Common Principles
 Object-oriented middleware uses object references to address
server objects
 We need to find a way to get hold of these object references
without assuming physical locations
 A name is a sequence of character strings that can be bound to
an object reference
 A name binding can be resolved to obtain the object reference
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Common Principles
 There may be many server objects in a distributed object system
 Server objects may have several names
 Leads to large number of name bindings
 Name space has to be arranged in a hierarchy to avoid
 Name clashes
 Poor performance when binding/resolving names
 Hierarchy achieved by naming contexts
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Common Principles: Naming
Contexts
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Common Principles: Composite
Names
 Names are composed of possibly more than one component
 Used to describe traversals across several naming contexts
 Example:
("UEFA","England","Premier","Chelsea").
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Common Principles: Name Server
Behaviour
 Name bindings are managed by name servers
 Not every server object needs a name
 Server objects may have several names (aliases)
 Name servers must store bindings persistently
 Name servers should be tuned towards efficiently resolving
name bindings
 Name server may itself be distributed
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CORBA Naming Service
 Supports bindings of names to CORBA object references.
 Names are scoped in naming contexts.
 Multiple names can be defined for object references.
 Not all object references need names.
 The CORBA naming service is used by
 Clients to locate CORBA objects
 Servers to advertise specific CORBA objects
 Bootstrap service, enabling clients to find other services
 Facilitate the initial connections between clients and servers
 Basic function: association of names with object references
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Naming Service
 Server creates associations between names and object references
for the CORBA objects (object binding)
 Client retrieve the object references by querying the naming
service and using the name as the key.
 NS: a database of object bindings
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Naming Service
Naming Service
2. resolve
1. Create binding
Server
Client
3. Use target object
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Binding Hierarchy
 Context binding: an association between a name and a naming
context
 Object binding: an association between a name and an object
reference
Initial Naming Service
New_York.region
London. region
Main.failover
StockExchange
Frankfurt.region
Bankup.failover
BrokerService
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Components of a Stringified Name
Stringified Name
Binds to
Empty string
Initial naming context
London.region
Naming context
London.region/Main.failover
Naming context
London.region/Main.failover/StockExchange Object reference
 Initial naming context is always present and serves as the entry point
to the naming service
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CORBA Names
 Names are composed of simple names.
 Simple names are value-kind pairs.
 Value attribute is used for resolving names.
 Kind attribute is used to provide information about the role of
the object.
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CORBA Name
 Name has two format:
 Stringified name format: intuitive format that is easy to read and pass from
place to place
London.region/Main.failover/StockExchange
 Raw name format: defined in terms of IDL complex types and can be used only
within a CORBA program
Module CosNaming {
typedef string Istring;
struct NameComponent {
Istring id;
Istring kind;
};
typedef sequence<NameComponent> Name;
…
};
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CORBA Name
 Kind field is intended to describe how a name component is used
(similar to the file extension, client.exe)
 Special cases:
 empty kind field (omitting kind and kind separator “.”)
 Empty id field (starting the name component with “.”)
 Empty id field and empty kind field (denoted by “.”)
 Example: ././.
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The IDL Interfaces
 Naming Service is specified by two IDL interfaces:
 NamingContext defines operations to bind objects to names and resolve name
bindings.
 BindingInterator defines operations to iterate over a set of names defined in a
naming context.
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Binding
Module cosNaming{
enum BindingType{nobject, ncontext};
struct Binding{
Name binding_name;
BindingType binding_type;
};
typedef sequence<Binding> BindingList;
};
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Binding and Unbinding Operations
Module cosNaming{
interface NamingContext{
void bind(in Name n, in Object obj)
raise(NotFound, InvalidName, AlreadyBound,
CannotProceed);
void rebind( in Name n, in Object obj)
raise(NotFound, InvalidName, CannotProceed);
void bind_context(in Name n, in NamingContext nc)
raise(NotFound, InvalidName, AlreadyBound,
CannotProceed);
void rebind_context(in Name n, in NamingContext nc)
raise(NotFound, InvalidName, CannotProceed);
void unbind(in Name n)
raise(NotFound, InvalidName, CannotProceed);
};
};
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bind() and rebind()
 Both operations create an object binding that associates the
name n (in raw format) with the naming context nc.
 Rebind(): it creates a new object binding for the given name n or
if a binding already exists with that name, overwrites the
existing binding.
 Bind(): it creates a new object binding for the given name n as
long as there is no existing binding with that name. If a binding
already exists, an AlreadyBound exception is thrown.
 These operations create object bindings relative to the naming
context on which they are invoked.
 Example: A/B/C/MyObj on initial naming context, or MyObj on
A/B/C naming context
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bind_context() and rebind_context()
 Parameter is context
 Similar semantics as bind() and re_bind()
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Create Context
Module cosNaming{
//interface NamingContext{
NamingContext new_context();
NamingContext bind_new_context(in Name n);
raise(NotFound, InvalidName, AlreadyBound,
CannotProceed);
//};
};
 A naming context can be created (new_context()) and then be
binded (bind_context())
 Bind_new_context() creates a new context and binds it as a
subcontext of the context on which the operation is invoked.
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Before Binding
Context1
Context2
Context4
Context3
Context5
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After Binding
Context1
Context2
Context3
Context4
Context5
MyName
IOR of Object
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Naming Scenario: Binding
Promote Bielefeld to German '1. Liga' and relegate Frankfurt
root: Naming
Context
c:Client
1L:Naming
Context
1L=resolve("UEFA","Germany","1. Liga")
bind("Arm. Bielefeld", bielefeld)
unbind("Eintr. Frankfurt")
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Resolve Names
Module cosNaming{
//interface NamingContext{
Object resolve(in Name n)
raise(NotFound, InvalidName, CannotProceed);
//};
};
 It resolves the first component of the name to an object reference.
 If there are no remaining components, it returns this object reference
to the caller.
 Otherwise, it narrows the object reference to a NamingContext and
passes the reminder of the name to its resolve operation
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NotFound Exception
Module cosNaming{
…
interface NamingContext{
…
enum NotFoundReason {missing_node, not_context, not_object};
exception NotFound {
NotFoundReason why;
Name rest_of_name;
};
};
};
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Browsing Contexts
Module cosNaming{
//interface NamingContext{
interface BindingIterator {
boolean next_one(out Binding b);
boolean next_n(in unsigned long how_many,
out BindingList bl);
void destroy();
};
void list (in unsigned long how_many, out BindlingList bl,
out BindingIterator bi);
//};
};
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Converting Names:
NamingContextExt
Module cosNaming{
interface NamingContextExt:NamingContext{
typedef string StringName;
typedef string Address;
typedef string URLString;
StringName to_string(in Name n) raises (InvalidName);
Name to_name(in StringName sn) raises (InvalidNames);
URLString to_url(in Address addrKey, in StringName sn)
raises(InvalidAddress, InvalidName);
Object resolve_str(in StringName sn)
raises(NotFound, CannotProceed, InvalidName);
};
};
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Federated Naming Service
 Naming service supports federation: distinct naming servers can
be linked together to create a single naming graph.
 A client can then navigate seamlessly throughout the naming
graph without being aware that it is federated.
 The support is through bind_context()
 Graph or hierarchy?
 Cycles in the graph
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Federated Naming Service
Naming Service X
Naming Service Y
A
B
C
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Federated Naming Service
V
A
B
U
Context A can be accessed by:
A,
A/B/V/U/A,
or
A/B/V/U/A/B/U/V/A
Avoid infinite loop
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Building Applications Using CORBA
 Room booking system:
 Booking rooms for meetings for a duration (from 9:00 am to 4:00 pm by hour);
 Canceling the bookings
 The number and names of rooms can change
 When booking a room, a purpose and the name of the person making the
booking should be given
 System design decisions:
 Rooms and meetings are CORBA objects
 A meeting object defines a purpose and the person responsible for the meeting
 A meeting factory creates meeting objects
 A room stores meetings indexed by time slots
 Room have a name and register themselves under this name with the Naming
Service
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Room Booking System
Client Application
Proxy
RoomServer
view()
RoomServer
RoomImpl
book()
cancelBooking()RoomServer
RoomImpl
resolve()
CORBA Naming Service
NamingContext
bind()
resolve()
bind()
MeetingFactoryServer
RoomImpl
CreateMeeting()
MeetingImpl
MeetingFactoryImpl
MeetingImpl
MeetingImpl
MeetingImpl
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Room Booking Naming Convention
Root Naming Context
BuildingApplications
Rooms
Meeting Factory
Board Room
Meeting Room
Chairmen’s Office
Context Object
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Source Code
 Chapter 8
 Client
 ClientApplication.java
 RoomBookingClient.java
(“main”)
(“Impl”)
 Server
 Room
– RoomServer.java
– RoomImpl.java
 MeetingFactory
– MeetingFactoryServer.java
– MeetingFactoryImpl.java
 Meeting
– MeetingImpl.java
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Source Code
 Client
 ClientApplication.java
(“main”)
 Create “impl” client = RoomBookingClient();
 Initialize NS: client.init_from_ns();
 View existing bookings: client.view()
 RoomBookingClient
(“impl”)
 Private:
– orb, root_context, room_context, meeting_factory, rooms, meetings, booked, ior
 Constructor:
– Initialize orb
– Get root_context
 Init_from_ns:
– From root_context get room_context
– From root_context get meeting_factory
 View:
– Get rooms,
– List meetings
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Source Code
 Server
 Room
 RoomServer.java
– Initialize
ORB orb = ORB.init( args, null );
POA poa = POAHelper.narrow( orb.resolve_initial_references( "RootPOA"));
poa.the_POAManager().activate();
– Create room object and export the object reference
org.omg.CORBA.Object room_obj =
poa.servant_to_reference ( new RoomImpl( args[0] ) );
– Register with naming service
NamingContextExt root = NamingContextExtHelper.narrow (
orb.resolve_initial_references("NameService"));
root.bind_new_context( root.to_name( context_name ));
str_name = context_name + "/" + args[0];
root.bind( root.to_name( str_name), room_obj );
– Wait for request
orb.run();
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Source Code
 Server
 Room
 RoomImpl.java
– Inherit from RoomPOA
public class RoomImpl extends RoomPOA
– Implement all the operations declared in the idl:
»
»
»
»
Attributes
View
Book
Cancelbooking
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Source Code
 Server
 MeetingFactory
 MeetingFactoryServer.java
– Initialize
ORB orb = ORB.init( args, null );
POA poa = POAHelper.narrow( orb.resolve_initial_references( "RootPOA"));
poa.the_POAManager().activate();
– Create room object and export the object reference
org.omg.CORBA.Object meeting_factory_obj =
poa.servant_to_reference ( new MeetingFactoryImpl( ) );
– Register with naming service
NamingContextExt root = NamingContextExtHelper.narrow (
orb.resolve_initial_references("NameService"));
root.bind_new_context( root.to_name( context_name ));
str_name = context_name + "/" + args[0];
root.bind( root.to_name( str_name), meeting_factory_obj );
– Wait for request
orb.run();
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Source Code
 Server
 MeetingFactory
 MeetingFactoryImpl.java
– Inherit from MeetingFactoryPOA
public class MeetingFactoryImpl extends MeetingFactoryPOA
– Implement all the operations declared in the idl:
» createMeeting
MeetingImpl meetingImpl =
new MeetingImpl(purpose, participants);
try
{
org.omg.CORBA.Object obj =
_poa().servant_to_reference(meetingImpl);
Meeting meeting = MeetingHelper.narrow(obj);
return meeting;
}
…
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Source Code
 Server
 Meeting
 MeetingImpl.java
– Inherit from MeetingPOA
public class MeetingImpl extends MeetingPOA
– Implement all the operations declared in the idl:
» Attributes
» Destory
 Not all Corba objects need to be registered with NS
 servant_to_reference(meetingImpl) is done by other entity
(MeetingFactoryImpl)
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Limitations of Naming Service
 Limitation of Naming in all approaches: Client always has to
identify the server by name.
 Inappropriate if client just wants to use a service at a certain
quality but does not know from who:
 Automatic cinema ticketing,
 Video on demand,
 Electronic commerce.
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