Mobile Agents

1
Mobile Agents

• Shyh-Kang Jeng
• Department of Electrical Engineering/
• Graduate Institute of Communication Engineering
• National Taiwan University

2
Reference

• D. B. Lange and M. Oshima, Programming and
  Deploying Java Mobile Agents with Aglets,
  Addison-Wesley, 1998

3
Mobile Agents

• A mobile agent is not bound to the system where
  it begins execution. It has the unique ability
  to transport itself from one system in a network
  to another. The ability to travel allows a
  mobile agent to move to a system that contains an
  object with which the agent wants to interact and
  then to take advantage of being in the same host
  or network as the object

4
Seven Good Reasons for Using Mobile Agents

• Reduce the network load
• Overcome network latency
• Encapsulate protocols
• Execute asynchronously and autonomously
• Adapt dynamically
• Naturally heterogeneous
• Robust and fault-tolerant

5
Network Load Reduction
Application
Service
Host A
Host B
Application
Service
6
Disconnected Operation
Application
Service
Application
Service
Application
Service
7
Client-Server Paradigm
Server
Client
Know- how
8
Code-on-Demand Paradigm
Server
Know- how
Client
Know- how
9
Mobile Agent Paradigm
10
Mobile Agent Attributes
Principals
Interface
Implementation
State
Identifier
11
State

• Needed for the agent to resume computation after
  traveling
• A snapshot of the agents execution
• Execution state
• Object state

12
Implementation

• Needed for location-independent agent execution
• Either taking the implementation code or going to
  the destination, seeing what code is already
  there, and retrieving any missing code over the
  network
• Executable at the destination host
• Safe for the host to execute

13
Interface

• Needed for agent communication
• Can be anything from a set of method signatures
  that allow other agents and applications to
  access to a messaging interface that allows
  agents to communicate in agentspeak such as KQML

14
Identifier and Principals

• Identifier
• Needed to recognize and locate traveling agents
• Unique during its lifetime (immutable)
• Principals
• Needed to determine legal and moral
  responsibility
• Can be an individual, an organization, or a
  corporation
• Two main principals Manufacturer and owner

15
Place
Host
Agent
Place
Resources
Engine
16
Engine

• Workhorse and virtual machine for one or more
  places
• Provides places and agents with links to the
  underlying network and other resources provided
  by the host
• In a Java-based mobile agent system, the engine
  is likely to be identical with the Java virtual
  machine and the operating system

17
Resources, Location, and Principals

• Resources
• Such as networks, databases, processors, memory,
  disks, and other services
• Location
• Combination of the name of the place and the
  network address of the engine
• Typically as a IP address and a port of the
  engine with a place name attribute
• Principals
• Manufacturer and the place master

18
Agent Creation

• Initiated either by another agent or nonagent
  system
• Three steps
• Instantiation and identifier assignment
• Initialization
• Autonomous execution

19
Agent Disposal

• Initiated by the agent itself, by another agent
  or nonagent system
• Can be disposed of by the system for
• End of lifetime
• No use
• Security violation
• Shutdown
• Two steps
• Preparing for disposal
• Suspension of execution

20
Agent Transfer
Receiver
Resume Execution
Network
21
Agent Class Transfer
Server
Class Code
Origin
Agent
Class Code
22
Agent Communication
Now-Type
Future-Type
One-Way-Type
Receiver
Receiver
Receiver
Sender
Sender
Sender
23
Aglet and Proxy

• Aglet
• A mobile Java object
• Autonomous
• Reactive
• Proxy
• Representative of an aglet
• Protects the aglet from direct access
• Provides location transparency for the aglet
• Identifier

24
Aglet and Proxy
25
Context

• An aglets workspace
• Provides a means for maintaining and managing
  running aglets in a uniform execution environment
  where the host system is secured against
  malicious aglets
• Can be located by the combination of the servers
  address and the contexts name

26
Context
Host
Context
Context
Context
Server Process (Engine)
Network
27
Aglet Life-Cycle Model
Context B
Context A
Dispatch
Aglet
Aglet
Dispose
Clone
Retract
Create
Deactivate
Activate
Class File
Disk Storage
28
Aglet Event Model
29
Aglet Communication Model
Message
Message
Aglet/ Application
Proxy
Aglet
Reply
Reply
30
Class Aglet (1)

• Some final methods
• Object clone()
• void dispatch()
• void dispose()
• void deactivate()
• void snapshot()
• void addCloneListener()
• void addMobilityListener()
• void addPersistencyListener()
• AgletInfo getAgletInfo()
• AgletID getAgletID()
• AgletProxy getProxy()
• AgletContext getAgletContext()

31
Class Aglet (2)

• Some methods
• void run()
• void onCreation()
• void onDisposing()
• void waitMessage()
• void notifyMessage()
• boolean handleMessage()

32
Interface AgletProxy

• Some methods
• Object clone()
• AgletProxy dispatch()
• void dispose()
• void deactivate()
• void activate()
• Object sendMessage()
• FutureReply sendFutureMessage()
• void sendOnewayMessage()
• boolean isRemote()

33
Interface AgletContext

• Some methods
• AgletProxy createAglet()
• AgletProxy retractAglet()
• ReplySet multicastMessage()
• Object getProperty()
• void setProperty()
• void start()
• void shutdown()

34
Class Message

• Some methods
• Message(String kind)
• Message(String kind, Object value)
• String getKind()
• boolean sameKind()
• long getTimeStamp()
• void sendReply()
• void getArg()
• void setArg()

35
Example of Message Handling

• Message myName new Message(my name, Jacob)
• Message yourName new Message(your name?)
• Proxy.sendMessage(myName)
• String name (String)proxy.sendMessage(yourName)
  
• -------------------------------------------------
• Public boolean handleMessage(Message msg)
• if(msg.sameKind(my name))
• String name (String)msg.getArg()
• return true
• else if(msg.sameKind(your name?))
• msg.sendReply(Yina)
• return true
• else
• return false

36
Class FutureReply

• Some methods
• Object getReply()
• boolean isAvailable()
• void waitForReply()
• Example
• FutureReply future
• proxy.sendFutureMessage(msg)
• while(!future.isAvailable())
• doPeriodicWork()
• Object reply future.getReply()

37
Remote File Update
Host
Host
Host
Aglet
Aglet
Aglet
Aglet
Host (Updating)
38
Remote File Update Client Agent

• Object args new Object
• new URL(atp//java.trl.ibm.co.jp),
• new File(C/public/test.dat),
• abc,
• xyz
• AgletContext context getAgletContext()
• Context.CreateAglet(null, UpdateFile, args)

39
Class UpdateFile (1)

• Public class UpdateFile extends Aglet
• URL destination null
• File file null
• String from null
• String to null
• public void onCreation(Object args)
• destination (URL)((Object)args)0
• file (File)((Object)args)1
• from (String)((Object)args)2
• to (String)((Object)args)3
• addMobilityListener(
• new MobilityAdapter()
• public void onArrival(MobilityEvent e)
• replace(file, from, to)
• dispose() )

40
Class UpdateFile (2)

• try
• dispatch(destination)
• catch (Exception e)
• System.out.println(Failed to dispatch)
• void replace(File file, String from,
• String to)
• // Open file and replace from with to

41
Aglet Creation
Context. createAglet()
Aglet. Aglet()
Aglet. onCreation()
Aglet. run()
42
Class CreationExample

• public class CreationExample extends Aglet
• public void run()
• try
• getAgletContext().createAglet(
• getCodeBase(), CreationChild, null )
• catch (Exceptioin e)
• System.out.println(e.getMessage())
• Public class CreationChild extends Aglet
• public CreateChild()
• public void onCreation(Object init)
• public void run()

43
Aglet Disposal
Aglet. dispose()
Aglet. onDisposing()
Aglet. run()
44
Aglet Event Model

• Events
• CloneEvent
• MobilityEvent
• PersistencyEvent
• Listeners
• CloneListener
• MobilityListener
• PersistencyListener
• Adapters
• CloneAdapter
• MobilityAdapter
• PersistencyAdapter

45
Cloning
Aglet. run()
CloneAdapter. onCloning()
Original
CloneAdapter. onCloned()
CloneAdapter. onClone()
Clone
Aglet. run()
46
Class CloningExample

• public class CloningExample extends Aglet
• boolean theClone false
• public void onCreation(Object o)
• addCloneListener(
• new CloneAdapter()
• public void onCloning(CloneEvent e)
• public void onClone(CloneEvent e)
• theClone true
• public void onCloned(CloneEvent e)
• )
• public void run()
• if(!theClone)
• try clone()
• catch( Exception e )
• else

47
Dispatching
Aglet. run()
Aglet. dispatch()
Origin
MobilityAdapter. onDispatching()
MobilityAdapter. onArrival()
Destination
Aglet. run()
48
Class DispatchingExample (1)

• public class DispatchingExample extends Aglet
• boolean theRemote false
• public void onCreation(object init)
• addMobilityListener(
• new MobilityAdapter()
• public void onDispatching(MobilityEvent
  e)
• //
• public void onArrival(MobilityEvent e)
• theRemote true
• //
• )

49
Class DispatchingExample (2)

• public void run()
• if(!theRemote)
• try
• URL destination new URL(
• (String)getAgletContext().
• getProperty(location) )
• dispatch( destination )
• catch (Exception e)
• System.out.println(e.getMessage())
• else
• // The remote aglet runs here

50
Retracting
AgletContext. retractAglet()
MobilityAdapter. onArrival()
Local
Aglet. run()
Aglet. run()
Remote
MobilityAdapter. onReverting()
51
Class RetractionExample

• public class RetractionExample extends Aglet
• public void run()
• try
• AgletProxy proxy
• getAgletContext().createAglet(null,
• RetractionChild, null)
• URL destination
• new URL(atp//some.host.com)
• AgletID aid proxy.getAgletID()
• proxy.dispatch(destination)
• getAgletContext().
• retractAglet(destination, aid)
• catch (Exception e)
• System.out.println(e.getMessage())

52
Class RetractionChild

• public class RetractionChild extends Aglet
• boolean reverted false
• boolean dispatched false
• public void onCreation(Object init)
• addMobilityListener(
• new MobilityAdapter()
• public void onReverting(MobilityEvent e)
• reverted true
• public void onArrival(MobilityEvent e)
• dispatched true
• )
• public void run()

53
Persistence
Aglet. run()
Aglet. Deactivate()
Before
PersistencyAdapter. onDeactivating()
PersistencyAdapter. onActivation()
After
Aglet. run()
54
Class DeactivationExample (1)

• public class DeactivationExample extends Aglet
• private static long SECOND 1000
• boolean activated false
• public void onCreation(Object init)
• addPersistencyListener(
• new PersistencyAdapter()
• public void onDeactivating(
• PersistencyEvent e)
• public void onActivation(
• PersistencyEvent e)
• activated true
• )

55
Class DeactivationExample (2)

• public void run()
• if(!activated)
• try
• deactivate( 10SECOND )
• catch (Exception e)
• System.out.println(e.getMessage())
• else
• //

56
Directory Listing
Remote Host
Origin
Dispatch
List Directory
Dispatch
Display List
57
Class ListingAglet (1)

• public class ListingAglet extends Aglet
• boolean back false
• File dir new File(C\\Public)
• String list
• URL origin null
• public void onCreation(Object o)
• addMobilityListener(
• new MobilityAdapter()
• public void onArrival(MobilityEvent me)
• if( back )
• for(int i0 iltlist.length )
• System.out.println(I
• listi)
• dispose()
• else

58
Class ListingAglet (2)

• try
• list dir.list()
• back true
• dispatch(origin)
• catch ( Exception e )
• dispose()
• )
• origin getAgletContext().getHostingURL()
• try
• dispatch(
• new URL(apt//killi.genmagic.com))
• catch( Exception e )

59
A Traveling Aglet
Origin
dest-1
dispatch(dest-1)
dest-2
dest-1
dispatch(dest-2)
dest-n
dest-
dispatch(dest-n)
(result)
dest-1
60
Class ControllerExample

• public class ControllerExample extends Aglet
• public void run()
• try
• AgletProxy proxy getAgletContext().
• createAglet(getCodeBase(),
• ControllerChild, null)
• Vector itinerary new Vector()
• itinerary.addElement(new URL(dest-1))
• itinerary.addElement(new URL(dest-2))
• itinerary.addElement(new URL(dest-n))
• Enumeration enum itinerary.elements()
• while(enum.hasMoreElements())
• proxy
• proxy.dispatch((URL)enum.nextElement())
  
• catch (Exception e)

61
Class ControllerChild

• public class ControllerChild extends Aglet
• public void onCreation(Object o)
• addMobilityListener(
• new MobilityAdapter()
• public void onArrival(MobilityEvent me)
• doTask()
• )
• void doTask()
• //

62
Finding an Aglet

• May be necessary when the aglet is autonomous
• Three possibilities
• Search
• Sending a search aglet
• Using multicast
• Logging
• Every aglet leaves an electronic fingerprint
• Registration
• Every aglet registers its current location in a
  database

63
AgletFinder Example
Traveler
Register (MobilityListener)
3 Hello
1Register
Traveler Finder
Aglet Finder
2Lookup
64
Class AgletFinder

• public class AgletFinder extends
• Hashtable database new Hashtable()
• public void onCreate(Object init)
• getAgletContext().setProperty(AgletFinder,
• getProxy())
• public boolean handleMessage(Message msg)
• if(msg.sameKind(Lookup))
• msg.sendReply(database.get(msg.getArg()))
• else if(msg.sameKind(Register))
• database.put(msg.getArg(NAME),
• msg.getArg(PROXY))
• else if(msg.sameKind(Unregister))
• database.remove(msg.getArg(NAME))
• return true

65
Class Register (1)

• public class Register extends MobilityAdapter
• AgletProxy finder
• Message msg new Message(Register)
• public Register(AgletProxy finder, String
  name)
• msg.setArg(NAME, name)
• this-gtfinder finder
• public void onArrival(MobilityEvent me)
• try
• msg.setArg(PROXY, me.getAgletProxy())
• finder.sendMessage(msg)
• doTask()
• catch(Exception e)

66
Class Register (2)

• public void unregister()
• try
• Message unregister
• new Message(Unregister)
• unregister.setArg(NAME,
• msg.getArg(NAME))
• finder.sendMessage(unregister)
• catch (Exception e)

67
Class Traveler

• public class Traveler extends Aglet
• Register register null
• public void onCreation(Object o)
• AgletProxy finder (AgletProxy)
  getAgletContext().getProperty(AgletFinder)
• register new Register(finder, Traveler)
• addMobilityListener( register )
• try
• dispatch(destination)
• catch (Exception e)
• public void onDisposing()
• register.unregister()
• public boolean handleMessage(Message msg)
• if (msg.sameKind(Hello)) return true

68
Class TravelerFinder

• public class TravelerFinder extends Aglet
• AgletProxy finder null
• public void onCreation(Object o)
• finder (AgletProxy) getAgletContext().
• getProperty(AgletFinder)
• public void run()
• try
• AgletProxy traveler
• (AgletProxy) finder.sendMessage(
• new Message(Lookup,Traveler) )
• traveler.sendMessage(new Message(Hello))
• catch (Exception e)

69
Aglets in Parallel Execution
Origin
dispatch(dest-n)
and sendFutureMessage(doTask)
Origin
70
Class Worker

• public class Worker extends Aglet
• public boolean handleMessage(Message msg)
• if (msg.sameKind(DoTask))
• msg.sendReply(doTask())
• dispose()
• return true

71
Class ParallelExample (1)

• public class ParallelExample extends Aglet
• public void onCreation(Object init)
• Vector itinerary new Vector()
• try
• itinerary.addElement(new URL(dest-1))
• itinerary.addElement(new URL(dest-2))
• itinerary.addElement(new URL(dest-n))
• catch (Exception e)
• AgletContext context getAgletContext()
• Message doTask new Message(DoTask)
• ReplySet replies new ReplySet()

72
Class ParallelExample (2)

• Enumeration enum itinerary.elements()
• while (enum.hasMoreElements())
• try
• AgletProxy worker context.createAglet(
• getCodeBase(), Worker, null)
• worker
• worker.dispatch((URL)enum.nextElement())
  
• replies.addFutureReply(worker.
• sendFutureMessage(doTask))
• catch (Exception e)
• while (replies.hasMoreFutureReplies())
• try
• FutureReply future
• replies.getNextFutureReply()
• accumulateResults(future.getResult())
• catch (Exception e)

73
Class Worker1

• public class Worker1 extends Aglet
• public void onCreation(Object init)
• AgletProxy parent (AgletProxy) init
• addMobilityListener(
• new MobilityAdapter()
• public void onArrival(MobilityEvent me)
• try
• parent.sendMessage(
• new Message(Result, doTask()))
• dispose()
• catch (Exception e)
• )

74
Class ParallelExample1 (1)

• public class ParallelExample1 extends Aglet
• public void onCreation(Object init)
• Vector itinerary new Vector()
• try
• itinerary.addElement(new URL(dest-1))
• itinerary.addElement(new URL(dest-2))
• itinerary.addElement(new URL(dest-n))
• catch (Exception e)
• AgletContext context getAgletContext()
• AgletProxy parent getProxy()
• Enumeration enum itinerary.elements()

75
Class ParallelExample1 (2)

• while (enum.hasMoreElements())
• try
• AgletProxy worker context.createAglet(
• getCodeBase(),
• Worker1,
• parent )
• catch (Exception e)
• public boolean handleMessage(Message msg)
• if (msg.sameKind(Result))
• accumulateResults(msg.getArg())
• return true

76
Traveling Pattern

• Itinerary
• Objectifies aglets itineraries and routing among
  destinations
• Forwarding
• Provides a way for a host to forward newly
  arrived aglets automatically to another host
• Ticket
• Objectifies a destination address and
  encapsulates the quality of service and
  permissions needed to dispatch an aglet to a host
  and execute it there

77
Task Pattern

• Master-Slave
• Defines a scheme whereby a master aglet can
  delegate a task to a slave aglet
• Plan
• Provides a way of defining the coordination of
  multiple tasks to be performed on multiple hosts

78
Interaction Pattern (1)

• Meeting
• Provides a way for two or more aglets to initiate
  local interaction at a given host
• Locker
• Defines a private storage space for data left by
  an aglet before it is temporarily dispatched
  (sent) to another destination

79
Interaction Pattern (2)

• Messenger
• Defines a surrogate aglet to carry a remote
  message from one aglet to another
• Finder
• Defines an aglet that provides services for
  naming and locating aglets with specific
  capabilities
• Organized Group
• Composes aglets into groups in which all members
  of a group travel together