Glossary

Event sourcing comes with its own vocabulary, and the Cratis stack adds a few terms of its own. This is the one place each term is defined — one term, one meaning, everywhere. Where a term has a fuller explanation, the name links to it.

Event sourcing

These are the core ideas behind Chronicle. The Concepts section explains how they fit together.

Term	Definition
Event	A fact — something that happened, named in the past tense (AccountOpened). Immutable, never nullable, never multipurpose.
Event type	The schema and identity of an event — its shape and name.
Event source	The thing an event happened to, identified by an id (an account, a book).
Event sequence	An ordered, append-only stream of events you can subscribe to. The event log is the primary one — your source of truth.
Sequence number	An event’s position within a sequence.
Event store	The database that holds event sequences.
Event sourcing	Storing state as the full history of events, deriving current state by replaying them. See when to use it.
Namespace	A partition of an event store, used for multi-tenancy.
Tenant	An isolated set of data for one customer — see namespaces.
Identity	Who or what caused an event.
Correlation	Links events that belong to the same logical operation.
Causation	Links an event to the event that caused it.
Tags	Labels on events for filtering and correlation — see also event metadata tags.
Aggregate	A consistency boundary that encapsulates behavior and produces events.

Turning events into state

How events become the things you read and the actions you take.

Term	Definition
Observer	Anything that watches events and acts — a projection, reducer, or reactor.
Projection	Builds a read model by mapping events declaratively.
Reducer	Builds a read model by folding events imperatively.
Reactor	Produces side effects (notifications, calls to other systems) in response to events.
Read model	A queryable view shaped for one screen or question, built from events.
Changeset	The set of changes an observer applies for a single event.
Eventual consistency	A read model that catches up shortly after an event is appended — the default.
Immediate consistency	A read model updated synchronously, before the append returns — for reads you must get right now.

Full-stack: CQRS and the frontend

The terms Arc and Components add on top of Chronicle.

Term	Definition
Command	An intent to change state — a record with a Handle() method that appends events.
Query	A read of data, exposed to the frontend as a typed proxy.
Observable query	A query that holds a live connection and pushes new results when the data changes.
CQRS	Command Query Responsibility Segregation — separating the write side (commands) from the read side (queries).
Proxy generation	Arc emitting a typed TypeScript client from your C# commands and queries at build time.
Concept	A strongly-typed wrapper around a primitive (AccountId over Guid) so the compiler catches mix-ups.
Vertical slice	Everything for one behavior — command, events, projection, UI, specs — kept together in one folder.

New to all this? Start with Why developers choose Cratis, then the Chronicle tutorial.