Projections
The Power of Turning a Stream of Events Into an Entity
When the store retrieves the entity, it runs the projection. But since the entity is event sourced, there's no fixed entity row to retrieve. Instead, the events that pertain to an entity are retrieved, and then processed in order, in order to construct a view of that entity per the things that have happened to it.
The store only retrieves the events that have been recorded since the last retrieval and passes them along to the projection.
The projection has the rules and logic for how an event affects the state of the entity.
Once the new events are applied to the entity, the store caches the current version of the entity in memory, and returns the entity to the caller (usually, a handler).
The store may also optionally cache the entity on disk as a snapshot. By caching the entity on disk, or snapshotting the entity, lengthy streams don't have to be read in their entirety. Instead, the snapshot is retrieved, and only events that have not been applied to that version of the entity are then read and applied.
For more, see the projection user guide.