Lecture 3 From process-oriented to agent-oriented programming. Distributed object systems. Bond objects. Communication. Asynchronous communication. Event handling. Semantic understanding of messages. Probes.

Bond basics Distributed-object system. Message passing distributed object system. Why? Other systems, e.g. Jini are based upon Java RMI.

Bond objects A Bond object extends the standard Java object with: Unique identifier. Communication support. Serialization and cloning. Dynamic properties. Multiple inheritance. Visual Editor. Light-weight objects (e.g. messages, shadows)

Bond resident Container Object. Directory. Each object is registered at the time of creation. Aliases. Communication Engine – runs at a known port on a system with a given BondIPAddress

Communication engine

Local shadow of a remote object We wish to communicate with a remote object as if it were local. To do so we need a local representative of the remote object. Proxy in Voyager Stub in RMI Stubs are used to create VONs Shadows are used to crfeate a local copy of a remote object.

Communication with remote objects -bond shadows

Virtual Object Networks, VONs

Object mobility

Message composition The system supports messages in KQML and XML format. Message composition – putting all pieces together: The contents of the message. The source. The destination. The sub-protocol – we’ll talk about them later. For now think of a subprotocol as a dialect that two objects understand…..

KQML performatives Categories of performatives: Queries: ask; Responses: tell; Informational: Generative: Capability: Networking:

Asynchronous communication using reply-waiting slots

Event handling Java objects use listeners abstractions to capture events. Corba uses an event service. Bond uses event waiting slots.

The subscribe-notify model. Event waiting slots.

Subprotocols Dialects. Closed sets of messages required for a “conversation”. Each message is stamped with the subprotocol it belongs to. Each bond object: has a property called subprotocols implemented that lists all the subprotocols it understands. Implements the bondAccessSubprotocol

Subprotocol inheritance Static inheritance Dynamic inheritance. Probes extend an object and allow it to speak a new protocol. Probes are dynamic properties.