---
url: https://nimbus.overlap.at/guide/glossary.md
description: >-
  Pinned vocabulary for Nimbus, CQRS, and Event Sourcing — use these definitions
  literally when reasoning about the framework.
---

# Glossary

## Terms

* **Aggregate**: The consistency boundary for a stream of events on one **subject**; rebuilt by replaying events in order. Nimbus does not ship an `Aggregate` class — the idea is expressed by **state**, **reducers**, pure-core command functions, shell **handlers**, and EventSourcingDB **preconditions** (see [A Note on Aggregates](/guide/in-depth-example.html#a-note-on-aggregates)).
* **Bounded Context**: A DDD boundary within which a particular domain model, vocabulary, and rules apply consistently; in Nimbus, typically represented by a top-level **domain** folder (e.g. `iam`) and its **modules** (see [In Depth Example](/guide/in-depth-example)).
* **CloudEvents**: Industry-standard envelope for **Commands**, **Events**, and **Queries** in Nimbus (`specversion`, `id`, `type`, `source`, `data`, etc.). See [cloudevents.io](https://cloudevents.io/).
* **Command**: A write-side message expressing intent to change state; routed and validated, then handled to produce **events** (see [Commands](/guide/core/commands)).
* **Correlation ID**: Identifier carried across messages and HTTP requests to trace a single operation through the system (see [CorrelationID Middleware](/guide/hono/correlationid)).
* **Core**: See **Pure Core** — command functions, **reducers**, and event definitions live here.
* **CQRS**: Command Query Responsibility Segregation — separate models and paths for writes (**commands**) and reads (**queries**).
* **Domain**: A top-level business area in Nimbus folder layout (e.g. `iam`, `billing`) that groups related **modules** and represents the domain's **bounded context** (see [In Depth Example](/guide/in-depth-example)).
* **Domain Driven Design (DDD)**: Organizing code around business **domains** and **modules** (e.g. `iam/users`) rather than technical layers.
* **Event**: An immutable fact that already happened; appended to the **event log** and used to derive **state** (see [Events](/guide/core/events)).
* **Event Bus**: In-process publish/subscribe for **events** inside one application via `NimbusEventBus` (see [Event Bus](/guide/core/event-bus)). In event-sourced deployments, EventSourcingDB **observers** often play this role across process restarts.
* **Event-Driven Architecture (EDA)**: Style where **commands**, **events**, and **queries** drive behavior and integration; Nimbus is built around this (see [What is Nimbus?](/guide/what-is-nimbus)).
* **Event Log**: Append-only sequence of **events**; the source of truth in **Event Sourcing**. Every other data shape is derived from it (see [In Depth Example](/guide/in-depth-example)).
* **Event Mapping**: Conversion between Nimbus **CloudEvents** and EventSourcingDB records, preserving **correlation ID** and schemas (see [Event Mapping](/guide/eventsourcingdb/event-mapping)).
* **Event Observer**: Background loop that receives matching events from EventSourcingDB, used to build **projections** (see [Event Observer](/guide/eventsourcingdb/event-observer)).
* **Event Sourcing**: Persisting state changes as a sequence of **events**; current **state** is derived by **replay**ing them.
* **EventSourcingDB**: External event store integrated via `@nimbus-cqrs/eventsourcingdb`; persists the **event log**, supports **preconditions**, and delivers events to **observers** (see [EventSourcingDB Package](/guide/eventsourcingdb/)).
* **Handler**: Function registered on a **router**, **event bus**, or **event observer** that processes a **command**, **query**, or **event** matched by **message type**.
* **Hexagonal Architecture**: Ports-and-adapters style; Nimbus **Pure Core** / **Imperative Shell** aligns with it (see [Architecture](/guide/architecture)).
* **Imperative Shell**: The I/O layer (HTTP, databases, messaging) that orchestrates the **pure core** (see [Architecture](/guide/architecture)).
* **Lower Bound**: Observer option marking where to resume reading the **event log** after reconnect or restart, avoiding full **replay** from the beginning (see [Event Observer](/guide/eventsourcingdb/event-observer)).
* **Message Type**: CloudEvents `type` field (reverse-DNS, e.g. `at.overlap.nimbus.invite-user`); the **router** uses it to select a **handler** (see [Commands](/guide/core/commands)).
* **Module**: A DDD slice inside a **domain** for one business concept (e.g. `iam/users`) (see [In Depth Example](/guide/in-depth-example)).
* **Nimbus Exception**: Structured error type in `@nimbus-cqrs/core` with optional HTTP status and details (see [Exceptions](/guide/core/exceptions)).
* **OpenTelemetry**: Industry-standard observability API used by Nimbus for logging, tracing, and metrics (see [Observability](/guide/core/observability)).
* **Preconditions**: EventSourcingDB checks that must pass before events are appended (e.g. `isSubjectPristine`, `isSubjectOnEventId`); enforce the **aggregate** consistency boundary atomically (see [Write Events](/guide/eventsourcingdb/write-events)).
* **Projection**: Read-side function that turns **events** into **read model** documents or views (see [In Depth Example](/guide/in-depth-example)).
* **Pure Core**: Side-effect-free domain logic (commands, **reducers**, event definitions) with no I/O (see [Architecture](/guide/architecture)).
* **Query**: A read-side message requesting data without changing state (see [Queries](/guide/core/queries)).
* **Read Model**: Query-optimized document or view derived from **events** via **projections** (see [Queries](/guide/core/queries)).
* **Read Side**: CQRS path that serves **queries** and runs **projections**; does not append to the **event log** (see [In Depth Example](/guide/in-depth-example)).
* **Reducer**: Pure function that folds an **event** into current **state** (e.g. `applyEventToUserState`) (see [In Depth Example](/guide/in-depth-example)).
* **Replay**: Reading **events** in order to rebuild **state** or refresh a **projection** (see [Read Events](/guide/eventsourcingdb/read-events)).
* **Repository**: Type-safe MongoDB access layer extending `MongoDBRepository` for **read model** persistence (see [Repository](/guide/mongodb/repository)).
* **Router**: Type-safe message dispatcher (`MessageRouter`) that validates and traces **commands**, **queries**, or **events** (see [Router](/guide/core/router)).
* **Shell**: See **Imperative Shell** — command **handlers**, HTTP routes, and persistence live here.
* **State**: Current shape of an **aggregate** after applying all relevant **events** via a **reducer**.
* **Subject**: CloudEvents `subject` — typically identifies the aggregate or entity stream an **event** belongs to (see [Events](/guide/core/events)).
* **Write Side**: CQRS path that accepts **commands**, enforces business rules, and appends **events** to the **event log** (see [In Depth Example](/guide/in-depth-example)).
* **Zod**: Schema library used to validate **commands**, **queries**, and **events** in Nimbus (see [Commands](/guide/core/commands)).