Revision as of 10:45, 23 March 2010

What is ZooDiscovery?

ZooDiscovery is a discovery mechanism that runs as an OSGi service. It leverage Apache ZooKeeper robustness and implements Eclipse ECF Discovery API.(Hence the name!). ZooDiscovery is flexible and easy to configure.
This work is funded and made open by Remain Software and Industrial-TSI

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The concept in some words

ZooDiscovery implements both ECF discovery interfaces: IDiscoveryAdvertiser and IDiscoveryLocator. That is, ZooDiscovery can publish our services and gets us noticed about discovered services. Perfect! But how? A ZooDiscovery instance running at your machine does its job by exchanging data with other ZooDiscovery instance(s) running elsewhere. So each running ZooDiscovery service must know where that other "elsewhere" exactly is. This is why we should first make our ZooDiscovery happy, giving it an IP address to play with.
To keep it smooth, let's take it step by step following these cases:

Learn by asking

How to tell ZooDiscovery the target I want to connect to?

/*We use the property "discovery.flavor.standalone" to specify we're going to run as standalone(more about this later)
 and its value is the remote location's IP address we intend to connect to . 
  Please replace with usable IP address(es) when applicable. A comma separated list of one or more IP addresses is valid as well. 
 Something like: "discovery.flavor.standalone=192.1.32.10,192.1.32.11" means our ZooDiscovery instance will connect to both address 192.1.32.10 and 192.1.32.11
 where other ZooDiscovery services are running.
*/
IContainer container = null;
try {
//"ecf.discovery.zookeeper" is the container name we want to initiate.
container = ContainerFactory.getDefault().createContainer("ecf.discovery.zookeeper");
} catch(ContainerCreateException e1){ // TODO 
} 
ID target = container.getConnectNamespace().createInstance(  
new String[] { "discovery.flavor.standalone=192.1.32.10" });

How to get ZooDiscovery service?

IContainer container = null;
try {
//"ecf.discovery.zookeeper" is the container name we want to initiate.
container = ContainerFactory.getDefault().createContainer("ecf.discovery.zookeeper");
} catch(ContainerCreateException e1){ // TODO 
} 

ID target = container.getConnectNamespace().createInstance(  
new String[] { "discovery.flavor.standalone=192.1.32.10" });
try {
//we then try and connect.
container.connect(target, null);
} catch (ContainerConnectException e1) { // TODO  
}
// Connected! Our provider is ready then.

// To advertise services we need adapting our container this way:
IDiscoveryAdvertiser discoveryAdvertiser = (IDiscoveryAdvertiser) container.getAdapter(IDiscoveryAdvertiser.class);
//then we enjoy calling IDiscoveryAdvertiser contract methods

// To localize/discover services we need adapting it this way:
IDiscoveryLocator discoveryLocator = (IDiscoveryLocator) container.getAdapter(IDiscoveryLocator.class);
//then we enjoy calling IDiscoveryLocator  contract methods

I want ZooDiscovery publishing my OSGi services automatically

OSGi services registered with property "osgi.remote.interface=*" are candidate for auto-publication. By default this option is disabled.
To make ZooDiscovery autopublish all services having the above property once connected, you basically use the same way as above (How to get ZooDiscovery service?)

with the property "discovery.publish.auto=true"  added when creating the target id.
ID serverId = container.getConnectNamespace().createInstance(  
new String[] { "discovery.flavor.standalone=192.1.32.10" ,"discovery.publish.auto=true"});
//...same as above
// Once connected ZooDiscovery will publish all OSGi services registered with "osgi.remote.interface=*". You can still adapt and use as above, though.

How to build a ServiceInfo object and publish it

//Some service location
URI uri = URI uri = URI.create("http://www.example.com/discovery");
//Some service priority
int priority = 0;
//Some service weight
int weight = 3;
//Some random service properties
ServiceProperties serviceProperties = new ServiceProperties();
serviceProperties.setProperty("foobar", new String("foobar"));
serviceProperties.setPropertyBytes("foobar1", new byte[] { 1, 2, 3 });
IServiceTypeID serviceTypeID = null;
try {
serviceTypeID = ServiceIDFactory.getDefault().createServiceTypeID(
DiscoveryContainer.getSingleton().getConnectNamespace(),new String[] {"service1","service2"}, new String[] {"someProtocol"});
} catch (IDCreateException e) {//TODO			
}
//build a service info instance to be published
ServiceInfo  serviceInfo = new ServiceInfo(uri, "myServiceName", serviceTypeID, priority, weight, serviceProperties);
//advertise the service
discoveryAdvertiser.registerService(serviceInfo);

I want to publish an OSGi service using its ServiceReference

//Some service:
public interface IFooService  {
String getName();
}

//Some implementation
public class FooService implements IFooService {
public String getName() {
return "Foo Service";
}
}

//Registration as an OSGi service  
public void start(BundleContext c){
Properties props = new Properties();
//props.put("osgi.remote.interfaces", "*");
ServiceRegistration registration = c.registerService(IFooService.class.getName(), new FooService(),props);				
IContainer container = null;
try {
//"ecf.discovery.zookeeper" is the container name we want to initiate.
container = ContainerFactory.getDefault().createContainer("ecf.discovery.zookeeper");
} catch(ContainerCreateException e1){ // TODO 
} 

ID serverId = container.getConnectNamespace().createInstance(  
new String[] { "discovery.flavor.standalone=192.1.32.10" });//replace with a valid ip address
try {
//we then try and connect.
container.connect(serverId, null);
} catch (ContainerConnectException e1) { // TODO  
}
IDiscoveryAdvertiser discoveryAdvertiser = (IDiscoveryAdvertiser) container.getAdapter(IDiscoveryAdvertiser.class);
IServiceInfo sinfo = new AdvertisedService(registration.getReference());
discoveryAdvertiser.registerService(sinfo);

I want to get notified about discovered services

You get notified about discovered services by registering yourself as an IServiceListener. Let's take it for a ride and make an inner listener class to see how lightweight its contract is:

IServiceListener sl = new IServiceListener() {
public void serviceUndiscovered(IServiceEvent anEvent) {
// service lost, do something..
}
public void serviceDiscovered(IServiceEvent anEvent) {
// new service is in, do something..
}
}; 
//register to get informed
discoveryLocator.addServiceListener(sl);

//To register for specific service types discoveries, you might consider registering under IServiceTypeListener

NOTE: ZooDiscovery tracks OSGi services being registered under IServiceListener or IServiceTypeListener, and add them as listeners so that (if you choose to) you don't have to add them explicitly the way we did just above. This is handy when your design is a bit more dynamic/component driven.

How to get this whole thing doing its job with less code

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Advanced configuration

ZooDiscovery Flavors

Standalone mode: discovery.flavor.standalone

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Centralized mode: discovery.flavor.centralized

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Replicated mode: discovery.flavor.replicated

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Fine tuning the underlying ZooKeeper

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Recipes

Automate proxying discovered services using R-OSGi and ZooDiscovery