🔗 Building a double-entry payment system in Elixir

Multi-tenant platforms which facilitate e-commerce transactions hold people's money. Getting that money to the right people, in the right amounts, at the right time, is the core trust contract with your partners. This is the most critical and sensitive component in our system as if it fails, trust erodes quickly.

We needed a payment engine that could collect funds from multiple sources, track what's owed across different transaction types, batch payments to minimize fees, and handle the messy reality of refunds, multi-currency, and timing mismatches.

This post covers how we:

1. Implementing double-entry bookkeeping in software: advantages, challenges, and key design decisions
2. Database schema design for ledgers, transactions, and journal entries
3. Transaction states, batching strategy, and idempotency
4. The complete payout pipeline from event completion to funds disbursement
5. Extensibility: how the system accommodates new transaction types cleanly through extension