An ExceptionSubscriber is an entity that is informed of exceptions and the
handling policy for exceptions caught by
The ExceptionSubscriber can then take some action, if desired.
For example, actors that communicate with outside clients may want to pass
along information about the error that has occurred, instead of returning
a generic message or a timeout error response.
This follows the Command design pattern, where the invoker is
, the client is
the Ptolemy developer (defines commands by dragging and dropping
attributes into the model), and the receiver is a Ptolemy entity
(such as an attribute that writes to a file, or an actor such as
which sends a retry
message upon exception with restart policy.