Ferrite Transactions, implemented by the Atomic Property Envelope (APE), let you update several keys with one commit decision. Readers never observe a partially applied envelope.
When to use transactions
Use a transaction for invariants that cross records: reserve inventory while creating an order, hold a booking slot while recording its owner, or update an account balance with its idempotency ledger.
inventory/product-42 stock: 8 → 7
order/order-91 absent → pending
idempotency/checkout absent → order-91All three changes commit or none do.
Keep the envelope bounded
One envelope can cover 1 to 64 keys. This is a deliberate boundary: model a domain decision, not an unbounded batch job. For large fan-out, commit the authoritative state and outbox record atomically, then process downstream work through a Queue.
Handle conflicts
Transactions use expected versions to reject stale decisions. On conflict, reload the current state and reevaluate the business rule. Do not retry an old decision indefinitely.
The shop uses transactions for guarded inventory and order state. The calendar uses them to ensure two requests cannot own the same time slot.
See Key-value Store for individual records and Workflows for long-running transitions.