Financial Invariants

Workflow

Map the domain: list financial objects in scope, trace where data enters, and document balance source of truth, lifecycle states, identifiers, rounding rules, and audit records.
State the invariants before editing (e.g. "posted ledger entries are immutable"). Inspect current behavior first; ask the user only when code/tests don't reveal the rule.
Test the invariants through public services/APIs/commands/jobs: normal posting, boundary amounts, currency mismatch, invalid state transitions, reversal/correction, retried operations.
Implement defensively: typed value objects/schemas/DB constraints, checks near the write boundary, DB transactions for multi-record writes, domain errors naming the invariant.