Actors systems can span complex communication graphs that make it hard to decide when actors are no longer needed. As a result, manually managing lifetime of actors is merely impossible. For this reason, CAF implements a garbage collection strategy for actors based on weak and strong reference counts.
Smart Pointers to Actors¶
In CAF, we use a different approach than the standard library because (1) we
always allocate actors along with their control block, (2) we need additional
information in the control block, and (3) we can store only a single raw
pointer internally instead of the two raw pointers
needs. The following figure summarizes the design of smart pointers to actors.
strong_actor_ptr instead of
weak_actor_ptr instead of
std::weak_ptr<...>. Unlike the counterparts from the standard
library, both smart pointer types only store a single pointer.
Also, the control block in CAF is not a template and stores the identity of an
node_id). This allows CAF to
access this information even after an actor died. The control block fits
exactly into a single cache line (64 Bytes). This makes sure no false
sharing occurs between an actor and other actors that have references to it.
Since the size of the control block is fixed and CAF guarantees the
memory layout enforced by
actor_storage, CAF can compute the
address of an actor from the pointer to its control block by offsetting it by
64 Bytes. Likewise, an actor can compute the address of its control block.
The smart pointer design in CAF relies on a few assumptions about actor types.
Most notably, the actor object is placed 64 Bytes after the control block. This
starting address is cast to
must be convertible to
reinterpret_cast. In practice, this means actor subclasses must not
use virtual inheritance, which is enforced in CAF with a
Strong and Weak References¶
A strong reference manipulates the
strong refs counter as shown above. An
actor is destroyed if there are zero strong references to it. If two actors
keep strong references to each other via member variable, neither actor can ever
be destroyed because they produce a cycle (see Breaking Cycles Manually). Strong
references are formed by
typed_actor<...> (see Actor References).
A weak reference manipulates the
weak refs counter. This counter keeps
track of how many references to the control block exist. The control block is
destroyed if there are zero weak references to an actor (which cannot occur
strong refs reached zero as well). No cycle occurs if two actors
keep weak references to each other, because the actor objects themselves can get
destroyed independently from their control block. A weak reference is only
actor_addr (see Address).
Converting Actor References with
actor_cast converts between actor pointers and
handles. The first common use case is to convert a
typed_actor<...> before being able
to send messages to an actor. The second common use case is to convert
strong_actor_ptr to upgrade a weak
reference to a strong reference. Note that casting
actor_addr to a
strong actor pointer or handle can result in invalid handles. The syntax for
actor_cast resembles builtin C++ casts. For example,
x to an handle of type
Breaking Cycles Manually¶
Cycles can occur only when using class-based actors when storing references to
other actors via member variable. Stateful actors (see Attaching Cleanup Code to Actors)
break cycles by destroying the state when an actor terminates, before the
destructor of the actor itself runs. This means an actor releases all references
to others automatically after calling
quit. However, class-based actors have
to break cycles manually, because references to others are not released until
the destructor of an actor runs. Two actors storing references to each other via
member variable produce a cycle and neither destructor can ever be called.
Class-based actors can break cycles manually by overriding
destroy(x) on each handle (see Handle). Using a handle
after destroying it is undefined behavior, but it is safe to assign a new value
to the handle.