Splitwise System Design — Visual Journey

What You Should Understand Now

• An expense is not a payment — it records who paid and who benefited; a balance is only the arithmetic consequence of recorded expenses and settlements, never a thing moved directly.
• Every split must sum to the expense total exactly, in integer minor currency units — never floating point — with a deterministic, documented rule for any rounding remainder.
• Net balance = amount paid on behalf of others minus allocated personal share, adjusted by settlements; within one closed currency ledger, every participant's net balance always sums to zero.
• Debt simplification preserves everyone's net position but not the original pairwise provenance — it is a suggestion that reduces transaction count, never an automatic transfer or cancellation of obligations.
• Corrections and deletes never silently overwrite financial history — they are reversal-plus-replacement events, so the full audit trail is always reconstructable.
• A transactional outbox plus idempotent, event-ID-keyed consumers make at-least-once event delivery safe — the immutable ledger stays the single source of truth, with balance projections as fast, rebuildable, honestly-versioned views of it.

Concepts in This Visual

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