Creating a communication protocol for agents

Agent Communication Language (ACL), proposed by the Foundation for Intelligent Physical Agents (FIPA), is a proposed standard language for agent communications.

The most popular ACLs are:

FIPA-ACL (by the FIPA, a standardization consortium)
Knowledge Query and Manipulation Language (KQML)

Both rely on speech act theory. A set of performatives is defined, also called Communicative Acts, and their meaning (e.g. ask-one). The content of the performative is not standardized, but varies from system to system.

To make agents understand each other they have to not only speak the same language, but also have a common ontology. An ontology is a part of the agent's knowledge base that describes what kind of things an agent can deal with and how they are related to each other.

SARL framework provides the support of the ACL (mostly according to the FIPA specifications). This part of the SARL API provides:

Implementation of protocols, from the initiator and participant points of view.
A conversation manager that is connected to the SARL agent.
Several encoding tools of messages in order to enable communication with other ACL-compliant frameworks, e.g. FIPA-OS or Jade.

This tutorial explains briefly the method for creating a new protocol that could be used by the agents. The example that is taken is the Ping-Pong protocol. The principle of the protocol is the following:

The Ping agent is sending a sarl-ping message to another agent, named the PongAgent agent.
The PongAgent agent is receiving the sarl-ping message, and replies with a sarl-pong message to the sender of the sarl-ping message.
The Ping agent is receiving a sarl-pong message and replies to the sender of the sarl-pong with a new sarl-ping message.

Definition of the protocol sequence

The first task to be done is to define the communication protocol, usually represented with a sequence diagram. The following diagram represents the Ping-Pong protocol:

The two life-lines in the sequence diagram represent the two actors in the communication protocol:

Initiator: the agent that initiates of the communication.
Participant: the agent that is the participant to the initiated communication.

The initiator sends the sarl-ping message to the target participant. After some treatment, the participant replies with a sarl-pong message.

Implementation of the protocol step-by-step

States of the Initiator and Participant

The first step is to define the states of the agents when they are involved into the communication. Basically, the states could be extracted from the previously sequence diagram:

In each life-line, a change of state corresponds to the sending or the receiving of a message. Each activation box corresponds to a specific state.

For the initiator point of view, the three following states are defined:

NOT_STARTED: the communication has not yet started.
WAITING_ANSWER: the initiator has sent a sarl-ping message, and is waiting for the participant's reply.
PONG_RECEIVED: the answer from the participant has been received.

For the participant point of view, the three following states are defined:

NOT_STARTED: the communication has not yet started.
BUILDING_ANSWER: the initiator has sent a sarl-ping message, and it is received by the participant. The initiator is waiting for the participant reply.
PONG_SENT: the participant has sent the sarl-pong.

The implemenation of the states are done inside an enumeration. The following code is the definition of this enumeration using the Java syntax:

public enum PingPongProtocolState implements ProtocolState {

	NOT_STARTED,
	CANCELED,
	BROKEN_PROTOCOL,

	WAITING_ANSWER,
	PONG_RECEIVED,
	BUILDING_ANSWER,
	PONG_SENT;

	public String getName() {
		return name();
	}
	
	public boolean isStarted() {
		return this != NOT_STARTED;
	}

	public boolean isFinished() {
		return this == PONG_RECEIVED
				|| this == PONG_SENT
				|| isCancelled() || isErrorneous();
	}

	public boolean isCancelled() {
		return this == CANCELED;
	}

	public boolean isErrorneous() {
		return isBrokenProtocol();
	}

	public boolean isBrokenProtocol() {
		return this == BROKEN_PROTOCOL;
	}

}

First the states are defined. Two more states are added in order to represent the facts that a communication protocol is cancelled by the initiator CANCELED and the protocol is broken BROKEN_PROTOCOL because one of the participants didn't follows the protocol's sequence.

Since an enumeration in Java is a specific type of class, it is possible to implement functions inside an enumeration. Please note that our state enumeration implements the ProtocolState type. This state represents a state into an ACL protocol. It is an interface that provides all the necessary functions that are used by the rest of the ACL API.

Based on this interface implementation, several functions must be implemented:

getName: replies the name of the state, i.e. the name of the enumeration constant.
isStarted: replies if the state indicates that the communication has started.
isFinished: replies if the state corresponds to a final state.
isCancelled: replies if the state indicates that the communication was cancelled.
isErrorneous: replies if the state indicates a protocol error.
isBrokenProtocol: replies if the state represents the broken protocol.

Events that are received by the agent

An agent could instances a conversation with a ConversationManager. This tool manages the conversations and links the agent to any change of state in the communication protocol. This link may take two forms:

Object-oriented event listeners/observers.
SARL events that are fired into the inner context of the agent.

The first method does not need a specific action. The second approach needs a specific implementation for the Ping-Pong communication protocol. Consequently, in order to enable the SARL agents to react to the changes of state in the protocol, specific events are defined and fired for each receiving of message.

Regarding the PingPong protocol, two events are defined:

PingReceived is fired into the participant when a sarl-ping is received.
PongReceived is fired into the initiator when a sarl-pong is received.

In the handler for the first event, the participant is able to react and reply. In the handler for the second event, the initiator is able to react to the completion of the protocol.

In addition to the defined events, an shared abstract event must be defined, namely AbstractSarlPingPongProtocolEvent. This abstract implementation permits to share the reference to the instance of SarlPingPongProtocol (that will be defind later) that is representing the instance of the communication protocol for the agent.

event AbstractSarlPingPongProtocolEvent extends AbstractAclProtocolEvent {
	val protocol : SarlPingPongProtocol
	new (protocol : SarlPingPongProtocol) {
		this.protocol = protocol
	}
}

Then, all the defined events must be implemented:

event PingReceived extends AbstractSarlPingPongProtocolEvent
event PongReceived extends AbstractSarlPingPongProtocolEvent

Definition of the PingPong Protocol

For defining the Ping-Pong protocol, a specific class must be defined for each initiator/participant. But, before defining these classes, a common super type must be defined.

Definition of the Abstract PingPong Protocol

The abstract implementation of the protocol contains all the definitions that are shared by the different participants' protocols. The SarlPingPongProtocol is the abstract implementation of the Ping-Pong protocol.

abstract class SarlPingPongProtocol extends OneToOneAclProtocol {

	new (owner : Agent) {
		super(owner)
		PingPongProtocolState::NOT_STARTED.change(null)
	}

	final override getAclProtocolId : AclProtocolId {
		AclProtocolId::NONE
	}

	final def getProtocolId : String {
		"sarl-pingpong"
	}

	final override isStarted : boolean {
		getState != PingPongProtocolState::NOT_STARTED
	}

	protected final override getCancelledState : ProtocolState {
		PingPongProtocolState::CANCELED
	}

	
	protected final override getErrorState : ProtocolState {
		PingPongProtocolState::BROKEN_PROTOCOL
	}

}

First the class extends the OneToOneAclProtocol type, which is one of the super types helping to create specific protocols. The OneToOneAclProtocol type represents a protocol involving an initiator and a single participant.

The constructor forces the state of the protocol to be set to NO_STARTED. This constructor takes a single argument that is the agent owning the protocol behavior, i.e. an ACL protocol is implemented as a SARL behavior owned by an agent.

The getAclProtocolId function replies the identifier of the protocol (from the FIPA specifications for several of them). Because the Ping-Pong protocol is not part of the FIPA specifications, the NONE value is replied.

The getProtocolId function replies the identifier of the protocol. By default, this function returns the name of the protocol from the FIPA specifications. Because the Ping-Pong protocol is not part of the FIPA specifications, we define the function in order to reply the name of the protocol.

The functions isStarted, getCancelledState and getErrorState reply if the conversation has been started, the "cancel" state and the "error" state, respectively.

Definition of the Initiator-side of the Protocol

The definition of the initiator-side of the communication protocol is based on the following class that is extending the super type defined in the previous section:

behavior InitiatorSarlPingPongProtocol extends SarlPingPongProtocol {
	
	final override isInitiatorSide : boolean {
		true
	}

	final override isParticipantSide : boolean {
		false
	}

	def cancel(reason : Object = null) {
		cancelProtocol(reason, null)
	}

The two functions isInitiatorSide and isParticipantSide specify if the protocol is for the initiator or the participant (the initiator here). The cancel function is defined in order to enable the initiator to cancel the communication. This function calls the cancelling function from the API that is defined into the super type. This function takes the reason of the cancellation as first argument, and the SARL event to be fired into the agent for notifying the cancellation (none/null in our example).

Now, each function that is provided to the initiator for starting the conversation are implemented. In the context of the Ping-Pong protocol, only the ping function is provided in order to send the acl-ping message.

The expected arguents of this function are the identifier of the participant (it may be unknown, i.e. null), and the application-dependent data to be put into the ping message.

def ping(participant : UUID = null, pingDescription : Object) {

The first action to do is to initialize the conversation as initiator. If the participant cannot be determined, the conversation must not be stared.

if (participant !== null) {
	participant.initiateAsInitiator
}
val part = this.participant
if (part === null) {
	reportError("Unspecified participant identifier")
} else {

Since the conversation is used in a parallel execution context, it is important to ensure that all the actions within the protocol are "synchronized". In the following code, we use the lock attribute that is defined into the ACL protocol super type in order to synchronize the protocol's code.

The following code tests if the ping function is invoked when the state of the protocol is NOT_STARTED.

val rlock = this.lock.readLock
rlock.lock
var st : ProtocolState
try {
	st = this.state
} finally {
	rlock.unlock
}
if (st == PingPongProtocolState::NOT_STARTED) {
	val wlock = this.lock.writeLock
	wlock.lock
	try {
		st = this.state
		if (st == PingPongProtocolState::NOT_STARTED) {

Then, the conversation could start. The following actions are done:

Reset any internal data that is used for storing the result of the conversation.
Start a parallel timer that is generating a conversation time-out.
Change to the expected starting state WAITING_ANSWER.
Send the sarl-ping message, with the PROPOSE performative (this performative is selected within the set of performatives defined by FIPA).

this.resultData = null
startTimeoutNotifier
PingPongProtocolState::WAITING_ANSWER.change(null)
sendMessage(proposeDescription, Performative::PROPOSE, part)

The rest of the ping function's code finalize the implementation of the functions with some unlocking mechanisms and error reporting.

				} else {
					reportError(MessageFormat::format("Cannot send a sarl-ping message in state {0}", state.name))
				}
			} finally {
				wlock.unlock
			}
		} else {
			reportError(MessageFormat::format("Cannot send a sarl-ping message in state {0}", state.name))
		}
	}
}

As briefly explained before, you may want to store the result of the conversation for further use. The most used practise is to defined an attribute of type ProtocolResult that is reset when the conversation is canceled. The value of this attribute is canceled into the ping function and is changed when the acl-pong message is received from the participant.

	var resultData : ProtocolResult

	protected override onCancelled {
		this.resultData = null
	}

The initiator must react to any message sent by the participant, if and only if the conversation has started and has not failed:

on AclMessage [isStarted && !hasFailed] {

In this SARL event handler, the first thing to do is to test if the state of the protocol is valid when receiving the sarl-pong message. If not, an error is reported.

	val st = getState
	if (st != PingPongProtocolState::WAITING_ANSWER) {
		reportError(MessageFormat::format("Cannot handle a participant message {0} in state {1}",
			occurrence.performative.name,
			st.name))
	} else {

The initiator checks if the expected performative is received (here AGREE). The result of the conversation is built. And, the finish function is called in order to mark the conversation as finished. The two arguments of this function are the final state of the conversation, and the instance of the event to be fired into the associated agent, (here, PongReceived).

switch (occurrence.performative) {
	case AGREE: {
		val wlock = this.lock.writeLock
		wlock.lock
		try {
			this.resultData = new ProtocolResult(occurrence.sender, occurrence.performative, occurrence.content.content)
			finish(PingPongProtocolState::PONG_RECEIVED, new PongReceived(this))
		} finally {
			wlock.unlock
		}
	}
	default: {
		occurrence.performative.reportUnpexectedPerformativeError
	}
}

Of course, if the recieved performative is not expected, an error is reported.

Finally, the function getAnswer is implemented in order to enable the initiator to get the result of the conversation. In order to be safe, the synchronization lock is used, and the conversation state is checked, before returned the conversation result.

def getAnswer : ProtocolResult {
	val rlock = this.lock.readLock
	rlock.lock
	try {
		if (this.state == PingPongProtocolState::PONG_RECEIVED) {
			return this.resultData
		}
	} finally {
		rlock.unlock
	}
	return null
}

Definition of the Participant-side of the Protocol

The participant side of the conversation is defined into the ParticipantSarlPingPongProtocol type. This type extends the shared SarlPingPongProtocol type. Additionnally, the utility functions are defined, a well as the attribute for storing the data stored into the received sarl-ping message.

behavior ParticipantSarlPingPongProtocol extends SarlPingPongProtocol {

	var pingData : ProtocolResult

	final override isInitiatorSide : boolean {
		false
	}

	final override isParticipantSide : boolean {
		true
	}

At the participant side, the notUnderstood function is usually defined in order to give the opportunity to the participant to indicate that it is not understanding the query. This function takes the reason of the mis-understanding as argument. It invokes the notUnderstoodProtocol function that is defined into the super-type API.

def notUnderstood(reason : Object = null) {
	notUnderstoodProtocol(reason)
}

The participant may need to get the data that is stored into the sarl-ping message.

def getPing : ProtocolResult {
	val rlock = this.lock.readLock
	rlock.lock
	try {
		if (this.numberOfErrors === 0) {
			return this.pingData
		}
	} finally {
		rlock.unlock
	}
	reportError("Cannot proceed two ping-pong queries in the same conversation")
	return null
}

Then, the pong function is provided to the participant is order to give to it the ability to reply. The implementation follows the same guidelines as the ones used for implementing the ping function above.

def pong(pongInformation : Object = null) {
	val rlock = this.lock.readLock
	rlock.lock
	var st : ProtocolState
	try {
		st = this.state
	} finally {
		rlock.unlock
	}
	if (st == PingPongProtocolState::BUILDING_ANSWER) {
		val wlock = this.lock.writeLock
		wlock.lock
		try {
			st = this.state
			if (st == PingPongProtocolState::BUILDING_ANSWER) {
				PingPongProtocolState::PONG_SENT.finish(null)
				sendMessage(pongInformation, Performative::AGREE, this.initiator)
			} else {
				reportError(MessageFormat::format("Pong cannot be sent in the state {0}", state.name))			
			}
		} finally {
			wlock.unlock
		}
	} else {
		reportError(MessageFormat::format("Pong cannot be sent in the state {0}", state.name))			
	}
}

The participant has to react to the receiving of the sarl-ping message. The following code is the SARL event handler that is dedicated to the implementation of the pong-reply by the participant. The implementation follows the same guidelines as for the initiator's event handler.

on AclMessage [!hasFailed] {
	switch (occurrence.performative) {
		case Performative::PROPOSE: {
			val data = getPing
			if (data === null) {
				val wlock = this.lock.writeLock
				wlock.lock
				try {
					this.pingData = new ProtocolResult(occurrence.sender, Performative::PROPOSE, occurrence.content.content)
					this.conversationId = occurrence.conversationId
					this.initiator = occurrence.sender
					PingPongProtocolState::BUILDING_ANSWER.change(new PingReceived(this))
					startTimeoutNotifier
				} finally {
					wlock.unlock
				}
			} else {
				reportError("Cannot proceed two ping-pong queries in the same conversation")
			}
		}
		default: {
			reportUnpexectedPerformativeError(occurrence.performative)
		}
	}
}

sarl/sarl-acl