Naming Service 1

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

Naming Service 2

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

Naming Service 3

Common Principles: Naming Contexts

Cup

Winners

Cup

Winners

1.FC

Ka

iser

slau

tern

1.FC

Ka

iser

slau

tern

Pre

mie

rP

rem

ier First

First

M

an

Uni

ted

M

an

Uni

ted

Chelsea

Chelsea

QP

RQ

PR

South End

United

South End

United

EnglandEnglandGermany

Germany

1. L

iga

1. L

iga 2. Liga

2. Liga

BV

BB

VB

Bayern

Bayern B

ochu

mB

ochu

m

LauternLautern

UEFAUEFA

Manchester

United

Manchester

United

Naming Service 4

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").

Naming Service 5

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

Naming Service 6

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

Naming Service 7

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

Naming Service 8

Naming Service

Naming Service

Client Server

1. Create binding

2. resolve

3. Use target object

Naming Service 9

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

London. regionNew_York.region

Frankfurt.region

Bankup.failoverMain.failover

StockExchange BrokerService

Naming Service 10

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

Naming Service 11

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.

Naming Service 12

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;

…

};

Naming Service 13

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: ././.

Naming Service 14

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.

Naming Service 15

Binding

Module cosNaming{

enum BindingType{nobject, ncontext};

struct Binding{

Name binding_name;

BindingType binding_type;

};

typedef sequence<Binding> BindingList;

};

Naming Service 16

Binding and Unbinding OperationsModule 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);

};};

Naming Service 17

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

Naming Service 18

bind_context() and rebind_context()

Parameter is context Similar semantics as bind() and re_bind()

Naming Service 19

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.

Naming Service 20

Before Binding

Context1

Context5Context4

Context3Context2

Naming Service 21

After Binding

Context1

Context5Context4

Context3Context2

MyName

IOR of Object

Naming Service 22

Naming Scenario: Binding

Promote Bielefeld to German '1. Liga' and relegate Frankfurt

1L=resolve("UEFA","Germany","1. Liga")1L=resolve("UEFA","Germany","1. Liga")

root: Namingroot: NamingContextContextc:Clientc:Client 1L:Naming1L:Naming

ContextContext

bind("Arm. Bielefeld", bielefeld)bind("Arm. Bielefeld", bielefeld)

unbind("Eintr. Frankfurt")unbind("Eintr. Frankfurt")

Naming Service 23

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

Naming Service 24

NotFound ExceptionModule cosNaming{

…

interface NamingContext{…

enum NotFoundReason {missing_node, not_context, not_object};

exception NotFound {

NotFoundReason why;

Name rest_of_name;

};

};

};

Naming Service 25

Browsing ContextsModule 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);

//};

};

Naming Service 26

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);

};};

Naming Service 27

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

Naming Service 28

Federated Naming Service

A

B

C

Naming Service X

Naming Service Y

Naming Service 29

Federated Naming Service

A

BU

V

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

Naming Service 30

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

Naming Service 31

Room Booking System

MeetingFactoryImpl

MeetingImpl MeetingImpl

MeetingImpl

MeetingImplMeetingFactoryServer

NamingContext

CORBA Naming ServiceRoomImpl

RoomServer

RoomImpl

RoomServer

RoomImpl

RoomServer

Proxy

Client Application

CreateMeeting()

bind()

resolve()

resolve() view()book()cancelBooking()

bind()

Naming Service 32

Room Booking Naming Convention

Root Naming Context

BuildingApplications

Rooms

Context Object

Board Room

Meeting Room

Chairmen’s Office

Object Name

Meeting Factory

Naming Service 33

Source Code Chapter 8

Client ClientApplication.java (“main”) RoomBookingClient.java (“Impl”)

Server Room

– RoomServer.java

– RoomImpl.java MeetingFactory

– MeetingFactoryServer.java

– MeetingFactoryImpl.java Meeting

– MeetingImpl.java

Naming Service 34

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

Naming Service 35

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();

Naming Service 36

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

Naming Service 37

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();

Naming Service 38

Source Code Server

MeetingFactory MeetingFactoryImpl.java

– Inherit from MeetingFactoryPOA

public class MeetingFactoryImpl extends MeetingFactoryPOA

– Implement all the operations declared in the idl:

» createMeetingMeetingImpl meetingImpl =

new MeetingImpl(purpose, participants);try{ org.omg.CORBA.Object obj = _poa().servant_to_reference(meetingImpl); Meeting meeting = MeetingHelper.narrow(obj); return meeting; }…

Naming Service 39

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)

Naming Service 40

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.