpay Balance te

Mercari's platform is a resource for developers and technology enthusiasts, offering insights into engineering challenges, technology stack, and culture at Mercari. Through articles, tech talks, and open-source contributions, Mercari offers insights into building innovative and scalable technology solutions. Developers can learn about Mercari's engineering culture, agile methodologies, and contributions to the broader tech community.

Mercari Engineering

Mercari's Balance team extended their double-entry bookkeeping system from JPY-only to multi-currency support. Key challenges included deciding where to store exchange rates (API at header level, database at source level for future flexibility), implementing flexible validation that accepts different rounding methods within reasonable margins, and designing reversal operations to use output-only exchange rates to prevent ledger imbalances. The team also recognized the importance of tracking both region and currency as separate dimensions, since currency codes alone don't uniquely identify legal jurisdictions.

Extending the Balance Service: Challenges in Implementing Multi-Currency

Challenge 1: Data Modeling for Exchange Rates

Challenge 2: Precision and Validation Logic

Challenge 3: Designing the Reversal Interface