Description

This law represent an improvement of the basic ping-pong law that maintains its fundamental behavior. A few features are added in this law:

  1. Improved fault tolerance. Whenever the destination of a ping message cannot be found, the pingTo term in the control state is removed and the source is informed. Another improvement reffers to the fact that the source of a message is informed whenever there is an attempt to send a message invalid under this policy. The policy also features the behavior of an agent that runs disconnected from its controller.
  2. User-friendly behavior. A set of messages are designed to provide the user with its own state on demand or periodically. The set of messages a user can send at a certain moment in time are also displayed.

The law

The Prolog version of this law follows: 

law(pingpongplus,language(prolog)).

/* the pingPong basic protocol*/
sent(X,ping(M),Y) :- not(pingTo(Y)@CS), do(add(pingTo(Y))),do(forward).

arrived(X,ping(M),Y) :- do(add(pingFrom(X))), do(deliver).

sent(X,pong(M),Y) :- pingFrom(Y)@CS, do(remove(pingFrom(Y))),do(forward).

arrived(X,pong(M),Y) :- do(remove(pingTo(X))), do(deliver).


sent(Self,getCS,Z) :- do(discloseCS(all)).

/*disconnection and reconnection */
disconnected :- do(imposeObligation(toQuit,10, min)).

reconnected :- do(repealObligation(toQuit)).

obligationDue(toQuit) :- do(quit).

/*automatic state monitoring */

sent(S,display(DT),Z) :- do(imposeObligation(display,DT,sec)), do(add(obTime(DT))),
    do(show(possibleEvent(all), cs(all))).

obligationDue(display) :- obTime(DT)@CS, do(imposeObligation(display,DT,sec)), 
    do(show(possibleEvent(all), cs(all))).

sent(S,stopDisplay,Z) :- obTime(T)@CS, do(remove(obTime(T))),
		      do(repealObligation(display)).

/*exception handling*/
exception(forward(Self,ping(M),[Y,L]),D) :- do(remove(pingTo(Y))),
    do(deliver(Self,exc(ping(M)),Y)).

exception(E,D) :- do(deliver(Self,exception(E,D),Self)).

/*sentinel rule*/
sent(X,N,Y) :- do(deliver(Self,messageIgnored,Self)).
The Prolog law can be found at:
http://www.moses.rutgers.edu/examples/pingpongplus/Pingpongplus.law

This is the Java version of the law: 


public class Pingpongplus extends Law {

    public void sent(String source, String message, String dest) {
        String pingToY = "pingTo(" + dest + ")";
        String pingFromY = "pingFrom(" + dest + ")";

        if (message.startsWith("ping(") && !CS.has(pingToY)) {
            doAdd(pingToY);
            doForward();
	    return;
        }
        if (message.startsWith("pong(") && CS.has(pingFromY)) {
            doRemove(pingFromY);
            doForward();
            return;
        }
        if (message.equals("getCS")) {
            doDiscloseAllCS();
            return;
        }
        if (message.startsWith("display(")) {
            int dt = Integer.parseInt(getContentFromMessage(message));
            doImposeObligation("display", dt, "sec");
            doAdd("obTime(" + dt + ")"); 
            doShowEvents("all", "all");
            return;
        }
        if (message.equals("stopDisplay")) {
            Term timeTerm = CS.findT("obTime(%A)");
            doRemove(timeTerm);
            doRepealObligation("display");
            return;
        }
        doDeliver(Self, "messageIgnored", Self);
    }

    public void arrived(String source, String message, String dest) {
        if (message.startsWith("ping(")) {
            doAdd("pingFrom(" + source + ")");
            doDeliver();
        }
        if (message.startsWith("pong(")) {
            doRemove("pingTo(" + source + ")");
            doDeliver();
        } 
    }

    public void disconnected() {
        doImposeObligation("toQuit", 10, "min");
    }

    public void reconnected() {
        doRepealObligation("toQuit");
    }

    public void obligationDue(Term obligationTerm) {

        if (obligationTerm.toString().equals("toQuit")) doQuit();

        if (obligationTerm.toString().equals("display")) {
            int dt = CS.fetchInt("obTime");
            doImposeObligation("display", dt, "sec");
            doShowEvents("all", "all");
        }
    }

    public void exception(Message m, String diagnostic) {
        if ((m.type == Const.FWD) && m.source.equals(Self) && m.s_payload.startsWith("ping")) {
            doRemove("pingTo(" + m.dest + ")");
            doDeliver("law", "exc(" + m.s_payload + ")", Self);
            return;
        }
        doDeliver("law", "exception(type(" + m.type + "),source(" + m.source + "),payload(" + m.s_payload 
            + "),dest(" + m.dest + "),diagnostic(" + diagnostic + "))", Self);
    }

    /*This is a helper method to retrieve the only argument of a regular expression*/

    public String getContentFromMessage(String anyMessage) { 
        int index = anyMessage.indexOf("("); 
        if (index == -1) return ""; 
        else return anyMessage.substring(index+1, anyMessage.length()-1); 
    }
}
This law can be found at:
http://www.moses.rutgers.edu/examples/pingpongplus/Pingpongplus.java1

Running the example

The basic behavior of this law can be observed by following the same scenario outlined in the ping-pong law example. In order to observe the behavior augmented by the new additions to the law note the following:

  • When sending an arbitrary message to any agent, there always is a messageIgnored answer to it.
  • Whenever a ping message is sent to an non-existing agent, an exception notification is provided. Also, in this case no trace is left in the control state.
  • In order to observe the control state at a certain moment of time, send the message getCS to anybody, and check the control state in the main communication window; in this case, the message is captured and handled directly by the controller.
  • When sending the message display(t), where t can be any positive integer, the control state and the repertoire will be displayed automatically every t seconds. The control state is displayed in the web-based agent text area called CS. Also, the repertoire text area displays all the possible messages the agent can send (feature available in the prolog version of the law). This way, one can observe the state of the agent during the interaction.
  • Stop the automatic monitor of the control state by sending a stopDisplay message.
  • Furthermore, observe that the actor (web-based interface) can disconnect from its controller, and reconnect. For this law, the grace period is 10 minutes. Also, observe that while the actor is disconnected, the state of its agent can be changed by the receiving of a message.