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Jini Lookup Discovery Service
Spec Index A Collection of Jini Technology Helper Utilities and Services Specifications

Version 2.0

LD - JiniTM Lookup Discovery Service

LD.1 Introduction

Part of the JiniTM Discovery and Join Specification is devoted to defining the discovery requirements for well-behaved Jini clients and services, called discovering entities, which are required to participate in the multicast discovery protocols. Discovering entities are required to send multicast discovery requests to lookup services with which the entities wish to interact. In addition, they must continuously listen for and act on announcements from the desired lookup services. Interactions with a discovered lookup service may involve registration with that lookup service, or may simply involve querying the lookup service for services of interest (or both). To find specific lookup services, discovering entities also need to be able to participate in the unicast discovery protocol.

Under certain circumstances, a discovering entity may find it useful to allow a third party to perform the entity's discovery duties. For example, an activatable entity that wishes to deactivate may wish to employ a special Jini technology-enabled service (Jini service)--referred to as a lookup discovery service--to perform discovery duties on its behalf. Such an entity may wish to deactivate for various reasons, one being to conserve computational resources. While the entity is inactive, the lookup discovery service, running on the same or a separate host, would employ the discovery protocols to find lookup services in which the entity has expressed interest and would notify the entity when a previously unavailable lookup service has become available.

The facilities of the lookup discovery service are of particular value in a scenario in which a new lookup service is added to a long-lived djinn containing multiple inactive services. Without the use of a lookup discovery service, the time frame over which the new lookup service is fully populated can be both unpredictable and unbounded.

To understand why this time frame can be unpredictable, consider the fact that an inactive service has no way of discovering a new lookup service. This means that each inactive service in the djinn that wishes to discover and join a new lookup service must first activate. Since activation of a service occurs when some client attempts to use the service, the amount of time that passes between the arrival of the new lookup service and the activation of the service can vary greatly over the range of services in the djinn. Thus, the time frame over which the lookup service becomes fully populated cannot be predicted because it could take arbitrarily long before all of the services activate and then discover and join the new lookup service.

In addition to being unpredictable, the time it takes for the lookup service to fully populate can also be unbounded. This is because there is no guarantee that the lookup service will send multicast announcements between the time the service activates and the time it deactivates. If the timing is right, it is possible that one or more of the services in the djinn may never discover and join the new lookup service. Thus, without the use of the lookup discovery service, the new lookup service may never fully populate.

As another example of a discovering entity that may find it useful to allow a third party to perform the entity's discovery duties, consider an entity that exists in an environment with one of the following characteristics:

If such an entity was provided with references to lookup services--located outside of the entity's multicast radius--that contain services needed by the entity, the entity could contact each lookup service and retrieve the desired service references. One way to provide the entity with access to those lookup services might be to configure the entity to find and use a lookup discovery service, operating beyond the entity's range, that can employ multicast discovery to find nearby lookup services belonging to groups in which the entity has expressed interest. After acquiring references to the targeted lookup services, the lookup discovery service would pass those references to the entity, providing the entity with access to the services registered with each lookup service. In this way, the entity participates in the multicast discovery protocols through a proxy relationship with the lookup discovery service, gaining access not only to lookup services outside of its own range, but also to all of the services registered with those lookup services.

Note that the scenario just described does not come without restrictions. For the lookup discovery service to be able to "link" an entity with lookup services in the way just described, the lookup discovery service must be registered with a lookup service having a location that either is known to the entity or is within the multicast radius of the entity. Furthermore, the lookup discovery service must be running on a host that is located within the multicast radius of the lookup services with which the entity wishes to be linked. That is, the entity must be able to find the lookup discovery service, and the lookup discovery service must be able to find the other desired lookup services.

To address these scenarios, the lookup discovery service participates in both the multicast discovery protocols and the unicast discovery protocol on behalf of a registered discovering entity or client. This service will listen for and process multicast announcement packets from Jini lookup services and will, until successful, repeatedly attempt to discover specific lookup services that the client is interested in finding.

Upon discovery of a previously undiscovered lookup service of interest, the lookup discovery service notifies all entities that have requested the discovery of that lookup service that such an event has occurred. The event mechanism employed by the lookup discovery service satisfies the requirements defined in the Jini Distributed Events Specification. Note that the entity that receives such an event notification does not have to be the client of the lookup discovery service; it may be a third-party event-handling service such as an event mailbox service. Once a client is notified of the discovery of a lookup service, it is left to the client to define the semantics of how it interacts with that lookup service. For example, the client may wish to join the lookup service, simply query it for other useful services, or both.

The lookup discovery service must be implemented as a well-behaved Jini service and must comply with all of the policies embodied in the Jini technology programming model. Thus, the resources granted by this service are leased, and implementations of this service must adhere to the distributed leasing model for Jini technology as defined in the Jini Distributed Leasing Specification. That is, the lookup discovery service will grant its services for only a limited period of time without an active expression of continuing interest on the part of the client.

LD.1.1 Goals and Requirements

The requirements of the interfaces and classes specified in this document are:

The goals of this document are as follows:

LD.1.2 Other Types

The types defined in the specification of the LookupDiscoveryService interface are in the net.jini.discovery package. The following object types may be referenced in this chapter. Whenever referenced, these object types will be referenced in unqualified form: 

net.jini.core.discovery.LookupLocator
net.jini.core.event.EventRegistration
net.jini.core.event.RemoteEventListener
net.jini.core.event.UnknownEventException
net.jini.core.lease.Lease
net.jini.core.lookup.ServiceID
net.jini.core.lookup.ServiceRegistrar
net.jini.discovery.DiscoveryEvent
net.jini.discovery.DiscoveryGroupManagement
net.jini.discovery.DiscoveryListener
java.io.IOException
java.rmi.MarshalledObject
java.rmi.NoSuchObjectException
java.rmi.RemoteException
java.util.Map

LD.2 The Interface

The LookupDiscoveryService interface defines the service--referred to as the lookup discovery service--previously introduced in this specification. Through this interface, other Jini services and clients may request that discovery processing be performed on their behalf. This interface belongs to the net.jini.discovery package, and any service implementing this interface must comply with the definition of a Jini service. This interface is not a remote interface; each implementation of this service exports a front end proxy object that implements this interface local to the client, using an implementation-specific protocol to communicate with the actual remote server (the back end). All of the proxy methods must obey normal Java Remote Method Invocation (RMI) remote interface semantics except where explicitly noted. Two proxy objects are equal (using the equals method) if they are proxies for the same lookup discovery service.

The one method defined in this interface throws a RemoteException, and requires only the default serialization semantics so that this interface can be implemented directly using Java RMI. 

package net.jini.discovery;

public interface LookupDiscoveryService {
    public LookupDiscoveryRegistration register(
                              String[] groups,
                              LookupLocator[] locators,
                              RemoteEventListener listener,
                              MarshalledObject handback,
                              long leaseDuration)
                                      throws RemoteException;
}

When requesting a registration with the lookup discovery service, the client indicates the lookup services it is interested in discovering by submitting two sets of objects. Each set may contain zero or more elements. One set consists of the names of the groups whose members are lookup services the client wishes to be discovered. The other set consists of LookupLocator objects, each corresponding to a specific lookup service the client wishes to be discovered.

For each successful registration the lookup discovery service will manage both the set of group names and the set of locators submitted. These sets will be referred to as the managed set of groups and the managed set of locators, respectively. The managed set of groups associated with a particular registration contains the names of the groups whose members consist of lookup services that the client wishes to be discovered through multicast discovery. Similarly, the managed set of locators contains instances of LookupLocator, each corresponding to a specific lookup service that the client wishes to be discovered through unicast discovery. The references to the lookup services that have been discovered will be maintained in a set referred to as the managed set of lookup services (or managed set of registrars).

Note that when the general term managed set is used, it should be clear from the context whether groups, locators, or registrars are being discussed. Furthermore, when the term group discovery or locator discovery is used, it should be taken to mean, respectively, the employment of either the multicast discovery protocols or the unicast discovery protocol to discover lookup services that correspond to members of the appropriate managed set.

Finally, with respect to any remote operation, two classes of exception are referenced in this document. The term indefinite exception refers to a class of exception where any such exception does not allow assertions to be made about the probability of success (or failure) of future attempts to perform the operation. A RemoteException caused by a transient communication failure is one such example of an exception that can be classified as an indefinite exception. As such, whenever an indefinite exception is encountered, retrying the operation at a later time may be advisable because the operation may succeed when retried.

The other class of exception to note is identified by the term definite exception, which refers to a class of exception where any such exception is indicative of a permanent failure. That is, when an operation fails as a result of an exception that can be classified as a definite exception, that exception allows one to assert that any future attempts to perform the failed operation will also be met with failure. In many cases, a SecurityException can be interpreted as a definite exception. Thus, whenever an operation fails as a result of a definite exception, because it is a virtual certainty that failure will again result on all future attempts to perform the operation, it is generally not advisable to retry the operation.

LD.3 The Semantics

To employ the lookup discovery service to perform discovery on its behalf, a client must first register with the lookup discovery service by invoking the register method defined in the LookupDiscoveryService interface. The register method is the only method specified by this interface.

LD.3.1 Registration Semantics

An invocation of the register method produces an object--referred to as a registration object (or simply a registration)--that is mutable. That is, the registration object contains methods through which it may be changed. Because registrations are mutable, each invocation of the register method produces a new registration object. Thus, the register method is not idempotent.

The register method may throw a RemoteException. Typically, this exception occurs when there is a communication failure between the client and the lookup discovery service. When this exception does occur, the registration may or may not have been successful.

Each registration with the lookup discovery service is persistent across restarts (or crashes) of the lookup discovery service until the lease on the registration expires or is cancelled.

The register method takes the following as arguments:

The register method returns an object that implements the LookupDiscoveryRegistration interface. It is through this returned object that the client interacts with the lookup discovery service. This interaction includes activities such as group and locator management, state retrieval, and discarding discovered but unavailable lookup services so that they are eligible for rediscovery (see Section LD.4.1, "The LookupDiscoveryRegistration Interface" for definition of the semantics of the methods of the LookupDiscoveryRegistration interface).

The groups argument takes a String array, none of whose elements may be null. Although it is acceptable to specify null (which is equivalent to DiscoveryGroupManagement.ALL_GROUPS) for the groups argument itself, if the argument contains one or more null elements, a NullPointerException is thrown. If the value is null, the lookup discovery service will attempt to discover all lookup services located within the multicast radius of the host on which the lookup discovery service is running. If an empty array (equivalent to DiscoveryGroupManagement.NO_GROUPS) is passed in, then no group discovery will be performed for the associated registration until the client, through the registration's setGroups or addGroups method, changes the contents of the managed set of groups to either a non-empty set of group names or null.

The locators argument takes an array of LookupLocator objects, none of whose elements may be null. If either the empty array or null is passed in as the locators argument, then no locator discovery will be performed for the associated registration until the client, through the registration's addLocators or setLocators method, changes the managed set of locators to a non-empty set of locators. Although it is acceptable to input null for the locators argument itself, if the argument contains one or more null elements, a NullPointerException is thrown.

If the register method is invoked with a set of group names and a set of locators in which either or both sets contain duplicate elements (where duplicate locators are determined by LookupLocator.equals), the invocation is equivalent to constructing this class with no duplicates in either set.

Upon discovery of a lookup service, through either group discovery or locator discovery, the lookup discovery service will send an event, referred to as a discovered event, to the listener associated with the registration produced by the call to register.

After initial discovery of a lookup service, the lookup discovery service will continue to monitor the group membership state reflected in the multicast announcements from that lookup service. Depending on the lookup service's current group membership, the lookup discovery service may send either a discovered event or an event referred to as a discarded event. The conditions under which either a discovered event or a discarded event will be sent are as follows:

It is important to note that when the lookup discovery service (passively) discards a lookup service, due to group membership changes (lost interest) or unavailability (communication failure), the discarded event will be sent to only the listeners of those registrations that have previously requested that the affected lookup service be discovered through at least group discovery. That is, the listener of any registration that is interested in the affected lookup service through only locator discovery will not be sent either type of passive discarded event. This is because the semantics of the lookup discovery service assume that since the client, through the registration request, expressed no interest in discovering the lookup service through its group membership, the client must also have no interest in any group-related changes in that lookup service's state.

A more detailed discussion of the event semantics of the lookup discovery service is presented in Section LD.3.2, "Event Semantics".

A valid parameter must be passed as the listener argument of the register method. If a null value is input to this argument, then a NullPointerException will be thrown and the registration fails.

Note that if an indefinite exception occurs while attempting to send a discovered or discarded event to a registration's listener, the lookup discovery service will continue to attempt to send the event until either the event is successfully delivered or the client's lease on that registration expires. If an UnknownEventException or a definite exception occurs while attempting to send a discovered or discarded event to a registration's listener, the lookup discovery service assumes that the client is in an unknown, possibly corrupt state, and will cancel the lease on the registration and clear the registration from its managed set.

The state information maintained by the lookup discovery service includes the set of group names, locators, and listeners submitted by each client through each invocation of the register method, with duplicates eliminated. This state information contains no knowledge of the clients that register with the lookup discovery service. Thus, there is no requirement that a client identify itself during the registration process.

LD.3.2 Event Semantics

For each registration created by the lookup discovery service, an event identifier will be generated that uniquely maps the registration to the listener as well as to the registration's managed set of groups and managed set of locators. This event identifier is returned as a part of the returned registration object and is unique across all other active registrations with the lookup discovery service.

Whenever the lookup discovery service finds a lookup service matching the discovery criteria of one or more of its registrations, it sends an instance of RemoteDiscoveryEvent (a subclass of RemoteEvent) to the listener corresponding to each such registration. The event sent to each listener will contain the appropriate event identifier.

Once an event signaling the discovery (by group or locator) of a desired lookup service has been sent, no other discovered events for that lookup service will be sent to a registration's listener until the lookup service is discarded (either actively, by the client through the registration, or passively by the lookup discovery service) and then rediscovered. Note that more information about what it means for a lookup service to be discarded is presented in Section LD.3.1, "Registration Semantics" and the section of this specification titled "Discarding Lookup Services".

If, between the time a lookup service is discarded and the time it is rediscovered, a new registration is requested having parameters indicating interest in that lookup service, upon rediscovery of the lookup service an event will also be sent to that new registration's listener.

The sequence numbers for a given event identifier are strictly increasing (as defined in the Jini Distributed Events Specification), which means that when any two such successive events have sequence numbers that differ by only a value of 1, then no events have been missed. On the other hand, when the set of received events is viewed in order, if the difference between the sequence numbers of two successive events is greater than 1, then one or more events may or may not have been missed. For example, a difference greater than 1 could occur if the lookup discovery service crashes, even if no events are lost because of the crash. When two such successive events have sequence numbers whose difference is greater than 1, there is said to be a gap between the events.

When a gap occurs between events, the local state (on the client) related to the discovered lookup services may or may not fall out of sync with the corresponding remote state maintained by the lookup discovery service. For example, if the gap corresponds to a missed event representing the (initial) discovery of a targeted lookup service, the remote state will reflect this discovery, whereas the client's local state will not. To allow clients to identify and correct such a situation, each registration object provides a method that returns a set consisting of the proxies to the lookup services that have been discovered for that registration. With this information the client can update its local state.

When requesting a registration with the lookup discovery service, a client may also supply (as a parameter to the register method) a reference to an object, wrapped in a MarshalledObject, referred to as a handback. When the lookup discovery service sends an event to a registration's listener, the event will also contain a reference to this handback object. The lookup discovery service will not change the handback object. That is, the handback object contained in the event sent by the lookup discovery service will be identical to the handback object registered by the client with the event mechanism.

The semantics of the object input to the handback argument are left to each client to define, although null may be input to this argument. The role of the handback object in the remote event mechanism is detailed in the Jini Distributed Events Specification.

LD.3.3 Leasing Semantics

When a client registers with the lookup discovery service, it is effectively requesting a lease on the resources provided by that service. The initial duration of the lease granted to a client by the lookup discovery service will be less than or equal to the requested duration reflected in the value input to the leaseDuration argument. That value must be positive, Lease.FOREVER, or Lease.ANY. If any other value is input to this argument, an IllegalArgumentException will be thrown. The client may obtain a reference to the Lease object granted by the lookup discovery service through the associated registration returned by the service (see Section LD.4.1, "The LookupDiscoveryRegistration Interface").

LD.4 Supporting Interfaces and Classes

The lookup discovery service depends on the LookupDiscoveryRegistration interface, as well as on the concrete classes RemoteDiscoveryEvent and LookupUnmarshalException.

LD.4.1 The LookupDiscoveryRegistration Interface

When a client requests a registration with the lookup discovery service, an object that implements the LookupDiscoveryRegistration interface is returned. It is through this interface that the client manages the state of its registration with the lookup discovery service. 

package net.jini.discovery;

public interface LookupDiscoveryRegistration {
    public EventRegistration getEventRegistration();
    public Lease getLease();
    public ServiceRegistrar[] getRegistrars()
                             throws LookupUnmarshalException,
                                    RemoteException;
    public String[] getGroups() throws RemoteException;
    public LookupLocator[] getLocators()
                                      throws RemoteException;
    public void addGroups(String[] groups)
                                      throws RemoteException;
    public void setGroups(String[] groups)
                                      throws RemoteException;
    public void removeGroups(String[] groups)
                                      throws RemoteException;
    public void addLocators(LookupLocator[] locators)
                                      throws RemoteException;
    public void setLocators(LookupLocator[] locators)
                                      throws RemoteException;
    public void removeLocators(LookupLocator[] locators)
                                      throws RemoteException;
    public void discard(ServiceRegistrar registrar)
                                      throws RemoteException;
}

As with the LookupDiscoveryService interface, the LookupDiscoveryRegistration interface is not a remote interface. Each implementation of the lookup discovery service exports proxy objects that implement this interface local to the client, using an implementation-specific protocol to communicate with the actual remote server. All of the proxy methods must obey normal Java RMI remote interface semantics except where explicitly noted. Two proxy objects are equal (using the equals method) if they are proxies for the same registration created by the same lookup discovery service.

The discovery facility of the lookup discovery service, together with its event mechanism, make up the set of resources clients register to use. Because the resources of the lookup discovery service are leased, access is granted for only a limited period of time unless there is an active expression of continuing interest on the part of the client.

When a client uses the registration process to request that a lookup discovery service perform discovery of a set of desired lookup services, the client is also registered with the service's event mechanism. Because of this implicit registration with the event mechanism, the lookup discovery service "bundles" both resources under a single lease. When that lease expires, both discovery processing and event notifications will cease with respect to the registration that resulted from the client's request.

To facilitate lease management and event handling, the LookupDiscoveryRegistration interface defines methods that allow the client to retrieve its event registration information. Additional methods defined by this interface allow the client to retrieve references to the registration's currently discovered lookup services, as well as to modify the managed sets of groups and locators.

If the client's registration with the lookup discovery service has expired or been cancelled, then any invocation of a remote method defined in this interface will result in a NoSuchObjectException. That is, any method that communicates with the back-end server of the lookup discovery service will throw a NoSuchObjectException if the registration on which the method is invoked no longer exists. Note that if a client receives a NoSuchObjectException as a result of an invocation of such a method, although the client can assume that the registration no longer exists, the client cannot assume that the lookup discovery service itself no longer exists.

Each remote method of this interface may throw a RemoteException. Typically, this exception occurs when there is a communication failure between the client and the lookup discovery service. Whenever this exception occurs as a result of the invocation of one of these methods, the method may or may not have completed its processing successfully.

LD.4.1.1 The Semantics

The methods defined by this interface are organized into a set of accessor methods, a set of group mutator methods, a set of locator mutator methods, and the discard method. Through the accessor methods, various elements of a registration's state can be retrieved. The mutator methods provide a mechanism for changing the set of groups and locators to be discovered for the registration. Through the discard method, a particular lookup service may be made eligible for rediscovery.

The Accessor Methods

The getEventRegistration method returns an EventRegistration object that encapsulates the information the client needs to identify a notification sent by the lookup discovery service to the registration's listener. This method is not remote and takes no arguments.

The getLease method returns the Lease object that controls a client's registration with the lookup discovery service. It is through the Lease object returned by this method that the client requests the renewal or cancellation of the registration with the lookup discovery service. This method is not remote and takes no arguments.

Note that the object returned by the getEventRegistration method also provides a getLease method. That method and the getLease method defined by the LookupDiscoveryRegistration interface both return the same Lease object. The getLease method defined here is provided as a convenience to avoid the indirection associated with the getLease method on the EventRegistration object, as well as to avoid the overhead of making two method calls.

The getRegistrars method returns a set of instances of the ServiceRegistrar interface. Each element in the set is a proxy to one of the lookup services that have already been discovered for the registration. Additionally, each element in the set will be unique with respect to all other elements in the set, as determined by the equals method provided by each element. The contents of the set make up the current remote state of the set of lookup services discovered for the registration. This method returns a new array on each invocation.

This method can be used to maintain synchronization between the set of discovered lookup services making up a registration's local state on the client and the registration's corresponding remote state maintained by the lookup discovery service. The local state can become unsynchronized with the remote state when a gap occurs in the events received by the registration's listener.

According to the event semantics of the lookup discovery service, if there is no gap between two sequence numbers, no events have been missed and the states remain synchronized with each other; if there is a gap, events may or may not have been missed. Therefore, upon finding gaps in the sequence of events, the client can invoke this method and use the returned information to synchronize the local state with the remote state.

To construct its return set, the getRegistrars method retrieves from the lookup discovery service the set of lookup service proxies making up the registration's current remote state. When the lookup discovery service sends the requested set of proxies, the set is sent as a set of marshalled instances of the ServiceRegistrar interface. The lookup discovery service individually marshals each proxy in the set that it sends because if it were not to do so, any deserialization failure on the set would result in an IOException, and failure would be declared for the whole deserialization process, not just an individual element. This would mean that all elements of the set sent by the lookup discovery service--even those that were successfully deserialized--would be unavailable to the client. Individually marshalling each element in the set minimizes the "all or nothing" aspect of the deserialization process, allowing the client to recover those proxies that can be successfully unmarshalled and to proceed with processing that might not be possible otherwise.

When constructing the return set, this method attempts to unmarshal each element of the set of marshalled proxy objects sent by the lookup discovery service. When failure occurs while attempting to unmarshal any of those elements, this method throws an exception of type LookupUnmarshalException (described later). It is through the contents of that exception that the client can recover any available proxies and perform error handling related to the unavailable proxies. The contents of the LookupUnmarshalException provide the client with the following useful information:

The type of exception that occurs when attempting to unmarshal an element of the set sent by the lookup discovery service is typically an IOException or a ClassNotFoundException (usually the more common of the two). A ClassNotFoundException occurs whenever a remote object on which the marshalled proxy depends cannot be retrieved and loaded, usually because the codebase of one of the object's classes or interfaces is currently "down." To address this situation, the client may wish to proceed with its processing using the successfully unmarshalled proxies, and attempt to unmarshal the unavailable proxies (or re-invoke this method) at some later time.

If the getRegistrars method returns successfully without throwing a LookupUnmarshalException, the client is guaranteed that all marshalled proxies belonging to the set sent by the lookup discovery service have each been successfully unmarshalled; the client then has a snapshot--relative to the point in time when this method is invoked--of the remote state of the lookup services discovered for the associated registration.

The getGroups method returns an array consisting of the group names from the registration's managed set; that is, the names of the groups the lookup discovery service is currently configured to discover for the associated registration. If the managed set of groups is empty, this method returns the empty array. If there is no managed set of groups associated with the registration (that is, the lookup discovery service is configured to discover DiscoveryGroupManagement.ALL_GROUPS for the registration), then null is returned.

The getLocators method returns an array consisting of the LookupLocator objects from the registration's managed set; that is, the locators of the specific lookup services the lookup discovery service is currently configured to discover for the associated registration. If the managed set of locators is empty, this method returns the empty array.

The Group Mutator Methods

With respect to a particular registration, the groups to be discovered may be modified using the methods described in this section. In each case, a set of groups is represented as a String array, none of whose elements may be null. If any set of groups input to one of these methods contains one or more null elements, a NullPointerException is thrown. The empty set is denoted by the empty array (DiscoveryGroupManagement.NO_GROUPS), and "no set" is indicated by null (DiscoveryGroupManagement.ALL_GROUPS). No set indicates that all lookup services within the multicast radius should be discovered, regardless of group membership. Invoking any of these methods with an input set of groups that contains duplicate names is equivalent to performing the invocation with the duplicate group names removed from the input set.

The addGroups method adds a set of group names to the registration's managed set. This method takes one argument: a String array consisting of the set of group names with which to augment the registration's managed set.

If the registration has no current managed set of groups to augment, this method throws an UnsupportedOperationException. If the parameter value is null, this method throws a NullPointerException. If the parameter value is the empty array, then the registration's managed set of groups will not change.

The setGroups method replaces all of the group names in the registration's managed set with names from a new set. This method takes one argument: a String array consisting of the set of group names with which to replace the current names in the registration's managed set.

If null is passed to setGroups, the lookup discovery service will attempt to discover any undiscovered lookup services located within range of the lookup discovery service, regardless of group membership.

If the empty set is passed to setGroups, then group discovery will be halted until the registration's managed set of groups is changed--through a subsequent call to this method or to addGroups--to a set that is either a non-empty set of group names or null.

The removeGroups method deletes a set of group names from the registration's managed set. This method takes one argument: a String array containing the set of group names to remove from the registration's managed set.

If the registration has no current managed set of groups from which to remove elements, this method throws an UnsupportedOperationException. If null is input, this method throws a NullPointerException. If the registration does have a managed set of groups from which to remove elements, but either the input set is empty or none of the elements in the input set match any element in the managed set, then the registration's managed set of groups will not change.

Once a new group name has been placed in the registration's managed set as a result of an invocation of either addGroups or setGroups, if there are lookup services belonging to that group that have already been discovered for that registration, no event will be sent to the registration's listener for those particular lookup services. However, attempts to discover any undiscovered lookup services belonging to that group will continue to be made on behalf of the registration.

Any already discovered lookup service that is a member of one or more of the groups removed from the registration's managed set as a result of an invocation of either setGroups or removeGroups will be discarded and will no longer be eligible for discovery (for that registration), but only if that lookup service satisfies both of the following conditions:

The Locator Mutator Methods

With respect to a particular registration, the set of locators to discover may be modified using the methods described in this section. In each case, a set of locators is represented as an array of LookupLocator objects, none of whose elements may be null. If any set of locators input to one of these methods contains one of more null elements, a NullPointerException is thrown. Invoking any of these methods with a set of locators that contains duplicate locators (as determined by LookupLocator.equals) is equivalent to performing the invocation with the duplicates removed from the input set.

The addLocators method adds a set of LookupLocator objects to the registration's managed set. This method takes one argument: an array consisting of the set of locators with which to augment the registration's managed set.

If null is passed to addLocators, a NullPointerException will be thrown. If the parameter value is the empty array, the registration's managed set of locators will not change.

The setLocators method replaces all of the locators in the registration's managed set with LookupLocator objects from a new set. This method takes one argument: an array consisting of the set of locators with which to replace the current locators in the registration's managed set.

If null is passed to setLocators, a NullPointerException will be thrown.

If the empty set is passed to setLocators, then locator discovery will be halted until the registration's managed set of locators is changed--through a subsequent call to this method or to addLocators--to a set that is non-null and non-empty.

The removeLocators method deletes a set of LookupLocator objects from the registration's managed set. This method takes one argument: an array containing the set of LookupLocator objects to remove from the registration's managed set.

If null is passed to removeLocators, a NullPointerException will be thrown. If any element of the set of locators to remove is not contained in the registration's managed set, removeLocators takes no action with respect to that element. If the parameter value is the empty array, the managed set of locators will not change.

Whenever a new locator is placed in the managed set as a result of an invocation of one of the locator mutator methods and that new locator equals none of the previously discovered locators (across all registrations), the lookup discovery service will attempt unicast discovery of the lookup service associated with the new locator.

If locator discovery is attempted for a registration, such discovery attempts will be repeated until one of the following events occurs:

Upon discovery of the lookup service corresponding to the new locator, or upon finding a match between the new locator and a previously discovered lookup service, a discovered event will be sent to the registration's listener, unless that lookup service was previously discovered for that registration through group discovery.

Any already discovered lookup service corresponding to a locator that is removed from the registration's managed set as a result of an invocation of either setLocators or removeLocators will be discarded and will no longer be eligible for discovery, but only if it is not currently eligible for discovery through group discovery--that is, only if the lookup service is not also a member of one or more of the groups in the registration's managed set of groups.

Discarding Lookup Services

When the lookup discovery service removes an already discovered lookup service from a registration's managed set of lookup services, the lookup service is said to be discarded.

There are a number of situations in which the lookup discovery service will discard a lookup service:

For each of these cases, whenever the lookup discovery service discards a lookup service, it will send an event to the registration's listener to notify it that the lookup service has been discarded.

The discard method provides a mechanism for registered clients to inform the lookup discovery service of the existence of an unavailable--or unreachable--lookup service, and to request that the lookup discovery service discard that lookup service and make it eligible for rediscovery.

The discard method takes a single argument: the proxy to the lookup service to discard. This method takes no action if the parameter to this method equals none of the proxies reflected in the managed set (using proxy equality as defined in the Jini Lookup Service Specification. If null is passed to discard, a NullPointerException is thrown.

Although the lookup discovery service monitors the multicast announcements from all discovered lookup services for indications of unavailability, it should be noted that there are conditions under which the lookup discovery service will not discard such a lookup service, even when the lookup service is found to be unreachable. Whether or not the lookup discovery service discards such an unreachable lookup service is dependent on how each registration is configured for discovery with respect to that lookup service. If every registration that is configured to discover the unreachable lookup service is configured to discover it through locator discovery only, the lookup discovery service will not discard the lookup service. In other words, in order for the lookup discovery service to discard a lookup service it has determined is unreachable, at least one registration must be configured for discovery of at least one group in which that lookup service is a member.

Thus, whenever a client determines that a previously discovered lookup service has become unreachable, it should not rely on the lookup discovery service to discard the lookup service. Instead, the client should inform the lookup discovery service--through the invocation of the registration's discard method--that the previously discovered lookup service is no longer available and that attempts should be made to rediscover that lookup service for the registration. Typically, a client determines that a lookup service is unavailable when the client attempts to use the lookup service but receives an exception (indefinite or definite) as a result of the attempt.

Note that the lookup discovery service may be acting on behalf of numerous clients that have access to the same lookup service. If that lookup service becomes unavailable, many of those clients may invoke discard between the time the lookup service becomes unavailable and the time it is rediscovered. Upon the first invocation of discard, the lookup discovery service will re-initiate discovery of the relevant lookup service for the registration of the client that made the invocation. For all other invocations made prior to rediscovery, the registrations through which the invocation is made are sent a discarded event, and added to the list of registrations that will be notified when rediscovery of the lookup service does occur. That is, upon rediscovery of the lookup service, only those registrations through which the discard method was invoked will be notified.

Upon successful completion of the discard method, the proxy requested to be discarded is guaranteed to have been removed from the managed set of the registration through which the invocation was made. No such guarantee is made with respect to when the discarded event is sent to each such registration's listener. That is, the event notifying the listeners that the lookup service has been discarded may or may not be sent asynchronously.

LD.4.2 The RemoteDiscoveryEvent Class

When the lookup discovery service discovers or discards a lookup service matching the criteria established through one of its registrations, the lookup discovery service sends an instance of the RemoteDiscoveryEvent class to the RemoteEventListener implemented by the client and registered with the lookup discovery service. 

package net.jini.discovery;

public class RemoteDiscoveryEvent extends RemoteEvent {
    public RemoteDiscoveryEvent(Object source,
                                long eventID,
                                long seqNum, 
                                MarshalledObject handback,
                                boolean discarded,
                                Map groups)
                                       throws IOException {...}

    public boolean isDiscarded() {...}
    public ServiceRegistrar[] getRegistrars()
                          throws LookupUnmarshalException {...}
    public Map getGroups() {...}
}

The RemoteDiscoveryEvent class provides an encapsulation of event information that the lookup discovery service uses to notify a registration of the occurrence of an event involving one or more ServiceRegistrar objects (lookup services) in which the registration has registered interest. The lookup discovery service passes an instance of this class to the registration's discovery listener when one of the following events occurs:

RemoteDiscoveryEvent is a subclass of RemoteEvent, adding the following additional items of abstract state:

Methods are defined through which this additional state may be retrieved upon receipt of an instance of this class.

Clients need to know not only when a targeted lookup service has been discovered, but also when it has been discarded. The lookup discovery service uses an instance of RemoteDiscoveryEvent to notify a registration when either of these events occurs, as indicated by the value of the boolean state variable. When the value of that variable is true, the event is referred to as a discarded event; when false, it is referred to as a discovered event.

LD.4.2.1 The Semantics

The constructor of the RemoteDiscoveryEvent class takes the following parameters as input:

If the groups parameter is empty, the constructor will throw an IllegalArgumentException. If null is input to the groups parameter, the constructor will throw a NullPointerException. If none of the proxies referenced in the groups parameter can be successfully serialized, the constructor will throw an IOException.

The isDiscarded method returns a boolean that indicates whether the event is a discovered event or a discarded event. If the event is a discovered event, then this method returns false. If the event is a discarded event, true is returned.

The getRegistrars method returns an array consisting of instances of the ServiceRegistrar interface. Each element in the returned set is a proxy to one of the newly discovered or discarded lookup services that caused a RemoteDiscoveryEvent to be sent. Additionally, each element in the returned set will be unique with respect to all other elements in the set, as determined by the equals method provided by each element. This method does not make a remote call. With respect to multiple invocations of this method, each invocation will return a new array.

When the lookup discovery service sends an instance of RemoteDiscoveryEvent to the listener of a client's registration, the set of lookup service proxies contained in the event consists of marshalled instances of the ServiceRegistrar interface. The lookup discovery service individually marshals each proxy associated with the event because if it were not to do so, any deserialization failure on the set would result in an IOException, and failure would be declared for the whole deserialization process, not just an individual element. This would mean that all elements of the set sent in the event--even those that can be successfully deserialized--would be unavailable to the client through this method. Just as with the getRegistrars method defined by the LookupDiscoveryRegistration interface, individually marshalling each element in the set minimizes the "all or nothing" aspect of the deserialization process, allowing the client to recover those proxies that can be successfully unmarshalled and to proceed with processing that might not be possible otherwise.

When constructing the return set, this method attempts to unmarshal each element of the set of marshalled proxy objects contained in the event. When failure occurs while attempting to unmarshal any of the elements of that set, this method throws an exception of type LookupUnmarshalException. It is through the contents of this exception that the client can recover any available proxies and perform error handling with respect to the unavailable proxies.

If the getRegistrars method returns successfully without throwing a LookupUnmarshalException, the client is guaranteed that all marshalled proxies sent in the event have each been successfully unmarshalled during that particular invocation. Furthermore, after the first such successful invocation, no more unmarshalling attempts will be made (because such attempts are no longer necessary), and all future invocations of this method are guaranteed to return an array with contents identical to the contents of the array returned by the first successful invocation.

Note that an array, rather than a single proxy, is returned by the getRegistrars method so that implementations of the lookup discovery service can choose to "batch" the information sent to a registration. With respect to discoveries, batching the information may be particularly useful when a client first registers with the lookup discovery service.

Upon initial registration, multiple lookup services are typically found over a short period of time, providing the lookup discovery service with the opportunity to send all of the initially discovered lookup services in only one event. Afterward, as so-called "late joiner" lookup services are found sporadically, the lookup discovery service may send events referencing only one lookup service.

Note that the event sequence numbers, as defined earlier in Section LD.3.2, "Event Semantics", are strictly increasing, even when the information is batched.

The getGroups method returns a Map in which the elements of the map's key set are the instances of ServiceID that correspond to each lookup service for which the event was constructed and sent. Each element of the returned map's value set is a String array containing the names of the member groups of the associated lookup service whose ServiceID equals to the corresponding key. This method does not make a remote call. On each invocation of this method, the same Map object is returned; that is, a copy is not made.

The Map returned by the getGroups method is keyed by the ServiceID of each lookup service in the event, rather than by the proxy of each lookup service to avoid the deserialization issues addressed by the getRegistrars method. Thus, client's wishing to retrieve the set of member groups corresponding to any element of the array returned by the getRegistrars method, must use the ServiceID of the desired element from that array as the key to the get method of the Map returned by this method and then cast to String[].

LD.4.2.2 Serialized Forms

Class
 serialVersionUID
 Serialized Fields
RemoteDiscoveryEvent
 -9171289945014585248L
 boolean discarded
ArrayList marshalledRegs
ServiceRegistrar[] regs
Map groups
LD.4.2.3 The LookupUnmarshalException Class

Recall that when unmarshalling an instance of MarshalledObject, one of the following checked exceptions is possible:

The LookupUnmarshalException class provides a mechanism that clients of the lookup discovery service may use for efficient handling of the exceptions that may occur when unmarshalling elements of a set of marshalled instances of the ServiceRegistrar interface. When elements in such a set are unmarshalled, the LookupUnmarshalException class may be used to collect and report pertinent information generated when failure occurs during the unmarshalling process. 

package net.jini.discovery;

public class LookupUnmarshalException extends Exception {
    public LookupUnmarshalException
                    (ServiceRegistrar[] registrars,
                     MarshalledObject[] marshalledRegistrars,
                     Throwable[] exceptions) {...}
    public LookupUnmarshalException
                    (ServiceRegistrar[] registrars,
                     MarshalledObject[] marshalledRegistrars,
                     Throwable[] exceptions,
                     String message) {...}
    public ServiceRegistrar[] getRegistrars() {...}
    public MarshalledObject[] getMarshalledRegistrars() {...}
    public Throwable[] getExceptions() {...}
}

The LookupUnmarshalException class is a subclass of Exception, adding the following additional items of abstract state:

When exceptional conditions occur while unmarshalling a set of marshalled instances of ServiceRegistrar, the LookupUnmarshalException class can be used not only to indicate that an exceptional condition has occurred, but also to provide information that can be used to perform error handling activities such as:

LD.4.2.4 The Semantics

The constructor of the LookupUnmarshalException class has two forms. The first form of the constructor takes the following parameters as input:

The second form of the constructor takes the same arguments as the first and one additional argument: a String describing the nature of the exception.

Each element in the exceptions parameter should be an instance of IOException, ClassNotFoundException, or some unchecked exception. Furthermore, there is a one-to-one correspondence between each element in the exceptions parameter and each element in the marshalledRegistrars parameter. That is, the element of the exceptions parameter corresponding to index i should be an instance of the exception that occurred while attempting to unmarshal the element at index i of the marshalledRegistrars parameter.

If the number of elements in the exceptions parameter does not equal the number of elements in the marshalledRegistrars parameter, the constructor will throw an IllegalArgumentException.

The getRegistrars method is an accessor method that returns an array consisting of instances of ServiceRegistrar, where each element of the array corresponds to a successfully unmarshalled object. Note that the same array is returned on each invocation of this method; that is, a copy is not made.

The getMarshalledRegistrars method is an accessor method that returns an array consisting of instances of MarshalledObject, where each element of the array is a marshalled instance of the ServiceRegistrar interface and corresponds to an object that could not be successfully unmarshalled. Note that the same array is returned on each invocation of this method; that is, a copy is not made.

The getExceptions method is an accessor method that returns an array consisting of instances of Throwable, where each element of the array corresponds to one of the exceptions that occurred during the unmarshalling process. Each element in the return set is an instance of IOException, ClassNotFoundException, or some unchecked exception. Additionally, there should be a one-to-one correspondence between each element in the array returned by this method and the array returned by the getMarshalledRegistrars method. Note that the same array is returned on each invocation of this method; that is, a copy is not made.

LD.4.2.5 Serialized Forms

Class
 serialVersionUID
 Serialized Fields
LookupUnmarshalException
 2956893184719950537L
 ServiceRegistrar[] registars
MarshalledObject[] marshalledRegistrars
Throwable[] exceptions

LD.5 History

Version Description
v1.0 Initial release of this specification
v2.0 Defined indefinite and definite exceptions, and removed references to bad object and bad invocation exceptions.
Miscellaneous corrections.

License

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Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
Spec Index A Collection of Jini Technology Helper Utilities and Services Specifications
Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.