Managed (Local) Projection

ph lm

As a specialization of ManagedProjection, a LocalManagedProjection lives within the application process and does not use shared external databases to maintain its state. Relying on the locality, locking and keeping track of the state (position in the event stream) is just a matter of synchronization and an additional field, all being implemented in the abstract class LocalManagedProjection that you are expected to extend.

public class UserCount extends LocalManagedProjection {

    private int users = 0;

    @Handler
    void apply(UserCreated created) {
        users++;
    }

    @Handler
    void apply(UserDeleted deleted) {
        users--;
    }

    int count() {
        return users;
    }

}

As you can see, handling the WriterToken and the state management are taken care of for you, so that you can just focus on the implementation of the projection.

Due to the simplicity of use, this kind of implementation would be attractive for starting with for non-aggregates, assuming the data held by the Projection is not huge.