CORBA

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2004-02-02 A. Ghodsi [email protected] 1

Common Object Request Broker Architecture

Ali [email protected]

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Goal of lecture

Go a bit more into depth on the corearchitecture of CORBA

Less breadthRead van Steen’s book

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Reading suggestionsTanenbaum & van Steen

CORBASection 2.3 page page 85-98Section 3.2.2 page 152-158Section 9.1

Read chapter 9 and compare other systems with CORBACompare RPC and DCE Remote Objects with CORBA

LinksNice CORBA tutorial:http://www.omg.org/gettingstarted/

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OutlookGeneral Overview

General InformationApplications

Quick Architectural OverviewOOP plus Distribution TransparencyCORBA main overview

Interface Definition Language (IDL)TypesExamplesMappings

ORBDII (and DSI)ORB interfaceObject ReferencePOAPersistent and Transient Objects

Conclusions

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General CORBA InformationDistributed Object Model (more later)

It is a middlewareDifference between Network OS Middleware?

Only a standard (v 2.7, 3.0)No reference implementation!Many independent implementations

OMG - Non-profit organization 800 members!Standardized UML and more…

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Real World Applications?Support ”dinosaurs”

Companies have invested years of development in projects done in ADA, C, Smalltalk…CORBA enables interoperability with new languages

Languages with small user-baseEg Erlang, again interoperability

Big ERM, ERP, ISMany different architectures, languages, platforms…

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OutlookGeneral OverviewQuick Architectural Overview

OOP with Distribution TransparencyCORBA overview

Interface Definition Language (IDL)TypesExamplesMappings

ORBConclusions

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CORBA builds on the DOMProvides a nice model

Encapsulation

Inheritance

Polymorphism

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Exploiting EncapsulationEncapsulation enables:

Distribution TransparencyHave stubs and skeletons that together with ORBsenable distribution*.

Inter-operability**Define interfaces in a standardised wayInterface Definition Language (IDL)

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Goal 1: Distribution Transparency

Encapsulation: black-box principleHas an interfaceImplementation detailshidden public interface MathBox {

int add(int x, int y);

}

public class MathBoxCLimplements MathBox {

MathBoxCL() {}

int add(int x, int y)

{ return x+y; }

}

…

MathBox obj = new MathBoxCL();

System.out.println(obj.add(10,20));

…

Transparentlydistribute

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Distribution Transparency

Client…

MathBox obj = new MathBoxCL();Integer result = obj.add(10,20);

…

Server Implementationint add(int x, int y) { return x+y; }

MathBoxCL (PROXY)int add(int x, int y) { Msg msg=new Msg();

msg.Marshal(x);msg.Marshal(y);SendReqMsg(HOST,IP,msg);

}

MathBoxCL (SKELETON)int invoke(msg msg){ int x, y;

x=msg.Unmarshal(INT);y=msg.Unmarshal(INT);res=serverImpl.add(x,y);Msg msg=new Msg();msg.marshal(res);SendRespMsg(HOST, IP, msg);

}

MathBoxCL (PROXY)int add(int x, int y) { Msg msg=new Msg();

msg.Marshall(x);msg.Marshall(y);SendReqMsg(HOST,IP,msg);

}

MathBoxCL (SKELETON)int invoke(msg msg) { int x, y;

x=msg.Unmarshall(INT);y=msg.Unmarshall(INT);res=serverImpl.add(x,y);Msg msg=new Msg();msg.marshall(res);SendRespMsg(HOST,IP,msg);

}

Missing parts:

• Marshalling

• Unmarshalling

• References

• Binding client to server

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Goal 2: Inter-operability

Use a language with standardized syntax to define the interfaceGenerate the stub and the skeleton

Programming Language Independent

MathBoxCL (STUB)int add(int x, int y) { Msg msg=new Msg();

msg.Marshal(x);msg.Marshal(y);SendReqMsg(HOST,IP,msg);

}

MathBoxCL (SKELETON)int invoke(Msg msg) { int x, y;

msg=GetMsg();x=msg.Unmarshal(INT);y=msg.Unmarshal(INT);res=serverImpl.add(x,y);Msg msg=new Msg();msg.marshal(res);SendRespMsg(HOST, IP, msg);

}

JAVAC++

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Overview

STUB

Client Object Implementationoperation()args + return

value

SKELETON Object Adapter

ORB-dependent implementationApplication specific Stub and Skeleton

ORB Core ORB Core

Same inteface. ORB-independent

Network

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OutlookGeneral OverviewArchitecture OverviewInterface Definition Language (IDL)

TypesExampleLanguage Mappings

ORBConclusions

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Interface Definition Language

Builds on OOP principle of encapsulationClear boundary between implementation and interface

IndependentProgramming Language (Only OO?)OSPlatformNetwork Connectionetc

Can be converted to a binary format and stored in a database (i.e. well-defined schema, iterators)

Interface Repository (IR)A Repository ID for each interface

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IDL’s Type System

Two levels:Interfaces for CORBA objects!

One interface per CORBA objectOfficial types for variables

integers, floatsstruct, enumarraystringbinary values…and more!

Scoped typesmodulesexceptionsInterfacesstructs

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Examples

DSLABS.IDL:

typedef string GroupMembers[4];

interface DS_project {long register_project(in long groupId, in string status, inout string date);long get_status(in long groupId, out string state, out string date, out

GroupMembers gm);

};

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IDL language mappingsOMG defines mappings to different languages

C, C++, Java, Smalltalk, COBOL, Ada, Lisp, PL/1, Python, and IDLscriptProprietary mappings exist for obscure languages, though not standardized!

Every ORB has an IDL compilerCreates

A STUB and A SKELETON

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OutlookGeneral OverviewArchitecture OverviewInterface Definition Language (IDL)ORB

DII (and DSI)ORB interfaceObject ReferencesPOAPersistent and Transient Objects

Conclusions

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Compile time vs Runtime?

What if interfaces change?Recompile everything? UnfeasableDynamic interface definitions required:

IS (Information Systems)ERM (Enterprise Resource Management systems)Batch Serviceetcetera

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Dynamic Invocation Interface (DII)

Generic run-time invocation

No compile-time knowledge of CORBA object interfacesNo stub and skeleton needed a-prioriInstead, a generic interface is used

Of course defined in IDL

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Dynamic Invocation Interface (DII) cont.

In essence:Search and fetch an IDL from an Interface Repository. (remember binary presentation of IDL)Construct a requestSpecify target object, operation, and parametersInvoke the request

C++ (not entirely true)invoke(remoteObj, ”getStatus”, paramters)

Java uses reflection/introspection (transparent):remoteObj.getStatus(paramters);

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Complete picture

StaticStub


value

StaticSkeletonDynamic

Invocation

DynamicSkeletonInterface

Object Adapter


ORB Core ORB Core


Network

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Object ReferencesRemote object references

Enable clients to invoke CORBA objects

Three incarnationsLanguage specific implementation

E.g. pointer to a stub in C++ implementing the IDLNot valid outside local computation space

Language independent ORB representationIOR, Inter-operable Object RefereneceSupported by all ORBs

Textual representationSend by e-mail, store in DB, textfiles and so on.

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Inter-operable Object References (IOR)

Remote Object Reference”Reference to an object on a serverProtocolHostname & Port

*GIOP, address, port ex: ”IIOP v1.0”,”ripper.it.kth.se”, 8765

Object Key(Adapter & Object Name)

*Which object adapter, which object?

ex: ”OA5”, ”_DSD”

Type Name(Repository ID)

Repository ID ex: ”IDL:KTH/imit/DSD:1.0”

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ORB Interface

ORBInterface


value

StaticStub


Invocation


Object Adapter


ORB Core ORB Core


Network

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ORB InterfaceStandard interface (defined in IDL)

All ORBs implement this interface

Important services provided:Bootstrapping, getting initial referencesConverting Object References to Strings and vice versaObject Reference Counting

Distributed garbage collection

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How do I get an IOR?All ORBs implement:

string_to_object()file, e-mail, phone :)

resolve_initial_references()Returns an IOR for naming service, interface

repositoryContinue to search for IOR’s in a namingservice

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Portable Object Adapter (POA)


value

Object Adapter

ORB Core ORB CoreNetwork

ORBInterfaceStatic

Stub


Invocation


ORB-dependent implementationApplication specific Stub and SkeletonSame inteface. ORB-independent

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Why Object Adapters?

Several clients call the same object, whatto do?

Demultiplex requests

Client 1dsObject.calculate();


ServerDsObject::calculate()

{

...

}

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Why Object Adapters? (2)

Queue requests or run in separate threads?Serialize all requestsOne thread per objectOne thread per invocationPool of threads




{

...

}

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Security between the objects?Sandboxing?Share methods, separate data?




{

...

}

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Lifespan policy:Transient objectsPersistent Objects

Continues to exist even if activated/deactivated?




{

...

}

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Portable Object Adapter – PL meetsORB!

POA is generic and CORBA object independent and implements different activation policies

POA keeps pointers to skeletons*

An Object Identifier is associated with object.

A table called Active Object Map maps betweenObject Identifers => Skeletons

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Portable Object Adapter

Server DemultiplexerDispatch requests

to the right POA

POA1

POA2

POA1 (policy1)Invoke right

skeleton

Active Object Map

OBJ2 -> skel2

OBJ1 -> skel1

OBJ 1

skel1

OBJ 2

skel2

POA2(policy2)Invoke the right

skeleton

Active Object Map

OBJ3 -> skel3

OBJ 3

skel3

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Transient Object Illustration

Stub

Object Adapter

ORB Core

Client_dsd->student_operation()

Object Implementation

ORB Core

Request message

Unique ID : ”13FABCDA” ”OA5”, ”_DSD”

student_operation() + *par

Active Object Maps

OA4:

_InfoSec

OA5:

_DSC

_DSD

Skeleton

Reply message

return variables, out parameters

Unique ID : ”13FABCDA”

STUB: Object Reference

”IIOP v1.0”,”ripper”, 8765 ”OA5”, ”_DSD”IDL:Institution/IT/DSD:1.0

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Persistent Objects

A IOR to a Persistent Object always points to the same object

Migration TransparencyLocation Transparency

Ceases to exist only if the CORBA object is logically destroyed

Might moveMight change IP, Port, MachineMight change POAetc

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Persistent Objects continuedIts life cycle is independent of the objects

I.e. its existence is independent of whether the object is in the local address-space.

ORBs can automatically startup objects implementing persistent CORBA objects

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How is this possible?Implementation repository (IMR) is usedKeeps information about

Object AdapterStartup CommandCurrent Server

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STUB: Object Reference

”IIOP v1.0”,”IMR”, 8765 ”OA5”, ”_DSD”IDL:KTH/imit/DSD:1.0

ORB Core

Persistent Objects Illustrated

Stub

ORB Core

Client_dsd->student_operation()

Object Implementation

ORB Core

Skeleton

Implem. Repository

IMR Table

ir:1444/startupyOA_2

ripper:313rsh ripper /runOA5

bored:131rsh x ”/bin/st”xOA_1

AddressStartupAdapter

Active Object Maps

OA4:

_InfoSec

OA5:

_DSC

_DSD

Request message

Unique ID : ”13FABCDA” ”OA5”, ”_DSD”

student_operation() + par

”IIOP v1.0”,”ripper”, 313

Location ForwardReply message

return variables, out parameters

Unique ID : ”13FABCDA”

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Failure and replication (IOR cont)

Protocol1Hostname1 & Port1

*GIOP, address, port ex: ”IIOP v1.0”,”ripper.it.kth.se”, 8765

Object1 Key(Adapter1 & Object1 Name)

Type Name(Repository ID)

Repository ID ex: ”IDL:KTH/imit/DSD:1.0”

HOST1/PORT2/ADAPTER2/OBJECT2 ex: ripper1/1234/oa1/obj1

HOST2/PORT2/ADAPTER2/OBJECT2 ex: ripper2/3233/oa3/obj6…

Remote Object Reference”Reference to an object on a serverProtocol2Hostname2 & Port2

Object2 Key(Adapter2 & Object2 Name)

Multiple locations in one reference. If an invocation fails, go to next location

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Summary-1CORBA is a standardization effort

Based on the the Distributed Object Model

Provides inter-operability

Uses proprietary interface language: IDLAll CORBA objects have an interface in IDL Most of the services offered by CORBA have an interface in IDL

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Summary-2

Provides both Dynamic and Static invocationsDII/DSI and STUBS/SKELETONS

Stubs/Skeletons talk to an ORB

The ORB uses a standardized protocol to exchangeMessages(Req/Resp), IORs (persistent/transient)

ORB uses a POA to implement different activation policiesThreading policyLifespan policy (Persistent vs. Transient Objects)Security (sandboxing of object implementations)

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What did I miss?A whole lot! ☺

CORBA facilities/servicesSynchronizationCachingReplicationFault-toleranceSecurity

Comparison of CORBA against.NET DCOMJava RMIetcetera

Please read chapter 9!

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The End

THANK YOU VERY MUCH!

CORBA

Technology

new msg msg

new msg x

msg sendreqmsghost

marshalxmsg msg

marshallx msg

marshally msg

new mathboxcl mathboxcl

unmarshalint int addint