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Naming Service

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Naming service

• The CORBA COS (Common Object Services) Naming Service provides a tree-like directory for object references – It is like a filesystem provides a directory

structure for files.

• The Naming Service provided with Java IDL is an implementation of the COS Naming Service specification.

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Naming service (cont.)

• A name-to-object association is called a name binding.

• A name binding is always defined relative to a naming context.

• A naming context is an object that contains a set of name bindings in which each name is unique.

• Different names can be bound to an object in the same or different contexts at the same time.

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Naming service (cont.)

• To resolve a name is to determine the object associated with the name in a given context.

• To bind a name is to create a name binding in a given context.

• A name is always resolved relative to a context - there are no absolute names.

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Naming service (cont.)

• Because a context is like any other object, it can also be bound to a name in a naming context.

• Binding contexts in other contexts creates a naming graph – – a directed graph with nodes and labeled

edges where the nodes are contexts.

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Naming service (cont.)

• A naming graph allows more complex names to reference an object.

• Given a context in a naming graph, a sequence of names can reference an object.

• This sequence of names (called a compound name) – defines a path in the naming graph to

navigate the resolution process.

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Naming service (cont.)

• For an application to use the COS Naming Service, its ORB must know– the port of a host running a

naming service or – have access to a stringified initial

naming context for that naming service.

• The naming service can either be the Java IDL naming service or another COS-compliant naming service.

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Naming service (cont.)

• Prior to running a client or a server, you will start ORBD.

• ORBD includes – a persistent Naming

Service and – a transient Naming

Service,

• both of which are an implementation of the COS Naming Service.

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Persistent Naming Service

• The Persistent Naming Service provides persistence for naming contexts. – information is persistent across service

shutdowns and startups, and – is recoverable in the event of a service

failure. – If ORBD is restarted, the Persistent Naming

Service will restore the naming context graph, so that the binding of all clients' and servers' names remains intact (persistent).

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Transient Naming Service

• For backward compatibility, tnameserv, a Transient Naming Service shipped with older versions of the JDK, is also included in J2SE v.1.4. – A transient naming service retains naming

contexts as long as it is running. – If there is a service interruption, the naming

context graph is lost.

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How to get the naming service?

• Both client and server should agree on which root naming context to use.

• The orb.resolve_initial_references(String name_of_service) method is used to obtain the root naming context.

• If you get a handle on a Naming Service by providing "NameService" as the name_of_service, you will get a persistent naming service, meaning that the naming context graph will be restored after a service failure.

• If you get the handle using "TNameService", you will get a transient naming service, meaning that you will need to re-obtain the root naming context using orb.resolve_initial_references(String name_of_service) if there is a service interruption.

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Code to get the naming service?

For example, // get the root naming context org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService"); NamingContextExt ncRef =

NamingContextExtHelper.narrow(objRef);

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Getting naming service

• The string "NameService" is defined for all CORBA ORBs.

• When you pass in this string, the ORB returns a naming context object that is an object reference for: – The persistent name service if you are using

the Naming Service that is part of ORBD. – The transient name service if you are using

tnameserv.

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Getting TNameService naming service in ORBD (cont.)

• To specify that you want to use the transient naming service with ORBD, pass in the string "TNameService".

• The string "TNameService" is a proprietary name.

• When you pass in this string, the ORB returns a naming context object that is an object reference for ORBD's transient name service.

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NamingContextExt

• As with all CORBA object references, objRef is a generic CORBA object.

• To use it as a NamingContextExt object, you must narrow it to its proper type.

• NamingContextExtHelper is an idlj-generated helper class, similar in function to HelloHelper.

• The ncRef object is now an org.omg.CosNaming.NamingContextExt and you can use it to access the naming service and register the server.

• NamingContextExt and NamingContextExtHelper are new to this release of J2SE.

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NamingContextExt (cont)

• NamingContextExt is the extension of NamingContext.

• It contains a set of name bindings in which each name is unique and

• It is part of the Interoperable Naming Service. • Different names can be bound to an object in the

same or different contexts at the same time.• Using NamingContextExt, you can use URL-

based names to bind and resolve. • NamingContextExtHelper provides additional

helper methods like narrow() that are type-specific but don't deal with business logic.

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Interoperable Naming Service (INS)

• INS is a URL-based naming system on top of the CORBA Naming Service, as well as – a common bootstrap mechanism that lets applications

share a common initial naming context. • The INS provides the following features:

– Capability to resolve using stringified names (e.g., a/b.c/d)

– URLs for CORBA object references (corbaloc: and corbaname: formats)

– Standard APIs in NamingContextExt for converting between CosNames, URLs, and Strings

– ORB arguments for bootstrapping (ORBInitRef and ORBDefaultInitRef)

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Interoperable Naming Service

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Interoperable Naming Service (INS)

• An object reference contains at least three pieces of information: – an address,– the name of the POA that created an object

reference, and – an Object ID.

• Using INS, you can provide a URL to access the CORBA object, – which is more readable than a stringified IOR.

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stringified object reference

• The following stringified object reference formats are allowed:

1. Interoperable Object References (IOR) – An IOR is an object reference that is

understood by ORBs that can interoperate using the OMG-defined protocols GIOP and IIOP.

– A client can obtain an object reference using orb.object_to_string(objRef), as shown in the Browsing the Namespace example, or as a result of an invocation on another object reference.

• Note: If an IOR contains multiple profiles, the J2SE v.1.4 ORB always uses the first one.

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stringified object reference (cont)

2. Human-Readable URL Formats for CORBA Object References

• corbaloc: • The corbaloc: format is useful for CORBA

client programs, and • is typically used to resolve the reference using

the GIOP LocateRequest or Request message.

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stringified object reference (cont)

corbaloc:(cont.)• For example, a corbaloc: object reference

might look like this: •

corbaloc:iiop:1.2@MyBank.com:2050/TraderService

• This example show how to get an object reference for TraderService from host myBank.com on port 2050.

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stringified object reference (cont)

• corbaname: • The corbaname: format provides a mechanism

for a client to bootstrap directly, and• is typically used to resolve the stringified name

from the root naming context.• For example, a corbaname: object reference

might look like this: • corbaname::myBank.com:2050#Personal/schedule• where myBank.com is the host, 2050 is the port.

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stringified object reference (cont)

• corbaname: (cont.)• The portion of the reference up to the hash mark

(corbaname::myBank.com:2050 ) is the URL that returns the root naming context.

• This example provides the URL to use to: a) locate the Naming Service, and, b) resolve the name Personal/schedule from the Naming Service.

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Bootstrap Options for the ORB

• INS provides ORB options for bootstrapping. • To bootstrap a CORBA system, you must give it

an object reference. • The ORB can be configured to return the

handle of a customized CORBA service from resolve_initial_references() using either ORBInitRef and/or ORBDefaultInitRef.

• For example, • -ORBInitRef TraderService =

corbaloc::myBank.com:2050/TraderService• -ORBDefaultInitRef corbaloc:iiop:1.2:myBank.com:2050

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Bootstrap Options for the ORB

• The order of resolution when these options are used is as follows:

1.Objects registered with register_initial_references

2.-ORBInitRef

3.-ORBDefaultInitRef

4.Proprietary Bootstrap (Sun ORBs only)

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Using the Naming Service

• To use the Naming Service, you must first write the server and client code that finds and/or creates the namespace and/or the objects in the namespace.

• Before running the client and server, you must start the Naming Service and tell the client and server where to find it.

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Using the Naming Service

• The following steps loosely suggest what happens when the client and server attempt to access the Naming Service: 1. The server invokes the bind or rebind method to

associate a logical name with an object reference. 2. The Naming Service adds this object

reference/name binding to its namespace database.

3. A client application invokes the resolve method to obtain an object reference with this name.

4. The client uses an object reference to invoke methods on the target object.

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Starting the Naming Service

• we use the Object Request Broker Daemon, orbd, which includes both a Persistent and a Transient Naming Service, and is available with every download of J2SE 1.4 and higher.

• For a caller (client, peer, or client application) to be able to invoke a method on a remote object, that caller must first obtain a reference to the remote object.

• Once a remote object is registered on the server, callers can look up the object by name, obtain a remote object reference, and then remotely invoke methods on the object.

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Stopping the Naming Service

• To stop the naming service, use the relevant operating system command, such as kill on Solaris, or Ctrl+C in the DOS window in which orbd is running.

• Note that names registered with the naming service may disappear when the service is terminated if the naming service is transient.

• The Java IDL naming service will run until it is explicitly stopped.

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How do I connect to a 3rd-party Naming Service from Sun's ORB?

• The recommended way is to use the Inter-Operable Naming Service (INS), if this option is supported by the 3rd-party Naming Service.

• To use the Sun ORB with another vendor's Naming Service, 1. Start the 3rd-party name server on Host and Port. 2. Pass the following argument to ORB.init(): -

ORBInitRef NameService=corbaloc:iiop:1.2@:/NameService

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How do I connect to a 3rd-party Naming Service from Sun's ORB?

• When you do orb.resolve_initial_references( "NameService" ), you should be able to connect to 3rd-party Name Service.

• If you are still not able to connect, try these troubleshooting hints: – Verify that the 3rd-party Name Service supports INS. – Verify that the host and port information is accurate. – Verify that the 3rd-party Name Service has been started

successfully. – Verify that the 3rd-party Name Service supports GIOP 1.2. If

not, refer to the Name Server's documentation for the correct the GIOP version, and modify the corbaloc: URL accordingly.

– Determine if the 3rd-party Name Service has a different object key it uses to contact NameService. If so, refer to the Name Server's documentation.

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References

• Naming service:URL:http://java.sun.com/j2se/1.4.2/docs/guide/idl/

jidlNaming.html

• Java IDL: Interoperable Naming Service (INS):

• URL:http://java.sun.com/j2se/1.4.2/docs/guide/idl/INStutorial.html