Event-Driven Ledgers Without Tears

Money systems fail in boring ways: double processing, partial writes and balances that drift. Three rules keep ours honest.

1. Append, never update

Every movement is a journal entry. Balances are derived, so history can always be replayed.

2. Idempotency keys everywhere

Consumers will see the same event twice. Store the key with the entry inside the same transaction so a duplicate is a no-op.

3. Reconcile daily

Compare your ledger with the bank's statement every night. Discrepancies should create tickets automatically, not surprise anyone at month-end.

Kafka gives you durability and ordering per partition; correctness is still your job.