External Callbacks enable you to add Listeners to an ObjectContainer for the following db4o events
QueryStarted and QueryFinished events accept QueryEventArgs as a parameter and can be used to gather query statistics information.
Created, Activated, Deactivated, Updated and Deleted events accept ObjectEventArgs and can be used to gather statistics information or to initiate some special behavior after the action has been taken.
Creating, Activating, Deactivating, Updating and Deleting events accept CancellableEventArgs, Their primary usage is to perform action validity check and to stop the execution if necessary.