Reading the Akka doc : http://doc.akka.io/docs/akka/2.2.3/AkkaScala.pdf its states in section
2.2.1 Hierarchical Structure
The only prerequisite is to know that each actor has exactly one supervisor,
which is the actor that created it.
But at the top of the hierarchy tree the parent actor has no supervisor ?
It is very well explained in akka doc (see The Top-Level Supervisors section), a little excerpt from it:
The root guardian is the grand-parent of all so-called “top-level” actors and supervises all the special actors mentioned in Top-Level Scopes for Actor Paths using the SupervisorStrategy.stoppingStrategy, whose purpose is to terminate the child upon any type of Exception. All other throwables will be escalated … but to whom? Since every real actor has a supervisor, the supervisor of the root guardian cannot be a real actor. And because this means that it is “outside of the bubble”, it is called the “bubble-walker”. This is a synthetic ActorRef which in effect stops its child upon the first sign of trouble and sets the actor system’s isTerminated status to true as soon as the root guardian is fully terminated (all children recursively stopped).
