Splitwise — Notes

Functional

• Add expense (payer + participants + split rule).
• Groups, friends, categories.
• Pair / group balances.
• Settle up (record payments, simplify debts).
• Receipts, comments, reminders.
• Multi-currency.

Non-functional

• Eventual consistency OK; offline-first.
• p99 add expense < 500 ms.
• Reliable balance correctness.

Capacity

• ~50M users, hundreds of millions of expenses.
• Per-user data is small (KB-MB).

Schema

• users(id, name, email, default_ccy)
• groups(id, name, members[])
• expenses(id, group_id, payer_id, amount, ccy, ts, split_method, shares[])
• journal(pair_a, pair_b, amount_ccy, expense_id)

Trade-offs

• Materialized pair balance vs on-the-fly sum: materialize for fast reads; recompute occasionally.
• Currency conversion uses snapshot at create, not display time.
• Concurrent edits: per-expense version + LWW; for groups, CRDT-ish merge.

Refs

• Splitwise engineering posts.
• "Building Splitwise" blog by founders.
• ByteByteGo "Design Splitwise".