[Glossary](https://blnkfinance.com/glossary) ACID # What is ACID in a ledger? ACID ACID is four rules that keep a database write trustworthy: atomicity, consistency, isolation, and durability. In a ledger they mean a post either lands on both sides or on neither, the book stays balanced, two posts cannot corrupt the same balance, and a committed write survives a crash. A payment API that returns 200 is not the same thing as an ACID apply. ## How does ACID work in a ledger? Atomicity keeps the debit and the credit together. Consistency keeps the invariants you set, such as “this wallet cannot go below zero.” Isolation stops two concurrent posts from reading a stale total and both applying. Durability means the applied row is on disk before you treat it as money. ## ACID vs a 200 from the API Many ledgers accept work, queue it, and apply it later. The accept is not the apply. If you credit a customer on the HTTP response, you are treating a receipt as a commit. Wait for the applied record, or hold the amount inflight until you decide. ## What breaks without it? Two payouts read the same available balance and both leave. A crash after the debit and before the credit invents money. A retry without a unique reference posts twice. ACID is the write. The reference is the door. Blnk accepts a transfer as queued, then a worker applies it under locks so the balance write is atomic. The applied row is the commit. See [transaction lifecycle](https://docs.blnkfinance.com/transactions/transaction-lifecycle) and [concurrency](https://docs.blnkfinance.com/guides/concurrency). Related terms - [Ledger](https://blnkfinance.com/glossary/ledger) - [Transaction](https://blnkfinance.com/glossary/transaction) - [Optimistic locking](https://blnkfinance.com/glossary/optimistic-locking) - [Idempotency](https://blnkfinance.com/glossary/idempotency) - [Immutability](https://blnkfinance.com/glossary/immutability)