Events and Commands

The heartbeat of Sharpino relies on two fundamental concepts: Commands (the intent to change state) and Events (the immutable record of that change).

Events

Events represent facts that have already occurred. Because they form the historical ledger of the system, they must be immutable and safely serializable. When an Aggregate processes a Command, it yields a list of Events. These events are appended to the Event Store and then processed to derive the Aggregate's new state.

Commands

Commands are functions that encapsulate business intent. They receive the current state of an Aggregate, along with any necessary parameters, validate the business rules, and return a Result. If successful, the Result contains the Events to be committed. Note that for aggregates we need to use AggregateCommand from Sharpino.Core instead of Command.

The De-emphasis of the "Undoer"

Previously, Sharpino supported an "Undoer" mechanism for Commands—an automatic way to generate reverse actions to rollback a command.

In modern implementations, the use of the "Undoer" is heavily de-emphasized. In practice, it is almost always set to None.

Relying on generic "undo" logic often breaks domain semantics. Instead, compensating actions or state reversions should be modeled explicitly as their own domain Commands (e.g., instead of undoing a PayOrder command, issue an explicit RefundOrder command). This keeps the domain model clear, explicit, and intention-revealing.