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JXTA DiscoveryJXTA Discovery

www.jxta.orgwww.jxta.org

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Learning Objectives

• This module will help you...– Understand how JXTA advertisements

are published, cached, and expired– Understand the JXTA discovery

mechanism– Gain familiarity with the JXTA

Discovery Service API

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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

CacheManager

Discovery Service

IndexIndex

StoreExpirationStoreExpiration

• Persistence– Locally– Remotely– Out of band

• Horizon– Where to publish– When not to

publish

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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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Peergroup Services

Services are accessible through the peergroup object:

JavadiscService = peerGroup.getDiscoveryService();

CJxta_PG_get_discovery_service(group, &discovery);

PeerGroup Services

MembershipMembership

PipePipe

DiscoveryDiscovery

RendezvousRendezvous

ResolverResolver

EndpointEndpoint

PeerInfoPeerInfo

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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

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.

Publish Advertisements

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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.

Discovery Service – Java APIDiscover and Flush 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.getRemoteAdvertisements(null, DiscoveryService.PEER, null, null, 5, null); try { Thread.sleep(10 * 1000); } .. } .. }

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Discovery Service – C API

Jxta_status discovery_service_publish(Jxta_discovery_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

Publish and Flush Advertisements

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Discovery Service – C API

Jxta_status discovery_service_get_local_advertisements(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_advertisements(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.

Discover and Retrieve Advertisements

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Discovery Service – C API

Jxta_status discovery_service_add_discovery_listener(Jxta_discovery_service *service, Jxta_discovery_listener *listener)

– Register a discovery listener, to be notified on discovery events.

Jxta_status discovery_service_remove_discovery_listener(Jxta_discovery_service *service, Jxta_discovery_listener *listener)

– Remove a discovery listener.

Discovery Listeners

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Discovery Example – C

Jxta_PG_get_discovery_service(group, &discovery);

discovery_service_get_local_advertisements(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 (i=0; i < 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 DiscoveryEnd – JXTA Discovery

www.jxta.orgwww.jxta.org