Corba. Corba services. The (very) global picture. Corba. Distributed Object Systems 3 CORBA/IDL. Corba. Features. Services

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Distributed Object Systems

– 3 –

CORBA/IDL

Piet van Oostrum

Sep 11, 2008

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Corba

I _{Common Object Request Broker Architecture}

I _{Middleware for communicating objects}

I _Context

I Object Management Group (OMG)

I _{Consortium of computer companies (Sun, HP, IBM, ...)} I Defines standards (interfaces) for support of (distributed) OO

applications

I _{Also for example UML}

I Based on concrete available technology, propositions by

consortium members

I _Status

I Many services defined

I _{But not all implemented by all distributions yet} I Many commercial and open source implementations I New services invented occasionally (e.g. Corba Components)

Piet van Oostrum DOS-3 1

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Corba

Distributed Object Systems 03/02/98 © Atze Dijkstra

Java RMI 3 Dept. of Comp.Science, UU

Corba

• The (very) global picture

Business Objects Object Services

"System Frameworks"

Object Bus CommonFacilities

"Application Frameworks"

Collaboration Interoperability

The (very) global picture

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OMG Object Management Architecture

Corba services

• To make collaboration go well

Information Management Systems Management Task Management User Interface

Vertical Common Facilities

Horizontal Common Facilities

Common Object Services Common Facilities

Application Objects

Object Request Broker (ORB)

Naming Events Persistence Life Cycle Transactions Externalization Query Properties Concurrency Time Collections Relationships Trader Security Licensing Change Management

-

OMG Object Management Architecture

-

Not all services yet defined

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Services

I _Naming I Referencing by name I _{Naming schemes} I _Events I Sending events

I _{Registering interest in events}

I _Persistence

I Storing objects

I _{Life Cycle}

I Creating, copying, moving, deleting components on the bus

I _Transactions

I Two-phase commit, rollback

Many more . . .

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Features

I _{Static and dynamic method invocation}

I Compile time or runtime definition of method invocation

I _{High-level language bindings}

I Interface Definition Language (IDL)

I Independent of language (C, C++, Smalltalk, Java, Python) I _{Translates to language code}

I _{Self-describing system}

I Interface Repository

I _{Contains information describing interfaces, methods (+}

parameters), etc. a server provides

I Can be interrogated at runtime

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Example Quoter

I _{Looking like normal use of service} I _{Notation used: C++}

int main( void ) {

// Use a factory to bind to any quoter

Quoter_var quoter = find_quoter_service("Quoter") ; const char *stock_name = "ACME ORB, Inc." ;

long value = quoter->get_quote(stock_name) ; cout << stock_name << " = " << value << endl ; return 0 ;

}

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Quoter interface

I _{Functionality defined in OMG IDL}

interface Quoter {

exception Invalid_Stock{} ;

long get_quote( in string stock_name ) raises (Invalid_Stock) ; };

I Interface Description Language I Used by both clients and servers I Language independent

I With compilers to specific languages I _{Location transparent}

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CORBA structure

Dynamic Invocation Interface Repository Skeleton InterfaceORB Object Adapter Dynamic

Interface identical for all ORB implementations May be multiple object adaptors

ORB dependent interface

Stubs and a skeleton for each object type Static IDL Impl Repository Object ORB Core Skeleton Client _{Implementation} StubsIDL Requests

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CORBA core components

I _{An Object Request Broker (ORB)}

I _{Basic or Portable Object Adapter (BOA/POA)} I _{An Interface Definition Language (IDL)} I _{A Static Invocation Interface (SII)} I _{A Dynamic Invocation Interface (DII)} I _{A Dynamic Skeleton Interface (DSI)} I _{Interface and Implementation Repositories} I _{Programming language mappings} I _{An Interoperability Spec (GIOP, IIOP)}

GIOP (General Inter-ORB Protocol) = protocol between local ORBs

IIOP (Internet Inter-ORB Protocol) = GIOP through Internet (TCP/IP)

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Core components

I _{Object Request Broker (ORB)}

I Central component

I _{Object location, creation, activation} I Message exchange

I Has its own interface for local services I _{Local ORBs + G/IIOP = object bus}

I NB. ORB is a distributed thing, but each local part is also

called ORB

I _{Usually implemented as a library inside the process}

(e.g. DLL or Java package)

I _{Static Invocation Interface (SII)}

I Client stubs (proxy pattern) I Static interface to object services

I _{Dynamic Invocation Interface (DII) is runtime variant,}

interface repository is used for construction of calls

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Core components

I _{Static skeleton}

I Framework through which calls are passed to object service I _{Dynamic Skeleton Interface (DSI) is dynamic variant}

I _{Object adapter}

I Provides mapping of object identifiers to implementation

instances

I Uses implementation repository

I _{Object identifiers}

I _{ORB translates local identifier to Interoperable Object}

References (IOR) and vice-versa

I IOR contains all information to locate the remote object

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Object Adapter

Tasks of an object adapter

Object Adaptor

• Tasks of an object adaptor

Object Implementation Up-call to method Register new classes Instantiate new objects Manage object references Handle incoming requests Broadcast services ID ID ID Object Adapter Interface A skeleton Interface B skeleton Impl. Repository

Object adapter maps object ID’s to object instances Skeleton maps methods names to method implementations

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Basic Object Adapter (BOA)

I _{Basic service}

I _{Barebones ORB} I Variety of server kinds

I _{Server kinds}

I _{Single process accepting calls, blocking while handling} I Server per object starts at first call on object I Server per method starts for each call I _{Persistent server already exists}

I _{BOA is deprecated}

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Portable Object Adapter (POA)

I _{BOA is too restricted}

I _{E.g. no support for moving objects or objects that are} ‘sleeping’ in a database.

I _{BOA is underspecified}

I _{Vendors made their own extensions}_→_non-portable I _{POA solves this}

I _{Many different ways to create/revive objects} I _{Will be treated in a later lecture.}

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Corba implementations

I _{There are many Corba implementations:}

I Several commercial implementations

I Open source implementations (see selection below)

I _C++:

I TAO (The Ace ORB) I OmniORB

I _{Orbit (used in Linux/Gnome)}

I _Java:

I built-in implementation in Java (called Java IDL) is not very

good

I JacORB is a better implementation I _{OpenORB (a bit antique)}

I _Python:

I Fnorb (written in Python) I _OmniORB

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Using IDL

Client IDL stubs Object Impl's Server IDL stubs Client IDL stubs Object Impl's Server IDL stubs Object Adapter Server Client Client IDL stubs Object Impl's Server IDL stubs Instantiate Compile Skeletons

Add server impl code Create IDL definitions

Interface Repository Impl. Repository Precompiler 6 7 4 3 5 2 1

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Using IDL

I _{From IDL to runtime info}

I Define object classes using IDL

I _{Precompile IDL file, creates stubs and skeletons} I Add implementation code to skeletons I Compile code, creating misc interfaces and info I _{Bind class definitions to interface repository (optional)} I Instantiate objects on server

I Register run-time objects with the implementation repository

(optional)

I _Calling

I Static interface as method invocation

Uses normalobject.methodsyntax

I Dynamic interface using special calls to construct message

method invocation is constructed at run-time.

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IDL compiler

srv.cpp x_skel.cpp x.cpp x_skel.h Client x.idl Client Executable app.java IIOP IDL Developer IDL-to-C++ Compiler IDL-to-Java Compiler x.java Java ORB

Run-Time Server C++ ORBRun-Time

Developer Developer Executable Server x.h generated file (source: Iona)

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IDL

I _{We use a subset of Corba IDL}

I _{IDL version 2 defines ‘simple’ interfaces} I _{IDL version 3 defines ‘components’}

I _{IDL can be translated to C, C++, Java, Python, etc.} I _{Translation generates interface definitions, stubs, skeletons,}

etc.

I _{Other IDL’s (e.g. Microsoft IDL) are similar.}

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IDL concepts – 1

I _{Module: namespace, like package in Java}

I _{Interface: Basic thing, between Java interface and class}

I _{Abstract interface (no objects)}

I Local interface (only for non-remote objects) I ‘Unconstrained’ interface

I _{Inheritance: Interface can inherit from one or more other} interfaces (multiple inheritance):

I _{interface Dog: Pet, Animal}

I Inherits all variables, methods, exceptions, . . .

I Unconstrained interface may not inherit from local interface

I _{All unconstrained interfaces inherit from}_{CORBA::Object}_(this is Corba Object, not Java Object).

I _Constants: _{const double pi=3.14159;} I _{Operations (methods)}

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IDL concepts – 2

I _{Attributes (instance variables)}

I Implemented by get/set methods I Can be declared readonly (no set)

I _{Type declarations: give a name to a type} typedef struct complex {float re, im;}; typedef sequence<complex> vector; I _{Exceptions: can have data members}

I System exceptions: can be raised even when not specified I _{User exceptions: must be declared with}_raises

I Standard exceptions: are defined in OMG documentation

(both system and user)

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IDL concepts – 3

I _{Names (identifiers) are}_{case insensitive}_{, so User, USER and} user are the same.

But you must useonly one form

I _{Scopes: Names can be qualified by the module and interface} in which they are defined.

e.g. Stock::Quoter::get_quote

If no module is used:::Quoter::get_quote

I _{Members of structs, unions, etc. can also be qualified with} the struct, etc. name

I _{Inside the same module the module part can be omitted,} inside the same interface the interface part can be omitted, etc.

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IDL types

I _{short – 16 bit integer} I _{long – 32 bit integer} I _{long long – 64 bit integer}

I signed and unsigned

I _boolean

I _{float, double and long double} I _{fixed – fixed point number} I _{char (8 bits character – iso-8859-1)}

I _{wchar (wide characters – implementation dependent)}

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IDL types

I _{string – strings consisting of chars} I _{wstring – strings consisting of wchars} I _{octet (8 bits – unchanged)}

I _{struct (like in C, C++, Pascal record)}

I _{union (can contain different types, chosen with a switch} construction)

I _{enum (enumeration constant)} I _{array – multidimensional, fixed bounds}

I _{sequence – array like, one-dimensional; no fixed length (max} may be specified)

I _{any – can contain any other type.}

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IDL Example

e

(in short ho lon raises g explanation}; /* interface Dog:Pet, ni { } /*

interface Cat: Animal { void Eat(); w_ } ; ; } ; unsigned long attribut def of Cat */ def of Dog */ age; module MyAnimals { ( i ; ) NotInterested) A mal

except on NotInterested {string

void Bark y mng ci ontext d o meth

CORBA c ass with e c n a t r e h n ) e l p it u m ( l i i l e p t : s e m a a and d rection n p r e a i t r

nst ar

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v

s

, ..

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i

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IDL Hello

module HelloApp { interface Hello { string sayHello(); }; interface HelloFactory {

Hello create(in string message); };

};

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IDL Quoter

module Stock {

// An exception is a combination of // an event and maybe some data. exception Invalid_Stock {} ; exception Invald_Quoter {} ;

// Interface like C++ class, Java interface interface Quoter {

long get_quote(in string stock_name) raises (Invalid_Stock) ; };

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IDL Quoter – continued

// Manage the lifecycle of a Quoter object interface Quoter_Factory {

// Returns a new Quoter selected by name // e.g. "MSFT"

Quoter create_quoter(in string name) raises (Invalid_Quoter); void destroy_quoter(in Quoter quoter) ; };

};

I _{Similiar to Java constructs}

I _{in, out, inout specify direction of parameters}

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Complex types in IDL

I _{Complex types should be given a}_name

I enum I _struct I union I sequence I _array I fixed I typedef typedef long int; typedef string<30> name;

typedef some_complex_type mytype; I _{Note: the name to be defined comes last!!}

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Enum

I _enum_{defines a series of named constants:} enum Color { Red, Green, Blue }; const Color favorite_color = Blue; module Mod {

enum Size { small, medium, large }; };

const Mod::Size MYSIZE = Mod::medium;

Error:

const Color another = Mod::medium; // is an error

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Struct

I _struct_{defines a type with a collection of fields} I _{Like a Java class without methods}

struct Point2D { float x; float y; }; struct Address { string<25> street; short number; string<20> city; }

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Struct

I _{Better to use names for the complex types:} I _{In some cases this is obligatory}

typedef string<25> streetname; typedef string<20> cityname; struct Address {

streetname street; short number; cityname city; };

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Sequence and array

I _{Arrays have fixed size and can be multidimensional:} I _{Define matrix as an array of 10*20 floats}

typedef double Point3D[3]; typedef float matrix[10][20];

I _{First the type of the elements, then the name of the new type,} then the dimensions!!!

I _{Sequences are onedimensional and are flexible in length} I _{They may have an upper bound}

typedef sequence<int> vector;

typedef sequence<int, 10> bounded_vector;

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union

I _union_{can contain any of a number of other types} I _{The choice is indicated by a}_{discriminator} I _{This must be some integer, char, bool or enum.}

union message switch (char) { case ’e’: string english; case ’c’: wstring chinese; };

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union

enum message_type {SMS, Email}; struct sms_message { long telephone_number; string<160> contents; }; struct email_message { string From; string To; string Subject; string contents; };

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union

union message switch (message_type) { case SMS: sms_message sms; case Email: email_message email; };

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IDL to Java mapping

I _{Language mapping}

I _{Most constructs map easily to Java} I Done by IDL compiler

I _{Example constant, array}

module Example {

const long aLong = -12345; const long ArrayBound = 42; struct ArrayContainer {

long array[ArrayBound]; };

};

I _gives

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IDL to Java mapping

package Example;

public final class aLong {

public static final int value = (int) (-12345L); }

public final class ArrayContainer { // instance variables

public int[] array; // constructors

public ArrayContainer() { }

public ArrayContainer(int[] __array) {...} }

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Another Example

module StatMod { interface Stat {

void init();

void addItem(in double item); double getAvg ();

long getNum ();

void getAll (out double avg, out long num); };

};

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Python crash course

Python is an elegantdynamicprogramming language: I _{No static type declarations}

I _{Statements can be entered and executed directly in the} interpreter

I _{High level datatypes: lists, tuples, dictionaries, classes, . . .} I _{Each python file is a module (namespace) and can be}

imported in other modules I _{Block structure by indentation}

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Python example

class Test:

#constructor, ’self’ is like ’this’ in Java def __init__(self, value):

self.value = value def getval(self, x):

return self.value, x # return a tuple x = Test(5)

print x.getval(3) # should print (5, 3)

I _{instance variables/methods can only be referenced with dot} notation (obj.var/obj.meth()

I _{without dot only local and global variables}

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Python server part 1

import sys

from Fnorb.orb import BOA, CORBA import StatMod_skel class StatServer(StatMod_skel.Stat_skel): def init(self): self.count = 0 self.sum = 0.0 def addItem(self, x): self.sum += x self.count += 1 def getNum(self): return self.count

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Python server part 2

def getAvg(self): try: return self.sum/self.count except: return 0 def getAll(self): return self.getAvg(),self.getNum()

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Python server part 3

# Initialise the ORB.

orb = CORBA.ORB_init(argv, CORBA.ORB_ID) # Initialise the BOA.

boa = BOA.BOA_init(argv, BOA.BOA_ID)

# Create an object reference (’fred’ is the object key). obj = boa.create(’fred’, StatServer._FNORB_ID)

# Create an instance of the implementation class. impl = StatServer()

boa.obj_is_ready(obj, impl)

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Python server part 3

# Write the stringified object reference to a file f = open(’Server.ref’, ’w’)

f.write(orb.object_to_string(obj)) f.close()

# Start the event loop. boa._fnorb_mainloop()

In this example we don’t use a Naming service, just write the object reference to a file.

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Python client

import sys

from Fnorb.orb import CORBA import StatMod

# Initialise the ORB.

orb = CORBA.ORB_init(sys.argv, CORBA.ORB_ID) # Read the server’s stringified IOR from a file stringified_ior = open(’Server.ref’).read()

# Convert the stringified IOR into an active object reference. server = orb.string_to_object(stringified_ior)

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Python client

# Make sure that the server is not a ’nil object reference’ # (represented in Python by the value None).

if server is None:

raise ’Nil object reference!’ # Call the server!

server.init() # server.addItem etc.

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Java client

import org.omg.CORBA.*; import java.util.*; import java.io.*; import StatMod.*; public class StatClient {

public static void main(String args[]) { Stat stat;

String ior; try {

ORB orb = ORB.init(args, null); BufferedReader ref = new BufferedReader

(new FileReader("Server.ref")); ior = ref.readLine();

stat = StatHelper.narrow

(orb.string_to_object(ior));

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Java client

stat.init(); stat.addItem(7); stat.addItem(4); System.out.println(stat.getAvg()); } catch (Throwable e) { System.err.println(e); e.printStackTrace(System.err); } } }

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Translation to Java

package StatMod;

public interface StatOperations {

void init ();

void addItem (double item); double getAvg ();

int getNum ();

void getAll (org.omg.CORBA.DoubleHolder avg, org.omg.CORBA.IntHolder num); }

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Java out parameters

IntHolder count = new IntHolder(); DoubleHolder avg = new DoubleHolder(); ...

stat.init(); stat.addItem(7); stat.addItem(4); // Use out parameters stat.getAll(avg, count);

System.out.println(avg.value+" "+count.value); Holder classes have avalueattribute that contains the actual value of the (in)out parameter.