移动 AGENT系统——Aglets

提纲 Aglets简介 Aglets模型 Aglets API Aglets样例

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AGLETS

Aglets IBM http://www.trl.ibm.com/aglets/ http://aglets.sourceforge.net/

Latest version 2.5-alpha Stable version: 2.0.2

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AGLETS

What are mobile agents? Mobile network agents are programs that can be

dispatched from one computer and transported to a remote computer for execution. Arriving at the remote computer, they present their credentials and obtain access to local services and data. The remote computer may also serve as a broker by bringing together agents with similar interests and compatible goals, thus providing a meeting place at which agents can interact.

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AGLETS

What is the Aglets Software Development Kit? The Aglets Software Development Kit is an

environment for programming mobile Internet agents in Java. (It is what we used to be called Aglets Workbench.)

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AGLETS

What is an aglet? The aglet represents the next leap forward in the

evolution of executable content on the Internet, introducing program code that can be transported along with state information. Aglets are Java objects that can move from one host on the Internet to another. That is, an aglet that executes on one host can suddenly halt execution, dispatch itself to a remote host, and resume execution there. When the aglet moves, it takes along its program code as well as its data.

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AGLETS

Aglet Mobile Agent Platform (Aglets MAP): is the core platform, able to manage mobile agents.

Tahiti: is the main server in charge of managing the mobility of agents. It comes with a GUI that helps administrators taking care of running agents.

Aglets Software Development Kit (Aglets SDK – ASDK): is a library that provides developers all the facilities required to write mobile agents compliant to the Aglets MAP.

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AGLETS

迁移机制通信机制安全保障

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MOBILITY

Strong Mobility vs. Weak Mobility

Example for (int i = 0; i<10; i++){ migrate ( remoteHosts[i] ); }

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MOBILITY

Aglets only support weak mobility!

In fact, almost all the Java MAP support only the weak mobility. No support in Java to get enough information

about the execution flow. The Thread class itself is not serializable, and so

cannot be transmitted over the network.

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MOBILITY

Aglets ensures that an agent will keep its Java state (i.e. the value of its inner variable) among migrations (at least if the Java state is serializable).

When the agent is going to be executed on the remote machine, the execution will start from a well defined entry point (a method), and no re-initialization will happen (in order to not override the current agent state).

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COMMUNICATION

Based on message passing!

Agents cannot directly reference other agents, so it is not possible for an agent to handle a “pointer” to another agent. For security reason

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COMMUNICATION

Proxy Pattern A proxy is an object that masquerades another

object (its owner) and that forwarders method calls to its owner.

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COMMUNICATION

Aglets are hidden by proxies, and other aglets can send messages or perform method calls against the proxy, having the proxy to forward such method calls on the owner agent.

Advantages: Aglets are always protected, being invisible and that

the system can create and manage a lot of proxies for the same aglet without breaking the agent protection.

The dispose of a proxy does not causes the disposing of the agent it is masquerading.

In Aglets, agents are created and managed by the MAP, while proxies represents “handles” to other agents, thanks to which agents can communicate each other.

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SECURITY

protection of the host against aglets, protection of other aglets, protection of the aglet from the host, and protection of the underlying network.

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SECURITY

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SECURITY

The Aglet security model supports the flexible definition of various security policies and describes how and where a secure system enforces these policies.

Security policies are defined in terms of a set of rules by one administrative authority.

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SECURITY POLICIES

the conditions under which aglets may access objects;

the authentication required of users and other principals, which actions an authenticated entity is allowed to perform, and whether entities can delegate their rights;

the communications security required between aglets and between contexts, including trust; and

the degree of accountability required for each security relevant activity. 1

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AGLET 体系结构

Interagent Communication Manager

AgentApplication Gateway

Agent Manager

Reliability Manager

Directory Manager

Security Manager

External Application

Generic Mobile Agent Server

Network

Generic Mobile Agent Server

Generic Mobile Agent Server 19

AGLET 模型 What is java aglet?

Lightweight agent Java object that can move from one host to

anotherA mobile java agent that supports concepts

of autonomous execution and dynamic routing on its itinerary. Has its own thread of control Is event-driven Communicates by message passing “Write once, go anywhere”

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AGLET MODEL(1)

Basic Elements:Aglet – a mobile Java objectProxy – a representative of an

agletContext – an aglet’s workplaceIdentifier – bound to each aglet

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RELATIONSHIP BETWEEN AGLET AND PROXY

Proxy Represents the Aglet A handle for local, remote or deactivated aglet

Shields Public Methods for Potential Misuse Can Hide the “actual” Location of Aglet Proxy and Aglet on Different Computing

Nodes Proxy ： Aglet < = > n : 1

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InteractionClients Proxy Aglet

InteractionClients Proxy

Aglet

H1

H2

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CONTEXT A context is an uniform execution

environment for aglet. a stationary objectprovides a means for maintaining and

managing running aglets One node in a computer network may run

multiple servers and each server may host multiple contexts.

Contexts are named and can thus be located by the combination of their server's address and their name.

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CONTEXT

Aglets must be instantiated within a context, and at any time belong to exactly one context.

Context can be used for: createAglet retractAglet getAgletProxies / getAgletProxy setProperty/getProperty

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RELATIONSHIP BETWEEN HOST, SERVER PROCESS(ENGINE), AND CONTEXTS

Context Context Context

Host

Server Process (Engine)

Network26

CONTEXT, PROXY, AGLET

Context

Proxyaglet

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AGLET MODEL(2)

Basic ElementsAglet : a mobile java objectProxy : a representative of an agletContext : an aglet’s workplaceIdentifier : bound to each agletMessage : an object exchanged between agletsFuture reply : an asynchronous message-

sending

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AGLET MODEL(2)

Messagean object exchanged between aglets. It allows

for synchronous as well as asynchronous message passing between aglets.

Future replyA future reply is used in asynchronous message-

sending as a handler to receive a result later asynchronously

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AGLET : COMMUNICATION MODEL

Message passing: Allows aglets to create and exchange

messages in flexible ways. Key Components:

Aglet, AgletProxy, Message, Future reply, Reply set

Aglet/

Application

Message

Reply

Message

ReplyProxy Aglet

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AGLET MODEL(2)

Context

Proxyaglet Proxy aglet

Message

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AGLET: OPERATIONS

Six Basic Operations1. Creation2. Cloning3. Dispatching4. Retraction5. Activation and Deactivation6. Disposal

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AGLET: OPERATIONS

Creation Initializes a New Aglet Assigns Aglet an Identifier

Cloning Produce an Identical Copy of an Aglet Differences Between Clone and Original Aglet

Identifier Place of Execution

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AGLET: OPERATIONS

DispatchingAglet is Transported (Pushed) From One

Context to Another Retraction

Concept that Returns an Aglet Back to its Original Context

Original Context Pulls Back the Aglet From the Current Context

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AGLET: OPERATIONS

Deactivation and Activation Deactivation Must Occur Before Activation Deactivation Halts the Execution of an Aglet Activation Starts Execution of an Aglet Activation Must Occur to Restore the Aglet

Disposal Stops Execution of an Aglet Removes an Aglet From its Current Context

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BASIC OPERATIONS

Classification of Six Basic Aglet Operations Classified as the Life Cycle of an Aglet

Observation of Six Basic Aglet OperationsContext Functions Just Like a Place

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AGENT LIFE-CYCLE MODEL

Aglet Aglet

Class File

DiskStorage

Create Deactivate Activate

DisposeClone

Context A Context B

Retract

Dispatch

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AGENT LIFE-CYCLE MODEL

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AGLET EVENT MODEL

Aglet Programming Model Similar to the Java Event Model Event Based Listeners Used to Perform Event Actions

Three Event Model Listeners Clone Listener Mobility Listener Persistence Listener

Actions Listen for the Event They are Named After Listeners Can Be Customized to Perform Specific

Actions When the Listener Event Occurs 39

RELATIONSHIP BETWEEN AGLET AND ITS LISTENERS

Aglet

Clone Events

Mobility Events

Persistence Events

CloneListener

MobilityListener

PersistenceListener

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EVENT MODEL LISTENERS

Clone Listener Listens for Cloning Events Actions Can Be Customized to Occur Before,

During, or After the Cloning Mobility Listener

Listens for Mobility Events Actions Can Be Customized to Occur When an

Aglet is About to Be Dispatched, Retracted, or Arrives in a New Context

Persistence Listener Listens for Persistence Events Actions Can Be Customized to Occur When an

Aglet is About to Be Deactivated or Has Been Activated

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EXAMPLE

Agent dispatch

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THE AGLET MODEL

Message Handling Aglets exploit a communication system based on

message passing. An agent that wants to explicitly manage

messages has to override the handleMessage(..) method, returning true in the case that the message is managed by the agent, and false otherwise.

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MESSAGE HANDLING

An aglet is not necessarily aware of message source, it just receives and handles messages. Message source should be included in some way in the message itself, since there is no other way to identify it.

Message handling is as complex as the handleMessage(…) implementation.

The method handleMessage(Message msg) has one parameter, the message received by the aglet, which can be used to define the aglet message handling behavior.

The message type is its classification. 44

MESSAGE HANDLING

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THE AGLET MODEL

Agent Design PatternsTravelingTaskInteraction

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AGENT DESIGN PATTERN (CONT.) Traveling

Itinerary: routes among destinations. Forwarding: a host forwards an arrived agent to

another host. Ticket: objectifies a destination address and

encapsulates the quality of service and permissions needed to dispatch an agent to a host address and to execute it there.

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AGENT DESIGN PATTERN (CONT.)

Task Master-Slave: an agent delegate a task to another Plan: organize multiple tasks to be performed in

sequence or in parallel by multiple agents.

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AGENT DESIGN PATTERN (CONT.)

Interaction Meeting: two or more agents initiate local

interaction at a given host. Locker: defines a storage and come back for data

later. Messenger: an agent carries a remote message

from one agent to another. Facilitator: an agent provides services for naming

and locating agents. Organized Group: travel together.

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Aglet Class & API

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AGLETS

Mobile Agents with Java Aglet Package (Classes)

Aglet Class Message Class AgletID Class AgletInfo Class AgletStub Class FutureReply Class QoC Class ReplySet Class

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AGLETS

Mobile Agents with JavaAglet Package (Interfaces)

AgletProxy InterfaceAgletContext InterfaceMessageManager Interface

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AGLET API: CLASSES

Aglet ClassContains All Methods Needed to Perform

the Basic Aglet OperationsBasis for Building All AgletsContains All the Elements of the Aglet

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AGLET CLASS

Aglet Creation Create a Customized Aglet

import com.ibm.aglet.*;public class MyFirstAglet extends Aglet{

//Put aglet’s methods here}

The Dispatch Method Dispatches an Aglet to a Remote Context The Method Call Contains the URL of the Remote

Contextdispatch(new URL(atp://remote.host.com/context”));

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AGLET CLASS

Overridden methods:public void onCreation(Object init){

//do some initialization here…

}

public void run() { //do something here;

} //main entry point e.g. call dispatch(new URL(“atp://some.host.com//context”));

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AGLET API :AGLET CLASS

What happens when “dispatch”?

StateByte Code

Host Host

Sending Receiving

StateByte Code

HostHost

Object Serialization

Transfer

Deserialization and re-create state

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MESSAGE CLASS

Message Objects are Sent Using the AgletProxy Class MethodsObject sendMessage(Message msg)void sendOnewayMessage(Message msg)

Code Exampleproxy.sendMessage(myName);

String name = (String)proxy.sendMessage(yourName);

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MESSAGE CLASS

Public boolean handleMessage(Message msg){ if(msg.samekind(“my name”)){ String name=(String)msg.getArg(); return true; //Yes, I handled this message } else if(msg.samekind(“your name?”)) { msg.sendReply(“Yina”); //Return its name return true; //Yes, I handled this message } else return false; //No, I did not handle this message}

handleMessage method:

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AGLET API : AGLETPROXYHow to get a proxy?

Get proxy from a newly created aglet: AgletContext.createAglet() AgletProxy.clone()

Get proxy from existing aglets Aglet.getProxy() – get its own proxy AgletContext.getAgletProxies() – retrieve an enum. of

proxies in a context AgletContext.getAgletProxy() – get an aglet proxy for a

given aglet ID AgletContext.setProperty – put proxy into context property

and share it.

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AGLET API : AGLETCONTEXT An aglet context is the equivalent of a

place. An aglet uses AgletContext interface

to:Get information about its environmentSend message to the environment,

including other aglets currently active in that environment

How it works:context=getAgletContext();context.createAglet();context.retractAglet(remoteContextURL,agletID);

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EXAMPLE

• Aglet Example: Remote File Update• Premise: Large Multiple Remote Files that Must be

Updated by Word Replacement• One Solution: Move Files to Central Server, Perform

Update, and Move Files Back• Another Solution: An Aglet that Updates Files by

Replacing All Occurrences of One Specified Word in the Files With Another Specified Word

• Distributes the Load of Updates to Multiple Servers

• We’re Moving “Code” Rather Than Files

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MULTIPLE AGLETS UPDATING FILES IN PARALLEL

Aglet AgletAglet

Aglet

Host (Updating)

Host HostHostF FF FF F

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UPDATE FILE AGLET

import com.ibm.aglet.*;import com.ibm.aglet.event.*;import java.net.*;import java.io.*;public class UpdateFile extends Aglet{

URL destination = null;File dir = null;String from = null;String to = null;public void onCreation(Object args){

destination = (URL)((Object[])args)[0];dir = (File)((Object[])args)[1];from = (String)((Object[])args)[2];to = (String)((Object[])args)[3];addMobilityListener(){

new MobilityAdapter(){63

UPDATE FILE AGLET

Public void onArrival(MobilityEvent e){replace(args.file,args.from,args.to);dispose(); }

}}try{

dispatch(args.destination);}catch (Exception e){

System.out.println(“Failed to dispatch.”);

}}void replace(File, file, String, from, Sting

to){//Open ‘file’ and replace ‘from’ with ‘to’

}

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APPLICATION OF AGLETS Marketplace Architecture

Shop agents go to a market from a shop owner’s terminal Customer agents travel around various markets to get more

information Market advertisers go to other markets to invite customer

agents

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ANATOMY OF AN AGLET

Creation & Disposal Clone Mobility

– Dispatch and Retract Persistence

– Activate and Deactive

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CREATION

public final AgletContext Aglet.getAgletContext()//Example: getAgletContext().createAglet(getCodeBase(),”SomeAglet”,nul

l);Methods that can be overridden

protected Aglet.Aglet()Public void Aglet.onCreation(Object init)Public void Aglet.run()

createAglet()Aglet()onCreationrun()

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CLONINGpublic final Object Aglet.clone()public final void Aglet.addCloneListener(CloneListener

listener)public final void

Aglet.removeCloneListener(CloneListener listener)

public CloneAdapter.CloneAdapter()public void CloneAdapter.onCloning(CloneEvent

event)public void CloneAdapter.onClone(CloneEvent event)public void CloneAdapter.onCloned(CloneEvent event)

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public class CloningExample extends Aglet{

boolean _theClone=false;

public void onCreation(Object o) {

addCloneListener(new CloneAdapter(){

public void onCloning(CloneEvent e){

// print to the console }

public void onClone(CloneEvent e){

// print to the console }

public void onCloned(CloneEvent e){

// print to the console }

} );

}

public void run() { if (! _theClone) { //the original runs here… try{ clone(); } catch (Exception e) { System.out.println(e.getMessage());

}

else {//the clone runs here…}

}

run()

onCloning()

onCloned()

onClone()

run()

original

clone

CLONING

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MOBILITY

-- Dispatching

run( )dispatch( )onDispatching( )

onArrival( )run( )

Origin

Destination

retractAglet( )onArrival( )run( )

run( )onReverting( )

Local

Remote

-- Retracting

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PERSISTENCE

Persistence

run( )deactivate( )onDeactivating( )

onActivation( )run( )

Before

After

Aglet Context

Deactivate

Activate

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