Technical Stream Session 7: Resource Discovery and Jini

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Technical Stream Session 7 Resource Discovery
and Jini
Distributed Systems

• CSC 253
• Paul Grace

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Overview of the Session

• Introduce the Jini platform
• Provide motivation for a new distributed services
  technology
• Illustrate example Jini scenarios
• Discuss approaches to resource discovery
• Examine the Jini architecture
• The discovery, join and lookup protocols
• Jini proxy-based service interaction
• Jini service leases
• Programming Jini services and applications
• Examples of advertising and searching for Jini
  applications

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Why do we need Jini?

• The motivation for Jini is to provide Plug and
  Play of Network Services
• Form a community of devices, users and services
  without pre-planning, installation or human
  intervention
• Create spontaneous distributed applications
• Example network services
• Storage
• Computation
• Hardware device
• Software service
• User
• Network-centric rather than disk-centric
  applications

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Plug and Play Scenarios
Camera finds light switch to turn on before
taking picture
Camera finds disk drive to save pictures
Camera finds network printer to print pictures

• Common themes
• Service/Resource discovery
• Service interaction

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What is Jini

• A runtime infrastructure to add, remove, locate,
  and access network services
• Developed by Sun Microsystems
• Adopts the Java programming model One language
  everywhere
• Utilises the benefits of mobile code in Java
• Jini supports
• Discovery of Jini services
• Find me a service that matches my requirements
• Service interaction
• Dynamic use of a found service
• Services leases
• Track active Jini services. Services live for a
  period of time
• Distributed events
• Services notify interested services of changes in
  their status

See lecture on mobile agents
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What is Resource Discovery?

• Find a resource that meets the application
  requirements
• Match resource type
• e.g. Printer, CPU, Coursework submission service
• Match attributes
• e.g. inkcolour, speedgt500MHz, course253
• Wildcard lookup
• Range queries over all known resources
• e.g. (ProcessorType) (Speed 300-500MHz)
• Well known problem in resource discovery
• An identical resource type and attribute match
  may not match the semantic requirements
• Ongoing research Semantic Web solutions
• Two styles of resource discovery
• Directory-based
• Multicast-based

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Directory-based Discovery

• Centralised directory services
• Service connects directly (using unicast
  protocols) and sends service advertisement
  information
• Client connects directly and sends lookup
  request, directory service responds
• Good
• Scalablility
• Minimises network traffic about discovery
• Bad
• Requires third-party administration
• Cannot support spontaneous interaction
• Requires directory to be available in the network
• Clients and services must know the location of
  the directory
• Examples
• RMIRegistry, DNS, Lightweight Directory Access
  Protocol (LDAP), and others

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Multicast-based Discovery

• Common multicast address
• Services listen for discovery requests sent by
  clients directly
• Respond if they have a matching services
• Good
• No third party administration
• Supports ad-hoc resource lookup
• Bad
• Additional Network traffic
• Client must deal with multiple results
• Examples
• Universal Plug and Play (UPnP)
• Service Location Protocol (SLP)
• Hybrid of the multicast and directory approach

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Joining and Using Jini Services

• Jini provides a suite of protocols to add,
  locate, and interact with Jini services
• Jini defines a discovery protocol to locate a
  nearby Jini lookup service
• Directory based discovery
• A Join protocol to enable a service to register
  itself with a lookup service
• A Lookup protocol to enable a client to query a
  lookup service for available services
• The result of lookup is a Jini service proxy
• Returns mobile code
• The client uses the proxy to interact directly
  with the service

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The Jini Discovery Protocol

• All entities must find a Jini Lookup service in
  the network
• Either local to the network or remote
• Three alternative approaches
• Multicast dynamic discovery
• Unicast discovery
• Multicast announcement
• In each approach the Lookup Service returns a
  ServiceRegistrar Proxy Object using RMI
• This is the reference to do service advertisement
  and service lookup

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Multicast Discovery Protocol

• Reasons for using multicast discovery
• Find a nearby lookup service
• Starting service has no knowledge of network
• The multicast discovery protocol

Discovering Entity
Lookup Server
1. Sets up a TCP server instance of the
multicast response service
2. Lookup servers run instances of the multicast
request service listens for requests from
discoverers
TCP response
UDP Multicast
3. Performs a multicast request to lookup services
4. Connects to the discoverers multicast
response server and sends a reference to itself
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Unicast Discovery Protocol

• Reasons for using unicast discovery
• Client knows location of a specific lookup
  service
• e.g. jini//lus.comp.lancs.ac.uk
• Client must access a Jini service in another
  network
• Multicast discovery only works in local network
• Unicast protocol
• Client directly establishes a TCP connection with
  the lookup service and downloads the registrar
  proxy

TCP connection established
Unicast request sent
Unicast response sent
Discovering entity
Lookup service
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Multicast Announcement Protocol

• Reasons to use multicast announcement
• Lookup services announce their existence
• E.g. when a new lookup service is started
• Recover from network failure
• Clients lose track of lookup service ? when
  lookup service is restarted they are made aware
  of its return
• The Announcement protocol

Discovering Entity
Lookup Server
UDP Multicast
1. Listens on multicast announcement address
2. Sends a multicast announcement
TCP request - response
3. Performs unicast discovery
4. Sends response to unicast discovery
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The Join Protocol

• Allows the Jini service to advertise itself with
  the Lookup Service
• Invoke register() passing a ServiceItem object
  and a lease duration

register(ServiceItem, long)

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

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Leasing in Jini

• When Services die
• How do you stop their references accumulating in
  the lookup servers?
• Solution Leases
• A service is provided for a given amount of time
  (LEASE)
• Services must renew their leases
• Avoids third-party management for Lookup services
• Support failure detection
• Removes unreachable services
• Programming leases
• 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
• Avoid!!

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The Lookup Protocol

• The client creates a service template
• Contains the search query
• Combination of 3 parameters
• ServiceID, ServiceType (Class), attributes
• Invoke Lookup(ServiceTemplate) on the Lookup
  Service
• Via the registrar proxy

Lookup(ServiceTemplate t)
Jini Client (Consumer)
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Example Service Templates

• Search by attribute only
• Search for a service with attribute
• Name MyServer

aeAttributes new Entry1 aeAttributes0
new Name ("MyServer") template new
ServiceTemplate (null, null, aeAttributes)

• Search by service type
• Search for a service with a specific class
• Class HelloWorldService.class

Class types HelloWorldService.class templat
e new ServiceTemplate(null, types, null)
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Interacting with Jini Services

• Jini Service Proxies are of three types
• Proxy performs the service itself
• Code is self contained
• Does not require any remote interactions
• Proxy is an RMI stub for a remote service
• Minimal code only the interface to the remote
  object
• Client invokes operations on the proxy RMI used
  to invoke operation on the services
• Proxy is a smart adaptor
• Contains code to interact with the service
• Sockets
• CORBA
• Proprietary protocol

Client
Proxy
Service
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Jini in Full
7. Jini client uses Java proxy to communicate and
use the Jini service
Jini Client (Consumer)
Jini Services
6. Invokes lookup() on ServiceRegistrar to locate
desired service (returns Java proxy)
3. invokes register() on ServiceRegistrar object
to register service
2. receives a ServiceRegistrar object
5. receives a ServiceRegistrar object
1. Jini service discovers LUS
4. Jini client discovers LUS
LUS Jini Lookup Service
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Delving Deeper

• http//www.jini.org
• Community of Jini developers
• Sample code downloads, Jini articles, and blogs
• http//java.sun.com/developer/products/jini/
• Download the Jini platform from Sun
• Books
• Core JINI, W. K. Edwards, Prentice Hall
• Covers technical aspects of Jini
• JINI Example by Example, W.K. Edwards and T.
  Rodden, Prentice Hall
• Learn to program in Jini

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UPnP A Related Technology

• Universal Plug and Play (UPnP) from Microsoft
• Includes discovery, interaction, and event
  notification
• Based upon Web technologies (http, SOAP, XML)
  rather than Java technologies
• No mobile code
• Multicast based lookup
• XML descriptions of devices and services
• Matches DeviceType, ServiceType, and attribute
  matches
• Service interaction
• Via the SOAP protocol

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Expected Learning Outcomes

• At the end of this Unit
• You should understand why resource discovery is
  important in distributed systems
• You should know the different styles of resource
  discovery available
• You should know the key architectural features of
  the Jini technology
• You should know the benefits that Jini provides
  distributed application developers