Jini Lecture

1
Jini Lecture

• RMI Overview
• What is RMI
• Serialization to disk, sockets and in RMI
• RMI in Action A Broad Overview
• Jini
• What is Jini
• Jini Scenario An Overview
• Types of Jini Services
• Jini in Operation Jini Protocols
• Discovery
• Join
• Lookup
• Jini Scenario Filling in the Gaps
• Jini Scenario More Detail
• Jini other
• Jini Event Model
• Jini Groups

2
What is RMI?

• To communicate across the network in Java, can
  use
• Sockets one to one, duplex connections
• Need to pack data for socket and unpack other
  side
• i.e. need to agree on format and internal
  protocol up front
• RPC (Remote Procedure Call)
• abstracts communication interface to the level of
  a procedure call
• programmer appears to be calling a local
  procedure
• encodes arguments and returns values using an
  external data representation e.g. XDR
• RMI Remote Method Invocation
• Similar to RPC but uses Java Serialization to
  pack objects

3
Serialization in Java
java.io.Serializable java.io.Externalizable
Java Interfaces

• For storing and retrieving objects
• represents the state of objects sufficient for
  reconstruction
• Objects to be saved in the stream may support
• Serializable interface default serialization
  mechanism
• Externalizable interface custom serialization

Java Object
Java Object
Disk File
Serialize (ByteStream/String)
Deserialize (ByteStream/String)
TCP Socket
RMI Argument/ Parameter
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Serialize/Deserialize to Disk File
Serialize today's date to a file
FileOutputStream f new FileOutputStream("tmp
") ObjectOutputStream s new
ObjectOutputStream(f) s.writeObject(new
Date()) s.flush()
Deserialize today's date from a file
FileInputStream in new FileInputStream("tmp"
) ObjectInputStream s new
ObjectInputStream(in) Date date
(Date)s.readObject()
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Serializing a Class
To Implement Default Serialization
import java.io.Serializable public class
PersistentClass implements Serializable
transient private String notToCopy private
String name public PersistentClass()
notToCopy whatever    name
PersitentObject   
To tag Instance variables which should not be
copied
6
RMI In Action A Broad Overview
4. Call Remote Function by using Proxy
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What is Jini?
Historically, operating systems rely on disk
drives
Key Features

• Written in Java
• Uses RMI and Java Object Serialization to enable
  Java objects to move around the network
• Offers network plug and play of services (java
  objects)

Differences with RMI

• Provides Discovery of Jini Services
• Published RMI stub -gt RMI proxy
• Distributed Service concepts e.g.
• Searching,
• Leasing and Grouping
• Distributed Events

8
Jini

• Jini Players
• Broad Overview
• Joining and Using a Jini Network
• Discovery
• Join
• Leasing
• Lookup
• A Jini Scenario Filling in the gaps
• Jini Scenario More Detail..

9
Jini Players
Jini defines a runtime infrastructure that
provides mechanisms that enable you to add,
remove, locate, and access services. There are
three main players
and yes, and the network connecting all three -
generally be running TCP/IP
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Jini Scenario Broad Overview
Jini Service
Jini Client (Consumer)
LUS Lookup Service
the network (TCP/IP)
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Using Jini Services

• Several ways proxies interact with the service
• Proxy performs the service itself Java proxy
  code is completely self-contained
• does not require any remote functionality
• Similar to ?
• Proxy is an RMI stub for a remote service
• proxy is a minimal piece of code - interface to
  the remote object
• client makes a call on the proxy object, then RMI
  transfers this call and arguments to the remote
  object on the service provider, where the actual
  execution is made
• Proxy is a Smart Adapter Proxy contains code
  that decides where to execute it e.g. could use
• sockets
• Corba
• JXTA
• gain access to hardware devices that have their
  own communication protocol
• contact services written in other programming
  languages

12
Jini Federations

• Federation of services available on the Jini
  network
• cleints enlist the help of services to
  accomplish their tasks

13
Joining and Using a Jini Network
Jini defines one network-level protocol
-discovery and two object-level protocols - join
and lookup Discovery - enables clients and
services to locate lookup services (LUS) Join -
enables a service to register itself in a lookup
service Lookup - enables a client to query a
lookup service for available services.
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Discovery

• Unicast Interaction
• Client knows location of Jini LUS e.g.
  jini//spectra.astro.cf.ac.uk
• Use when static connections are needed
• Multicast dynamic discovery
• Uses the multicast request protocol
• Sends multicast request message to a well known
  multicast address
• LookUp service gets the request and connects to
  the requesting service.
• Sends a unicast message which contains proxy for
  the lookup service.

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The Join Process
Allows services to register as Jini services with
the LUS
register(ServiceItem, long)
Jini Service

• Pass register() a ServiceItem object lease
  duration (next.)

• ServiceItem Class
• ID for the service (automatically generated by
  LUS)
• object implementing the actual Jini Service
• attributes for service - list of objects

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Leasing in Jini

• used to keep track of active Jini services
• can be set by service or controlled by LUS
• services use leasing to register that they are
  alive, but allow themselves to be timed out''
  if they have failed or if they are unreachable.
• lookup service acts as the granter of the lease.

Setting Lease Duration

• Services can use defaults or set manually
• Lease.ANY - the service lets the lookup service
  decide on the time
• Lease.FOREVER - the request is for a lease that
  never expires

Grant Lease
4 Second Lease Scenario
2
4
2
4
2
4
1
3
1
3
1
3
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Lookup

• When a service has registered - its available
  for use by clients
• Clients use lookup to find services

Lookup(ServiceTemplate t)
Jini Client (Consumer)
(Obtain ServiceRegistrar using Discovery)

• ServiceTemplate
• contains the search query
• typically search using Java type (i.e. interface)

public interface JiniPrinter // print a
document contained in String or throw
PrinterException public void print(String
text) throws PrinterException public Class
LaserPrinter implements JiniPrinter public
int print(String text) throws PrinterException
// implement Laser-specific code here or throw
exception
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Jini Scenario Filling the Gaps
Jini Service
Jini Client (Consumer)
LUS Lookup Service
the network (TCP/IP)
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Jini Scenario More Detail.
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Jini Event Model

• Jini objects may also be interested in changes in
  other Jini objects i.e. want to be listeners for
  such changes.
• Networked nature of Jini means need a different
  event model
• Network delivery is unreliable
• Network delivery is time-dependent
• A remote listener may have disappeared by the
  time the event occurs. Listeners have to be
  allowed to time out'', like services do.
• Uses one event type RemoteEvent and small number
  of sublclasses are serializable

21
Organization of Jini Services

• Jini Services are organized as a federations of
  services i.e. communities
• Grouping
• similar to communities groups are the names
  used to represent communities
• Based around LUS -gt client can search for
  specific groups
• LUS can be members of many groups
• not globally unique

22
Jini Lecture

• RMI Overview
• What is RMI, Serialization, Broad Overview
• Jini
• What is Jini
• Jini Scenario An Overview
• Types of Jini Services
• Jini in Operation Jini Protocols
• Discovery
• Join
• Lookup
• Jini Scenario Filling in the Gaps
• Jini Scenario More Detail
• Jini other
• Jini Event Model
• Jini Groups
• Focus
• Detailed Jini Scenario, details of protocols