Message Handlers

As described in the preceding section, the conventional way for an actor to consume messages delivered to its ports is for threads to express receive requests explicitly on those ports. An alternative to this scheme is the use of message handlers. Instead of creating threads explicitly, 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 in 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 returns an error.

The use of message handlers is restricted to supervisor actors. This allows significant optimization of the RPC protocol when both the client and server reside on the same site, avoiding thread context switches (from the microkernel point of view, the client thread is used to run the handler) and memory copies (copying the message into microkernel buffers is avoided). The way messages are consumed by the threads or the handler is totally transparent to the client, the message sender. The strategy is selected by the server only.