Message Handlers

The conventional way for an actor to receive messages delivered to its ports is to have threads explicitly express receive requests. An alternative to this method is the use of message handlers. Instead of explicitly creating threads, an actor can attach a handler (a routine in its address space) to the port. When a message is delivered to the port, the handler is executed within the context of a thread provided by the microkernel.

Message handlers and explicit receive requests are exclusive. When a message handler has been attached to a port, any attempt by a thread to receive a message on that port will return an error.

The use of message handlers is restricted to supervisor actors. Message handlers enable significant optimization of the RPC protocol when both client and server reside on the same site, thereby avoiding thread context switches and memory copies. From the point of view of the microkernel, the client thread is used to run the handler and copying of the message into microkernel buffers is avoided.

The method in which messages are consumed (threads or handlers) is completely transparent to the client (the sender of the message). The strategy is selected on the server only.