JavaBased Distributed Models

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Java-Based Distributed Models
Charles E. Hughes ceh_at_cs.ucf.edu http//www.cs.ucf
.edu/ceh Professor, School of Computer
Science University of Central Florida
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Common DS Paradigms

• Channels send / receive
• Messages to single or collection of recipients
• Distributed Objects invoke services on remote
  objects
• Requires object serialization and
  un-serialization
• Tuple space write, read, take
• A space for reliable communication and
  coordination

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IP Network Communication

• Each node has an address (IP address)
• Each node has many mailboxes (ports)
• Messages (packets) are received at and sent
  through ports

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UDP Simplest Communication

• Like regular mail
• I send messages with a return address and mailbox
  
• Theres no guarantee of delivery or order of
  receipt
• If all goes well you receive my mail at your
  mailbox
• You can respond using my address and mailbox
  number

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Sample UDP Service

• CLIENT (sender)socket new DatagramSocket()
  packet new DatagramPacket(sendBuf,256,host,5500
  ) socket.send(packet)
• SERVER (receiver)socket new DatagramSocket(5500
  )packet new DatagramPacket(buf, 256)
  socket.receive(packet)address
  packet.getAddress()port packet.getPort()s
  new String(packet.getData(),packet.getlength())

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TCP/IP Common Protocol

• Like a phone call
• I place a call your address and ask for you
  (port)
• You pick up and automatically have my address and
  port
• We have a two-way channel (stream) over which we
  communicate
• We need to be sure our messages are arriving
  reliably and in order

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Sample TCP/IP Service

• CLIENT (sender, then just participant)connection
  new Socket( host, 5000 ) in
  connection.getInputStream()out
  connection.getOutputStream()
• SERVER (receiver, then just participant) server
  new ServerSocket( 5000, 2 )connection
  server.accept()in connection.getInputStream()
  out connection.getOutputStream()

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Remote Messages

• Personalized at the object level
• Messages are at a high-level of abstraction
• Interpretation of messages is at receiver end

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Sketch of Sample RMI Service

• INTERFACEpublic interface Hughes extends Remote
  ResType service() throws RemoteException
• CLIENTobj (Hughes)Naming.lookup(//host/Charles
  Server)result obj.service()
• SERVER obj new HughesImpl()Naming.rebind("//h
  ost/CharlesServer", obj)

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Tuple Space

• Tuples are like entries in a database
• lt Mexico City, Querétaro, 180 gtmeans it
  takes 180 min. to travel from Mexico City to
  Querétaro
• We can match this with many templates
• lt Mexico City, Querétaro, Integer gt
• lt String, Querétaro , 180 gt

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Tuple Match

• Template is of a chosen class
• Template fields can have values or null
• Template can match only tuples of class or
  subclass
• Values are exact matches nulls are type matches

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Sketch of JavaSpace Service

• TEMPLATEpublic class Atlas implements Entry
  public String startCity public String
  endCity public Integer travelTime
• PRODUCERJavaSpace space SpaceAccessor.getSpace(
  )Atlas msg new Atlas(Mexico City",
  Querétaro, 180)space.write(msg, null,
  Lease.FOREVER)
• CONSUMER JavaSpace space SpaceAccessor.getSpace
  ()Atlas template new Message() endCity
  QuerétaroAtlas msg space.take(template,
  null, Long.MAX_VALUE)

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Tuple Space Bid for Airline Ticket

• Customer write bid to proxys tuple space
• p.writeltme, home, to, leave, return, bidgt
• Customer takes bids from tuple space
• p.takelt?airline, me, home, to, ?leave, ?return,
  ?pricegt
• Customer accepts some bid
• p. writeltairline,me,home,to,leave,return,price,cre
  ditgt

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Proxy to Airlines

• Proxy takes bid from tuple space
• p.takelt?you, ?home, ?to, ?leave, ?return, ?bidgt
• Proxy replicates (writes) bid to each airline (a)
• a.writeltairline, you, home, to, leave, return,
  bidgt
• Proxy awaits and relays prices to you
• a.readlt?airline, you, home, to, leave, return,
  pricegt
• p.writeltairline, you, home, to, leave, return,
  pricegt

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Completing the Purchase

• Proxy passes your acceptance on
• Airline sends confirmation which is relayed to
  you
• Uses multiple tuple spaces
• Uses class (type) of template as part of match

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Atomic Operations

• Want to treat group of actions as atomic
• At some point we either
• Commit and roll all changes into visible state
  or
• Abort and roll back state to condition before
  group

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Transactions

• Some client starts a transaction
• Others join the transaction
• Client asks for a vote from participants
• Each responds prepared, nochanges, or abort

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Leases

• Crashes could be real bad with deserted tuples
• Each tuple leases space
• You must renew lease or tuple is deleted
  automatically

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Completing Transactions

• All participants keep state at start
• All prepared participants save prepared state
• Client reports roll back if anyone voted abort
• Client reports roll forward if no aborts
• Nochanges do not participate, but should pause

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Jini Architecture

• Discovery
• Find Jini lookup service (acts as broker)
• Join (if providing new service)
• Lookup services (if required)

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Discovery Parameters

• Protocol version number ( 1 )
• Response port
• Groups to discover (or empty for all)
• Set of lookup services that already responded
• All packed into at most 512 bytes for single UDP
  packet

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Discovery Process
Multicast request client
Multicast request server
Multicast response server
Multicast response client
Discovering Entity
Lookup Service
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Discovery Protocol

• Use multicast to request local lookup service
• Use unicast to request specific lookup service

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Join Parameters

• Service ID assigned when service is new
• Attributes that describe service
• Groups in which service participates
• Lookup services at which this is (to be)
  registered

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Avoiding a Packet Storm

• Wait a random time (up to 15 seconds)
• Then register service at lookup sites
• Random wait is useful after a crash
• If didnt do this, then all servers would start
  simultaneously
• Technique similar to that used after Ethernet
  collision

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Lookup

• Use lookup service to get desired service
• Can choose by group and attributes
• Print service that prints in color at 1440 dpi
• Three-D rendering object that imports VRML
• Service object can be remote or a local proxy