Actor Garbage Definition

The definition of actor garbage relates to meaningful computation. Meaningful computation is defined as having the ability to communicate with any of the root actors, that is, to access any resource or public service. The widely used definition of live actors is described in [4]. Conceptually, an actor is live if it is a root or it can either potentially: 1) receive messages from the root actors or 2) send messages to the root actors. The set of actor garbage is then defined as the complement of the set of live actors. To formally describe the SALSA actor garbage collection (GC) model, we introduce the following definitions:

• Blocked actor: An actor is blocked if it has no pending messages in its message box and it is not processing any message. Otherwise it is unblocked.

• Reference: A reference indicates an address of an actor. Actor can only send messages to actor if has a reference pointing to .

• Inverse reference: An inverse reference is a conceptual reference from the target of a reference to its source.

• Acquaintance: Let actor have a reference pointing to actor . is an acquaintance of , and is an inverse acquaintance of .

• Root actor: An actor is a root actor if it encapsulates a resource, or if it is a public service -- such as I/O devices, web services, and databases (i.e., a service actor).

The original definition of live actors is denotational because it uses the concept of ``potential'' message delivery and reception. To make it more operational, we use the term ``potentially live'' [2] to define live actors.

• Potentially live actors:
  • Every unblocked actor and root actor is potentially live.
  • Every acquaintance of a potentially live actor is potentially live.
• Live actors:
  • A root actor is live.
  • Every acquaintance of a live actor is live.
  • Every potentially live, inverse acquaintance of a live actor is live.