JXTA%20Training%20-%20Discovery

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JXTA Discovery
www.jxta.org
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Advertisements

• XML documents
• Provide platform-independent representation of
  objects that can be exchanged between different
  platforms (e.g., Java and C)
• Common operations
• Create
• Publish
• Discover/search

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Publishing Advertisements

• Persistence
• Locally
• Remotely
• Out of band
• Horizon
• Where to publish
• When not to publish

Discovery Service
Cache Manager
Index
Store Expiration
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Advertisement Caching

• Advertisements may be cached by peers
• Protocols don't specify how peers cache info
• Standard JXTA API bindings automatically perform
  caching
• Each advertisement published with a time-to-live
• Relative expiration date
• When expired, advertisements are removed
• Advertisements can be re-published

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Publish versus RemotePublish

• Publish()
• Stores advertisement in persistent local cache
• Exposes advertisements within peergroup
• Can specify lifetime and expiration for others
  (DEFAULT_LIFETIME and DEFAULT_EXPIRATION)
• RemotePublish()
• Publishes advertisement within local subnet and
  to connected rendezvous peers within the context
  of the group
• Advertisements held in other peers' cache for
  specified duration (DEFAULT_EXPIRATION)

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Rendezvous Peers

• Each peer group has a set of rendezvous peers
• Rendezvous peers cache indices to edge peer
  advertisements
• Discovery queries routed between rendezvous peers
• Any peer may become a rendezvous peer
• Peers can be pre-configured to know certain
  rendezvous peers or can discover them dynamically

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Discovery Service

• Asynchronous mechanism for discovering
  advertisements (peers, peer groups, pipes,
  services)
• Implements Peer Discovery Protocol (PDP)
• Can retrieve advertisements in local cache
• Can send Discovery Query Message
• To a specific peer
• Propagated to JXTA network

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Asynchronous Discovery Model

• Peer propagates discovery request within a
  peergroup
• Responses are received asynchronously
• Can query local cache (advertisements are stored
  asynchronously as they arrive)
• Can use event-based notification (implement
  discovery listener)
• No guarantees when/if a response will be received

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Guidelines

• Discovery
• Limit discovery within NetPeerGroup to
  boot-strapping operation (i.e. Finding group,
  etc.)
• Scope discovery of other resources within a
  smaller peergroup
• Publishing Advertisements
• Should search for advertisements before creating
  may already be published
• Don't introduce inconsistencies (e.g.,
  advertisements are uniquely identified by an ID,
  and some thought should be given when discovering
  by name)

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Discovery Service Java API
Publish Advertisements

• void publish(Advertisement advertisement, int
  type)
• Publish an advertisement with default lifetime
  and expiration for others.
• void publish(Advertisement adv, int type, long
  lifetime, long lifetimeForOthers)
• Publish an advertisement that will expire after a
  certain time.
• void remotePublish(Advertisement advertisement,
  int type)
• Remote publish an advertisement will attempt to
  remote publish adv on all configured transports.
  .
• void remotePublish(Advertisement adv, int type,
  long lifetime)
• Remote publish an advertisement will attempt to
  remote publish adv on all configured transports.
  The Advertisement will carry a a expiration of
  lifetime.

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Discovery Service Java API
Discover and Flush Advertisements

• Enumeration getLocalAdvertisements(int type,
  String attribute String value)
• Retrieve stored peer, group, and general JXTA
  advertisements.
• void getRemoteAdvertisements(String peerid, int
  type,String attribute, String value, int
  threshold, net.jxta.discovery.DiscoveryListener
  listener)
• Discovery JXTA peer, group, and general JXTA
  advertisements.
• void addDiscoveryListener(DiscoveryListener
  listener)
• Add a discovery listener, to be notified on
  discovery events..
• boolean removeDiscoveryListener(DiscoveryListener
  listener)
• Remove a discovery listener.
• void flushAdvertisements(String id, int type)
• Flush stored advertisements.

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Discovery Example Java
public class DiscoveryDemo implements
DiscoveryListener static PeerGroup
netPeerGroup null private
DiscoveryService discovery PeerAdvertisement
peerAdv static public void main(String
args) DiscoveryDemo myapp new
DiscoveryDemo() myapp.startJxta()
myapp.run() // method to start the
JXTA platform private void startJxta()
try netPeerGroup
PeerGroupFactory.newNetPeerGroup()
catch ( PeerGroupException e)
... // Get the
discovery service from our peer group
discovery netPeerGroup.getDiscoveryService()

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Discovery Example Java (continued)
Public void discoveryEvent(DiscoveryEvent ev)
DiscoveryResponseMsg res
ev.getResponse() String name
unknown // Get the responding peer's
advertisement // get the peer
advertisement PeerAdvertisement peerAdv
res.getPeerAdvertisement() if (peerAdv
! null) name peerAdv.getName()
System.out.println("
Got a Discovery Response "
res.getResponseCount() " elements from peer
" name " ") // get the
discovered Advertisements....
Enumeration responses res.getAdvertisements()

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Discovery Example Java (continued)
public void run() try
discovery.addDiscoveryListener(this)
while (true)
System.out.println("Sending a Peer Discovery
Message") discovery.getRemoteAdve
rtisements(null, DiscoveryService.PEER,
null, null, 5, null) try
Thread.sleep(10 1000)
..
..
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Discovery Service C API
Publish and Flush Advertisements

• Jxta_status discovery_service_publish(Jxta_discove
  ry_service service, Jxta_advertisement
  adv, short type, long lifetime,
  long lifetimeForOthers)
• Publish an advertisement that will expire after a
  certain time.
• Jxta_status discovery_service_remote_publish(Jxta_
  discovery_service service, Jxta_id
  peerid, Jxta_advertisement adv, short
  type, long expirationtime)
• Remote publish an advertisement will attempt to
  remote publish adv on all configured transports
  with the specified expiration.
• Jxta_status discovery_service_flush_advertisements
  (Jxta_discovery_service service,
  char id, short type)
• Flush stored advertisement

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Discovery Service C API
Discover and Retrieve Advertisements

• Jxta_status discovery_service_get_local_advertisem
  ents (Jxta_discovery_service service,
  short type, const char attribute, const
  char value, Jxta_vector advertisements)
• Retrieve stored peer, group and general JXTA
  advertisements.
• Jxta_status discovery_service_get_remote_advertise
  ments (Jxta_discovery_service service,
  Jxta_id peerid, short type, const char
  attribute, const char value, int
  threshold, Jxta_discovery_listener listener)
• Discover peer, group, and general JXTA
  advertisements.

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Discovery Service C API
Discovery Listeners

• Jxta_status discovery_service_add_discovery_listen
  er (Jxta_discovery_service service,
  Jxta_discovery_listener listener)
• Register a discovery listener, to be notified on
  discovery events.
• Jxta_status discovery_service_remove_discovery_li
  stener (Jxta_discovery_service service,
  Jxta_discovery_listener listener)
• Remove a discovery listener.

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Discovery Example C

Jxta_PG_get_discovery_service(group,
discovery) discovery_service_get_local_advert
isements(discovery,
DISC_GROUP, (char
)jstring_get_string(attr),
(char )jstring_get_string(value),
res_vec) if (res_vec ! NULL ) int
i printf("restored d group advertisement(s)
\n",jxta_vector_size(res_vec)) for (i0 i lt
jxta_vector_size(res_vec) i )
Jxta_PGA pgadv Jstring xml
jxta_vector_get_object_at (res_vec,
(Jxta_object)pgadv, i)
jxta_PGA_getxml( pgadv, xml ) printf(
(char )jstring_get_string(xml) )
JXTA_OBJECT_RELEASE(pgadv)
JXTA_OBJECT_RELEASE(xml)
JXTA_OBJECT_RELEASE(res_vec)
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End JXTA Discovery
www.jxta.org