# Author a product `one.product@1` is One's concise omission-first language for complete product meaning. Its declaration heads are ordinary domain exports, not parser keywords. Other domains may inherit the whole language or expose a different vocabulary through the same `.one` envelope. ## Define nominal values and faults ```one type OrderId@1 = Text @Length(1..=64) enum OrderStatus@1 pending placed cancelled record LineItem@1 sku: Sku quantity: UInt @Value(1..=100) unit_price: Money record Order@1 tenant: TenantId @Key @Scope id: OrderId @Key items: List @Cardinality(1..=100) total: Money status: OrderStatus @Invariant(total == sum(items[*].unit_price * items[*].quantity)) error PlaceOrderFault@1 duplicate invalid_catalog_item(sku: Sku) price_changed(expected: Money, actual: Money) ``` Types and declarations are nominal. Structurally identical records under different owner-qualified identities remain different. Faults are part of the operation contract and stay distinct from denial, timeout, cancellation, provider failure, defects, and unknown outcomes. ## Declare operations and services ```one service Orders@1 query get input tenant: TenantId input id: OrderId returns Order faults GetOrderFault command place input request: PlaceOrder returns Order faults PlaceOrderFault ``` Queries and commands state semantic behavior, inputs, outputs, faults, effects, authority, idempotency, deadlines, and recovery where applicable. They do not choose HTTP, messaging, a process boundary, a provider, or placement. ## Declare state and events ```one state OrdersState@1 stores Order consistency: serializable durability: durable event OrderPlaced@1(Order) event OrderCancelled@1(Order) ``` State declares logical keys, scope, transactions, queries, change behavior, retention, evolution, continuity, and recovery. It does not name tables, indexes, partitions, SQL, storage files, or database handles. An event records owned meaning; publication semantics and delivery guarantees remain explicit Communication contracts. ## Coordinate durable work ```one workflow FulfilOrder@1 input order: Order returns Shipment reservation = Inventory.reserve(order) payment = Billing.capture(order) compensate reservation with Inventory.release(reservation) if payment is domain_fault shipment = Shipping.create(order, reservation) return shipment recovery BillingCaptureRecovery@1 operation: Billing.capture correlation: Billing.capture.order_id observe: Billing.lookup ``` A workflow preserves exact version, steps, nondeterministic inputs, effects, outcomes, timers, approvals, checkpoints, compensation, and reconciliation. Compensation is a new effect, not erased history. Recovery definitions explain how to resolve an uncertain dispatch without repeating it blindly. Jobs, schedules, streams, human tasks, and agents use the same explicit completion, cancellation, budget, authority, and evidence model. An agent's model output is untrusted proposal data; tools are typed operations rather than credentials. ## Compose components, roots, and boundaries ```one component OrdersApi@1 realizes Orders component OrderWorkers@1 realizes FulfilOrder boundary PublicOrdersHttp@1 projects Json(Orders) authentication: CommercePolicy base_path: "/v1" ``` The system/root declaration lives in its own owner-governed semantic item: ```one one 1 semantic acme.system domain one.system@1 component CommerceApi@1 owner acme.principal#Commerce@1 provide service acme.orders#Orders@1 require one.deployment.local.release#Apply@1 require one.history.durable#Append@1 system Commerce@1 root commerce_api include CommerceApi build development build release build reproducible ``` Components state uses and realization obligations without choosing local, process, Wasm, or remote topology. A boundary deliberately projects an owned contract to HTTP, messaging, a client, an SDK, or another physical form and retains every conversion and loss. Each application, worker, migration, operator, client, and tool root receives an independent closure. ## State policy, privacy, and tenancy ```one policy CommercePolicy@1 profile TenantRoles allow roles owner operator operations Orders.get Orders.place privacy CustomerOrderPrivacy@1 classify Order.customer as personal purpose: order_fulfilment retain Order for 7y because legal_obligation erase lineage_from Order.customer on valid_request export_within: 30d ``` Policies declare requirements and decisions; they do not issue grants. Privacy, tenancy, retention, residency, erasure, export, and audit obligations remain explicit product meaning and flow into Planning, Authority, Data, History, and observation requirements. ## Define environments and objectives ```one environment Local@1 profile LocalDevelopment expose PublicOrdersHttp at http("127.0.0.1:8080") environment Production@1 profile EuProduction expose PublicOrdersHttp at https("api.acme.example") monthly_cost: <= 5_000USD availability: >= 99.95% release_requires: BillingUnknownOutcome ``` Environments contain portable desired state, quality, cost, assurance, placement, observation, and release requirements. They do not contain secret bytes, issued grants, live provider handles, or mutable observations. Profiles package repeated owner meaning but never hide weaker guarantees or choose a provider instance. ## Author tests, simulations, and release gates ```one simulation BillingUnknownOutcome@1 interrupt Billing.capture after dispatch before acknowledgement then Billing.lookup returns committed assert FulfilOrder does_not_redispatch Billing.capture ``` Tests and simulations bind exact source, lock, fixtures, time, entropy, scheduler, faults, provider responses, assumptions, and expected owner results. They prove only their declared scope. Release gates may require conformance, security, accessibility, performance, recovery, and observation evidence for the exact release. ## Keep omission honest Omission means the selected exact domain rule supplies a deterministic default or derivation. It never means “whatever the provider does.” Inference can prove consequences and add conservative requirements; it cannot invent business identity, consent, legal basis, provider selection, authority, accepted loss, or absence outside a proven boundary. Next: learn how product declarations lower into [contracts and components](/one/model/contracts-and-components).