How IPC Works

The IPC feature enables threads to exchange messages in either asynchronous or demand/response mode. The demand/response mode is also known as a Remote Procedure Call (RPC).

In asynchronous mode, the sender of an asynchronous message is only blocked during the time of local processing of the message. The system does not guarantee that the message has been deposited at the destination location.

In RPC mode, the RPC protocol enables the construction of client-server applications using a demand/response protocol for the management of transactions. The client is blocked until a response is returned from the server, or a user-defined (optional) timeout occurs. RPC ensures at-most-once semantics for the delivery of the request. It also ensures that the response received by a client originates from the server. The response received by the client must also correspond to the request (and not to a former request to which the response might have been lost).

RPC also enables a client to be unblocked (which will generate an error) if the server is unreachable or has crashed before emitting a response.

Finally, the RPC protocol supports abort propagation. When a thread that is waiting for an RPC reply is aborted, the event will be propagated to the thread which is currently servicing the client request.

A thread that attempts to receive a message on a port is blocked until the new message is received or a until user-defined (optional) time-out occurs.

A thread can attempt to receive a message on several ports at a time. Among the set of ports attached to an actor, a subset of enabled ports is defined. A thread can also attempt to receive a message sent to any of the enabled ports in its actors.

Ports attached to an actor can be dynamically enabled or disabled. When a port is enabled, it receives a priority value. If several of the enabled ports hold a message when a thread attempts to receive the message on the enabled set of ports, the port with the highest priority will be selected.

An actor's default port is not automatically enabled. If a port has not been enabled, it is automatically disabled. This does not mean that the port cannot be used to send or to receive messages. It means that the port cannot be used in multiple port receive requests because the default value is disabled.