Delete handles "delete" events.

Insert handles "insert" events.

Update handles "update" events.

AggregateID is the global AggregateID for Device Aggregate.

DatabaseError is when some operation related to Database, such as insert or find, goes wrong and the task cannot proceed.

InternalError represents an error when something goes wrong, and its our fault.

UserError occurs when there's an error because of user's action.

Device defines the Device Aggregate.