CORBA

2004-02-02 A. Ghodsi aligh@imit.kth.se 1

Common Object Request Broker Architecture

Ali Ghodsialigh@imit.kth.se

Goal of lecture

Go a bit more into depth on the corearchitecture of CORBA

Less breadthRead van Steen’s book

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/

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

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…

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…

OutlookGeneral OverviewQuick Architectural Overview

OOP with Distribution TransparencyCORBA overview

Interface Definition Language (IDL)TypesExamplesMappings

ORBConclusions

CORBA builds on the DOMProvides a nice model

Encapsulation

Inheritance

Polymorphism

Exploiting EncapsulationEncapsulation enables:

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

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

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

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

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++

Overview

Client Object Implementationoperation()args + return

SKELETON Object Adapter

ORB-dependent implementationApplication specific Stub and Skeleton

ORB Core ORB Core

Same inteface. ORB-independent

Network

OutlookGeneral OverviewArchitecture OverviewInterface Definition Language (IDL)

TypesExampleLanguage Mappings

ORBConclusions

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

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

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);

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

OutlookGeneral OverviewArchitecture OverviewInterface Definition Language (IDL)ORB

DII (and DSI)ORB interfaceObject ReferencesPOAPersistent and Transient Objects

Conclusions

Compile time vs Runtime?

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

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

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

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);

Complete picture

StaticStub

StaticSkeletonDynamic

Invocation

DynamicSkeletonInterface

Object Adapter

ORB Core ORB Core

Network

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.

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”

ORB Interface

ORBInterface

StaticStub

Invocation

Object Adapter

ORB Core ORB Core

Network

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

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

Portable Object Adapter (POA)

Object Adapter

ORB Core ORB CoreNetwork

ORBInterfaceStatic

Invocation

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

Why Object Adapters?

Several clients call the same object, whatto do?

Demultiplex requests

Client 1dsObject.calculate();

ServerDsObject::calculate()

Why Object Adapters? (2)

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

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

Lifespan policy:Transient objectsPersistent Objects

Continues to exist even if activated/deactivated?

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

Portable Object Adapter

Server DemultiplexerDispatch requests

to the right POA

POA1 (policy1)Invoke right

skeleton

Active Object Map

OBJ2 -> skel2

OBJ1 -> skel1

POA2(policy2)Invoke the right

skeleton

Active Object Map

OBJ3 -> skel3

Transient Object Illustration

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

_InfoSec

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

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

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

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

Object AdapterStartup CommandCurrent Server

STUB: Object Reference

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

ORB Core

Persistent Objects Illustrated

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

_InfoSec

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”

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

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

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)

What did I miss?A whole lot! ☺

CORBA facilities/servicesSynchronizationCachingReplicationFault-toleranceSecurity

Comparison of CORBA against.NET DCOMJava RMIetcetera

Please read chapter 9!

The End

THANK YOU VERY MUCH!

CORBA

Technology

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