--- title: Glossary description: One precise line per Arc.Kotlin term, cross-referenced to the guide or reference page that covers it in full. --- ## Commands and queries - **Command** — a class or record annotated `@Command` expressing an imperative intent; its public `handle` method produces a response, appends events, or both. See [Create and validate commands](/arc/backend/kotlin/guides/commands/). - **Handle** — the public instance method KSP requires on a `@Command`. May be synchronous, `suspend`, or return a Java `CompletionStage`. - **Provide** — the optional method that runs after validation and authorization, before `handle`, to fetch or compute data `handle` needs. See [Prepare handler values](/arc/backend/kotlin/guides/commands/#prepare-handler-values). - **Command key (`@CommandKey`)** — the property that identifies the command's subject; required for a command to return a plain Chronicle event. See [Return an event](/arc/backend/kotlin/guides/chronicle/#return-an-event). - **Read model** — a class or record annotated `@ReadModel` whose static or companion methods are queries. See [Expose one-shot and observable queries](/arc/backend/kotlin/guides/queries/). - **Query** — a static (Java) or `@JvmStatic` companion (Kotlin) method on a `@ReadModel`. One-shot queries return a value; observable queries return `Flow` or `Flow.Publisher`. - **Observable query** — a query whose return type is `Flow`/`Flow>` or `Flow.Publisher`/`Publisher>` (or, with the optional `arc-rxjava3` artifact, an RxJava 3 type). Arc hosts it as an HTTP snapshot, direct SSE, direct WebSocket, or a multiplexed hub. See [Declare observable queries](/arc/backend/kotlin/guides/observable-queries/). - **`@FromServices`** — marks a query parameter as a dependency resolved from Spring, distinguishing it from an unannotated caller argument. ## Generated artifacts - **KSP** — [Kotlin Symbol Processing](https://kotlinlang.org/docs/ksp-overview.html), the compiler plugin that reads `@Command`/`@ReadModel` declarations and generates handlers, performers, and the artifact manifest at compile time. Arc's processor is `io.cratis:arc-ksp`. See [Configuration reference](/arc/backend/kotlin/reference/configuration/) for the application-facing surface. - **Artifact manifest** — the language-neutral JSON document (`META-INF/cratis/arc/*.json`) KSP emits per module, consumed by the TypeScript proxy generator. See [Configuration reference](/arc/backend/kotlin/reference/configuration/). - **Proxy** — the generated TypeScript client for a command, query, model, interface, or enum, carrying a `// @generated by Cratis` header. See [Generate TypeScript proxies](/arc/backend/kotlin/guides/typescript-proxies/). - **`ARCKSP` diagnostic** — a stable compile-time error or warning code KSP reports, for example `ARCKSP0101` (unsupported command declaration). Cataloged in [KSP diagnostics](/arc/backend/kotlin/reference/diagnostics/). - **Concept (`ConceptAs`)** — a Kotlin class or Java record implementing `ConceptAs` that wraps a single scalar value. Serializes as its underlying value, not as a wrapper object, across commands, queries, models, validation, TypeScript, OpenAPI, and Chronicle command keys. ## Validation - **`ConceptValidator`** — a reusable, server-only rule applied everywhere a specific concept type appears in a command or query graph. See [Add validation](/arc/backend/kotlin/guides/commands/#add-validation). - **`ModelValidator`** — a server-only rule for one exact runtime model class, applied to command roots, nested models, and supplied query arguments. See [Reuse model validation](/arc/backend/kotlin/guides/commands/#reuse-model-validation). - **`FluentModelValidator`** — a bounded, literal-rule validator whose rules are also emitted into the generated TypeScript client, so the same constraint runs in the browser and on the server. See [Share fluent validation](/arc/backend/kotlin/guides/validation/). - **`@IgnoreValidation`** — cuts one member's validation edge before access while leaving serialization and binding unchanged. See [Ignore a validation member edge](/arc/backend/kotlin/reference/validation/#ignore-a-validation-member-edge). ## Identity, tenancy, and transport - **`AuthenticationHandler`** — an ordered Kotlin or Java (`AsyncAuthenticationHandler`) bean that inspects a request and returns an authenticated, failed, or anonymous result. See [Register a Kotlin authentication handler](/arc/backend/kotlin/guides/security/#register-a-kotlin-authentication-handler). - **`TenantIdResolver`** — resolves the current request's tenant from a header, query parameter, claim, subdomain, or fixed/development value. See [Configuration reference](/arc/backend/kotlin/reference/configuration/). - **`CommandResult` / `QueryResult`** — the JSON envelopes every command and query response uses. See the [HTTP contract reference](/arc/backend/kotlin/reference/http-contract/). - **Correlation ID** — the `X-Correlation-ID` value the Spring Boot starter establishes once per request, for every route in the host, Arc-owned or not. See [Correlation](/arc/backend/kotlin/reference/http-contract/#correlation). ## Chronicle (optional integration) - **Chronicle** — [Cratis's event-sourcing database and runtime](https://github.com/Cratis/Chronicle), consumed here through [Chronicle.Kotlin](https://github.com/Cratis/Chronicle.Kotlin). Optional: `io.cratis:arc` has no dependency on it. See [Integrate commands and read models with Chronicle](/arc/backend/kotlin/guides/chronicle/). - **Event** — an immutable Chronicle `@EventType` fact returned from a command's `handle` and appended to an event stream. - **Reactor** — a Chronicle artifact that observes events; `ChronicleCommandSideEffectHandler` lets one hand registered Arc command values to the real command pipeline as a side effect. See [Execute reactor command side effects](/arc/backend/kotlin/guides/chronicle/#execute-reactor-command-side-effects). - **`EventsWithConcurrencyScopes`** — a typed command response that attaches exact per-event-source Chronicle concurrency scopes to a batch of routed events. See [Attach exact concurrency scopes](/arc/backend/kotlin/guides/chronicle/#attach-exact-concurrency-scopes). ## Where to go next - [Feature parity reference](/arc/backend/kotlin/reference/parity/) — the evidence-backed status of every one of these terms in the current JVM implementation. - [Annotation reference](/arc/backend/kotlin/reference/annotations/) — the exact contract of every annotation named above.