Commands

Chronicle extends the Arc command pipeline with event sourcing behavior. The same model-bound command patterns work here — [Command] records, Handle() methods, CommandValidator — with Chronicle adding automatic event appending, event source identity resolution, and metadata-driven concurrency control.

What Chronicle Adds

A model-bound command handler in the Chronicle context can return events directly from Handle(), and Chronicle appends those events to the event log automatically. This keeps command handlers focused on decisions and domain logic rather than event log plumbing.

Chronicle also resolves the event source identity, event stream metadata, and concurrency scope from the command record itself — either by convention or via explicit attributes and interfaces. This means the same record that defines your command’s shape also carries all the information Chronicle needs to append events correctly.

What Handle() returns decides what happens

Chronicle inspects the value Handle() returns and appends accordingly. Anything it doesn’t recognize as an event (or event metadata) becomes the command’s response to the caller — so a tuple lets you append a fact and hand something back without ever calling the event log yourself.

Handle() returns	What Chronicle does
void / Task	Nothing is appended — the command just ran.
An [EventType] event	Appended to the resolved event source’s log.
IEnumerable<object> of events	Each is appended, in order.
EventForEventSourceId (or a collection of them)	Appended to the event source id carried in the value, overriding the resolved one — for writing to a different or several streams.
Tuple (event, result)	The event is appended; the other element is returned to the caller as the response.
Tuple (EventSourceId, event)	The EventSourceId sets the stream; the event is appended. See Returning EventSourceId.
Tuple (event, Subject)	The event is appended; the Subject is attached as compliance metadata, not returned.
Result<TResult, TError> (e.g. Result<TEvent, ValidationResult>)	A failure short-circuits the command (nothing appended); a success is unwrapped and handled like the rows above.

The rule of thumb: return the fact that happened. The event source id (see Resolving EventSourceId) decides which stream it lands on.

Topics

Topic	Description
Events	Returning events from commands and how Chronicle appends them automatically, including stream metadata.
Setting Subject	Supplying a compliance subject on the command or by returning it from Handle().
Returning EventSourceId	Explicitly deciding the event source id by returning it from a command tuple.
Concurrency	Declaring a concurrency scope on commands using metadata attributes and interfaces.