freeeの会計データ整合性 — 仕訳バランスの自動検証

freee is a cloud accounting service used widely from sole proprietors to mid-sized companies. Accounting data must not be off by even one yen, and accuracy that withstands tax audits and external audits is required. This article organizes how freee guarantees the integrity of journal entry data.

The basic principle of double-entry bookkeeping is that every transaction is recorded with the same amount on both the debit and credit sides. At freee, when writing a journal entry, both database constraints and the application layer verify within the same transaction that debit totals match credit totals.

When importing data from bank APIs or payment services, there is a risk that the same transaction could be registered twice due to network failures or retries. freee applies a unique constraint on the external system's transaction ID as an idempotency key, designed so that no matter how many times a request arrives, the result converges to a single posting.

Even when accounting data is edited or deleted, its history must remain completely. freee adopts append-only event logs and retains all changes in immutable form. The ability to fully reproduce later who changed which field, when, and from what value to what value is the precondition of audit compliance.

Errors are sometimes found after monthly or annual closings have ended. Directly rewriting an already-finalized period is forbidden; instead, the correct handling is to add a "correction journal entry" in the current period. freee's closing-lock feature excludes writes to past periods, and corrections are always recorded as new journal entries.

In addition to write-time verification, freee runs a nightly batch that recalculates account balances for all tenants and compares them against the existing balance table. When a difference occurs, an alert fires immediately and the cause is traced. This is the last line of defense to "notice even when there is a bug in the write path."

When a reconciliation difference appears, thoughtlessly rewriting the balance table is absolutely avoided. The journal entry causing the discrepancy is always identified and repaired through the normal correction flow. The more manual intervention is inserted, the higher the risk that data consistency is impaired.

To safely handle simultaneous writes to the same account, freee uses row locks keyed by tenant ID and account ID. Balance updates are always performed after acquiring this lock, designed so that even if interruptions occur between read and write, the result is not broken.

freee's integrity strategy is not a single silver bullet, but a combination of layered defenses: write-time verification, idempotency keys, append-only audit logs, closing locks, and nightly recalculation. In systems handling financial data, multi-layered defense — where other layers can catch failures even if one layer fails — is essential.