Ports

Messages are not addressed directly to threads, but to intermediate entities called ports. Ports are named using unique identifiers. A port is an address to which messages can be sent, and which has a queue holding the messages received by the port but not yet consumed by the threads. Port queues have a fixed maximum size, set as a system parameter.

For a thread to be able to consume the messages received by a port, this port must be attached to the actor that supports the thread. When a port is created by a thread, the thread attaches the port to an actor (possibly different from the one that supports the thread). The port receives a local identifier, relative to the actor to which it is attached.

A port can only be attached to a single actor at a time, but can be attached successively to different actors: a port can migrate from one actor to another. This migration can be accompanied, or not, by the messages already received by the port and not yet consumed by a thread. The concept of port provides the basis for dynamic reconfiguration. The extra level of indirection (the ports) between any two communicating threads means that the threads supplying a given service can be changed from a thread of one actor to a thread of another actor. This is done by changing the attachment of the appropriate port from the first thread's actor to the new thread's actor.

When an actor is created, a first port is attached to it automatically and is the actor's default port. The actor's default port cannot be migrated or deleted.