CORBA and Java u by Alex Chaffee u

CORBA and Java u by Alex Chaffee u alex@jguru. com u http: //www. jguru. com/ u Java online resources u http: //www. purpletech. com/ u Java training and consulting Copyright © 1998 Purple Technology, Inc

Abstract u CORBA loves Java! CORBA provides a platform-independent, language-independent way to write applications that can invoke objects that live across the room or across the planet. Java is an object-oriented language that's ideal for writing the clients and servers living on the Object Bus. In this session, we examine the ways they interoperate programmatically, as we walk you stepby-step from a CORBA IDL, to a server and client both written in Java, running on top of a 100%-Java ORB. We also discuss the relationship between CORBA and RMI, and discuss some of the real-world issues involved in deploying a CORBA-based application. Recommended: some knowledge of CORBA, ability to read Java source code. Copyright © 1998 Purple Technology, Inc

Introduction u Purple Technology u http: //www. purpletech. com u Java u Alex Training

Part I: CORBA Overview Copyright © 1998 Purple Technology, Inc.

What is CORBA? u Common Object Request Broker Architecture u Communication infrastructure for distributed objects u Allows a heterogeneous, distributed collection of objects to collaborate transparently Copyright © 1998 Purple Technology, Inc

What is CORBA good for? u Developing distributed applications u Locating remote objects on a network u Sending messages to those objects u Common interface for transactions, security, etc. u CORBA Services (more later) Copyright © 1998 Purple Technology, Inc

Why Distributed Applications? u Data is distributed u Administrative and ownership reasons u Heterogeneous systems u Shared by multiple applications u Scalability Copyright © 1998 Purple Technology, Inc

Why Distributed Applications? u Computation is distributed u Scalability: multiprocessing u Take computation to data u Heterogeneous architectures u Users are distributed u Multiple users interacting and communicating via distributed applications Copyright © 1998 Purple Technology, Inc

Distributed Object Systems u All entities are modeled as objects u Systems support location transparency u Interfaces, not implementations, define objects u Good distributed object systems are open, federated systems Copyright © 1998 Purple Technology, Inc

What is the OMG? u Designers of CORBA u Consortium of 700+ companies u Not including Microsoft u Members: u platform vendors u database u software tool developers u corporate u software vendors developers application vendors Copyright © 1998 Purple Technology, Inc

It’s Just A Spec u Has never been fully implemented u Probably never will be u Industry moves quickly and spec has to keep up u Interoperability vs. portability u Pass-by-value Copyright © 1998 Purple Technology, Inc

Basic CORBA Architecture Server Client response request ORB “Object Bus” Copyright © 1998 Purple

CORBA Objects u Examples u Service u Client u Component u Business object u CORBA objects approach universal accessibility u Any Language u Any Host on network u Any Platform Copyright © 1998 Purple Technology, Inc

CORBA Elements u 1. ORB u 2. CORBA Services u 3. CORBA Facilities u

ORB u Object Request Broker u “Object u Handles Bus” all communication among objects u Each host (machine) has its own ORB u ORBs know how to talk to each other u ORB also provides basic services to client Copyright © 1998 Purple Technology, Inc

ORB Responsibilities u Find the object implementation for the request u Prepare the object implementation to receive the request u Communicate the data making up the request u Retrieve results of request Copyright © 1998 Purple Technology, Inc

Network of ORBs u There’s an ORB on the server too u ORB receives

IIOP u Internet Inter-Orb Protocol u Network or “wire” protocol u Works across TCP/IP

ORB Features u Method invocations u Static and Dynamic u Remote objects or CORBA services u High-level language bindings u Use your favorite language; ORB translates u Self-describing u Provides services metadata for all objects and Copyright © 1998 Purple Technology, Inc

ORB Features u Local or remote u Same API wherever target object lives u Preserves context u Distributed security and transactions u Coexistence u Just with legacy code provide a wrapper object Copyright © 1998 Purple Technology, Inc

What is an ORB really? u Not a separate process u Library code that executes in-process u Listens to TCP ports for connections u One port per local object u Opens u. N TCP sockets to other objects ports per remote machine Copyright © 1998 Purple Technology, Inc

IDL u Interface Definition Language u Defines protocol to access objects u Like a

IDL Example module Calc { interface Adder { long add(in long x, in long

Stubs and Skeletons u Stub u lives on client u pretends to be remote object u Skeleton u lives on server u receives requests from stub u talks to true remote object u delivers response to stub Copyright © 1997 Alex Chaffee

Stubs and Skeletons (Fig. ) Client Host Machine Server Host Machine Client Object Remote

Client vs. Server u in CORBA, a client is a client relative to a particular object u i. e. an object with a reference to a “server” object u A client may also act as a server u If it has an IDL and stubs and skeletons u Technically, a CORBA server contains one or more CORBA objects Copyright © 1998 Purple Technology, Inc

Different Meanings of “Server” u Host machine u Program running on host machine u CORBA object running inside program u has IDL, stub, skeleton u Sometimes called a Servant Copyright © 1998 Purple Technology, Inc

Stubs and Skeletons -> Platform Independence u Client code has no knowledge of the implementation of the object or which ORB is used to access the implementation. Copyright © 1998 Purple Technology, Inc

CORBA Services u APIs for low-level, common tasks u Life Cycle Service u creating, copying, moving, removing objects u Naming Service u Register objects with a name u Look up objects by name Copyright © 1998 Purple Technology, Inc

CORBA Services u Concurrency u Obtain Control Service and release exclusive locks u Transaction Service u Two-phase commit coordination u Supports nested transactions u Persistence Service u Storing objects in a variety of databases u RDBMS, OODBMS, file systems Copyright © 1998 Purple Technology, Inc

CORBA Services u Security Service u Authentication, u Event ACLs, encryption, etc. Service u

CORBA Services u Relationship u Externalization u u u Query Licensing Properties u Time Trader u Collection u … and so on… u u See what I mean about it never being Copyright © 1998 Purple Technology, Inc implemented?

CORBA Facilities u Frameworks for specialized applications u Distributed Document Component Facility u Open. Doc u In progress: u Agents u Business Objects u Internationalization Copyright © 1998 Purple Technology, Inc

N-Tier Design with CORBA Storage “Tier” ORB DB ORB ORB DB Data Object ORB ORB Client Tier TP Monitor ORB Business Object Tier (after diagram in Orfali et al. ) Service “Tier” Copyright © 1998 Purple Technology, Inc

Three Tiers u User Interface Tier u Business Logic Tier u Data Storage Tier

Part II: Java IDL - Using CORBA from Java Copyright © 1998 Purple Technology,

Java CORBA Products u The Java 2 ORB u Visi. Broker for Java u

Java IDL u Should be named “Java CORBA” u More than just IDL u Full (? ) implementation of CORBA in 100% Java u Three Parts u ORB u Naming Service u idltojava compiler u Ships with JDK 1. 2 Copyright © 1998 Purple Technology, Inc

Transparent API u Java. IDL turns IDL into direct method calls u Easy to

The Java ORB u 100% Java u Generic u Allows Java IDL applications to run either as stand-alone Java applications, or as applets within Java-enabled browsers u Uses IIOP Copyright © 1998 Purple Technology, Inc

Other Java ORBs u Visigenic (Inprise) Visi. Broker u Netscape Communicator u Oracle Web

IDL to Java Mapping u Defined by OMG u Translates IDL concepts into Java

IDL to Java Type Mapping IDL Type boolean char / wchar octet short / unsigned short long / unsigned long float double string / wstring Java Type boolean char byte short int long float double String Copyright © 1998 Purple Technology, Inc

IDL vs. Java vs. C++ concepts IDL module Java package C++ namespace interface abstract class operation method member function attribute pair of methods pair of functions Copyright © 1998 Purple Technology, Inc

IDL Modules u Map to Java packages u Unfortunately, it has the root level name of the module u Clutters up your package hierarchy u e. g. module Calc -> u package Calc u interface Calc. Adder u not package ORG. omg. CORBA. modules. Calc Copyright © 1998 Purple Technology, Inc

IDL Interfaces u Map to Java interfaces Copyright © 1998 Purple Technology, Inc

IDL Operations u Map to Java methods Copyright © 1998 Purple Technology, Inc

IDL Attributes u Map to pair of functions u IDL u string name; u

idltojava u Development tool u Automatically generates java stubs, skeletons, helpers, holders, . .

Stubs u Java objects call stub methods u Stubs communicate with CORBA objects u

Skeletons u ORB passes request to skeleton (like a stub) u Skeleton calls local

Remote Interfaces and Stubs IDL Interface implements extends Client Stub Skeleton Remote Object (Server)

Show Me The Source Code u OK, here it comes. . . Copyright ©

idltojava input u Calc. idl module Calc { interface Adder { long add(in long

idltojava output u idltojava Calc. idl · Adder. java (a Java interface that maps the IDL interface) · _Adder. Stub. java (a client stub) · _Adder. Impl. Base. java (a server skeleton) · Adder. Helper. java (a helper class) · Adder. Holder. java (a holder class) u Must compile these files and put them in your CLASSPATH Copyright © 1998 Purple Technology, Inc

Adder. java u Created for you by idltojava package Calc; public interface Adder extends

Implementing a server u Extend base class u Implement u Provide u Create all methods declared in IDL a main method an ORB instance u Create a server instance u Inform the ORB about the instance u Acquire a Naming Context u Register the instance in that Naming Context under some name Copyright © 1998 Purple Technology, Inc

$Adder. Server. java class Adder. Server extends _Adder. Impl. Base { public int add($

Adder. Server. java class Adder. Server extends _Adder. Impl. Base { public int add( int x, int y ) { System. out. println(x + " + y + " = " + (x+y)); return x + y; } Copyright © 1998 Purple Technology, Inc

Adder. Server. java (cont. ) public static void main(String args[]) { // create and initialize the ORB orb = ORB. init(args, null); // create server and register it with the ORB Adder. Server adder. Ref = new Adder. Server(); orb. connect(adder. Ref); Copyright © 1998 Purple Technology, Inc

Adder. Server. java (cont. ) // get the root naming context org. omg. CORBA. Object obj. Ref = orb. resolve_initial_references("Name. Ser vice"); Naming. Context nc. Ref = Naming. Context. Helper. narrow(obj. Ref); // bind the Object Reference in Naming Name. Component nc = new Name. Component("Adder", ""); Name. Component path[] = {nc}; nc. Ref. rebind(path, adder. Ref); } Copyright © 1998 Purple Technology, Inc

Implementing a client u Create an ORB u Get a reference to the Naming Context u Look up the correct name in that Naming Context u Receive an object reference to that object u Actually, u Invoke to its Stub methods on the reference Copyright © 1998 Purple Technology, Inc

Adder. Client. java Copyright © 1998 Purple Technology, Inc

Object Reference u Two Meanings u 1. An abstract concept referring to a specific object living on a specific host, attached to a specific ORB u 2. A local Java reference to an object that relays messages to that object u Obtained from u new keyword u a Factory Object u the Naming Service Copyright © 1998 Purple Technology, Inc

Naming Service u tnameserv ut is for “transient” u Maps name to object reference u An implementation of the CORBA Object Service (COS) name service u to run the Naming Service u UNIX: tnameserv & u Win 32: start /m tnameserv Copyright © 1998 Purple Technology, Inc

Fire it up u start /m tnameserv u start java Adder. Server u java

CORBA Flow Client Virtual Machine Server Virtual Machine Client Remote Object Skeleton Stub Server

CORBA Flow 1. Server Creates Remote Object Client Virtual Machine 2. Server Registers Remote Object Client Server Virtual Machine Remote Object 1 Skeleton Stub Server 2 “Fred” Name Server Virtual Machine Copyright © 1997 Alex Chaffee

CORBA Flow Client Virtual Machine 3. Client requests object from. Remote Name Server 4. Name Server returns remote reference Object (and stub gets created) Client Skeleton Stub 3 Server Virtual Machine Server 4 “Fred” Name Server Virtual Machine Copyright © 1997 Alex Chaffee

RMI Flow Client Virtual Machine Server Virtual Machine Client Remote Object 5 7 6 Stub Skeleton Server 5. Client invokes stub method 6. Stub talks to skeleton 7. Skeleton invokes remote object “Fred” method Name Server Virtual Machine Copyright © 1997 Alex Chaffee

Pseudo-objects u The ORB is a pseudo-object u It works just like a remote

The Basic Object Adapter (BOA) u Another pseudo-object u Helps register objects with the ORB u Functions u Maintain Implementation Repository u Generate and interpret object references u Activate and deactivate implementation objects u Invoke methods via skeletons Copyright © 1998 Purple Technology, Inc

Why do you need both an ORB and a BOA? u I’m not really sure u Allows vendors to optimize or enhance functionality u register many objects en masse u cache object state elsewhere u E. g. Object database Copyright © 1998 Purple Technology, Inc

Using the BOA u Slightly different procedure for initializing objects u Hides name service from you u Ask the BOA to register the object u Ask the Helper object to bind the object u Once the object is created, interface is identical u Just call methods using normal Java syntax Copyright © 1998 Purple Technology, Inc

BOA Object Activation u Shared server u Multiple objects, one server u Normal procedure u Multithreaded (handled by CORBA implementation) u Unshared u New server process for each object Copyright © 1998 Purple Technology, Inc

BOA Object Activation (cont. ) u Server-per-method u Batch processing u Persistent u Shared

BOA Object Activation Scenario u Server u obj creates object instance = new My. Object(); u Server registers object with BOA u boa. create(interface_name, implementation_name, reference_data) u Visi. Broker does this for you in superclass constructor Copyright © 1998 Purple Technology, Inc

BOA Object Activation Scenario (cont. ) u Server tells BOA the object is ready u boa. obj_is_ready(obj) u Server tells BOA that all objects are ready u boa. impl_is_ready() u Objects shut down u boa. deactivate_obj(this) u Server shuts down u boa. deactivate_impl() Copyright © 1998 Purple Technology, Inc

Threads in Java. IDL u Remote invocations happen on a separate thread u Must

Callbacks u Pass in a reference to some CORBA object u The server object can invoke methods on that reference u ORB automatically generates a stub on the server side u The server has become the client, and vice versa Copyright © 1998 Purple Technology, Inc

Obtaining Object References u From the ORB u orb. resolve_initial_references(“Name. Servi ce”) u From a Naming Context u From a Stringified reference u From another object u From a parameter to your remote method Copyright © 1998 Purple Technology, Inc

Helper Objects u How CORBA does casting u narrow method changes the type of an object reference // get the root naming context org. omg. CORBA. Object obj. Ref = orb. resolve_initial_references("Name. Ser vice"); Naming. Context nc. Ref = Naming. Context. Helper. narrow(obj. Ref); Copyright © 1998 Purple Technology, Inc

Naming Contexts u Directory structure for CORBA u Naming Component = file u Naming Context = directory u Can add a Context as a Component u like subdirectories u Can add a Context on a different name server u like symbolic links Copyright © 1998 Purple Technology, Inc

Stringification u Stringified reference is ORBindependent u Object to string org. omg. CORBA. Object obj =. . . String str = orb. object_to_string(obj); u String to object u org. omg. CORBA. Object obj = orb. string_to_object(str); Copyright © 1998 Purple Technology, Inc

Visigenic: Caffeine u Java to IDL compiler Copyright © 1998 Purple Technology, Inc

Symantec: Visual Café 3. 0 Enterprise Suite u Java to IDL compiler u Automatic client adapter bean generation u Makes a Java. Bean that’s a proxy to a CORBA object u Distributed debugging u Follow call chain, examine variables, et al. u Across many VMs, many hosts, many OSs Copyright © 1998 Purple Technology, Inc

Part III: Example Application Copyright © 1998 Purple Technology, Inc

Publish-Subscribe System u Channel: an avenue for real-time data exchange u Consumers can subscribe to arbitrary data channels u Producers can publish data on channels u Invokes callback method on consumer Copyright © 1998 Purple Technology, Inc

Object Model u PSServer u Channel u creates get. Channel(String channel. Name) channel if it doesn’t already exist u Channel u void subscribe(Subscriber sub) u void publish(Object data) u Subscriber u void data. Arrived(Object data) Copyright © 1998 Purple Technology, Inc

Flow: Subscriber-side u Client acquires PSServer reference u Client acquires channel from PSServer u Client creates subscriber locally, passing reference to channel u Subscriber registers self with channel u channel. subscribe(this) u Client waits for callback to its data. Arrived method Copyright © 1998 Purple Technology, Inc

Flow: Publisher-side u Client acquires PSServer reference u Client acquires channel from PSServer u Client creates publisher locally, passing reference to channel u Publisher acquires data u From a database u From a data feed u Publisher calls channel. publish(data) Copyright © 1998 Purple Technology, Inc

Example Application: Stock data u Channel name = stock symbol name u Source code

Design issues u Name space for channels u TIBCO et al. use hierarchical directory structure u Caching prior messages u Subscribe should spawn a thread that sends previous N messages Copyright © 1998 Purple Technology, Inc

Design issues u Maintain local cache of data sent u Don’t want to go

Part IV: Advanced Topics Copyright © 1998 Purple Technology, Inc

Exceptions u CORBA exceptions are mapped to Java exceptions u System Exceptions u They’re subclasses of Runtime. Exception, so the compiler won’t complain if you forget to catch them u Minor codes u additional information u vendor-specific : -( u Completion Status Copyright © 1998 Purple Technology, Inc

Exceptions (cont. ) u User Exceptions u Generated by idltojava u Subclasses of Exception

Dynamic Invocation Interface (DII) u CORBA is fully self-aware u Methods for acquiring u List of all services u List of known objects u Interfaces of known objects u Etc. u Well-documented elsewhere Copyright © 1998 Purple Technology, Inc

Interface Repositories u Java. IDL clients do not typically need or use an Interface Repository u All of the needed type information is available in the stubs or is resolved by an object's server u Java. IDL supports access to Interface Repository information made available by other CORBA object implementations. Copyright © 1998 Purple Technology, Inc

Implementation Repository u contains information that allows the ORB to locate and activate implementations of objects u also store additional information associated with implementations of ORB objects. u Debugging information u Administrative control u Resource allocation u Security Copyright © 1998 Purple Technology, Inc

Value Objects u CORBA 3. 0 u Blobs u Avoids network latency of pass-byreference

RMI over IIOP u In Development u Uses IIOP as transport protocol u Uses

CORBA via RMI u Possible using middleware server u RMI from client to middleware

CORBA and EJB u Transport u EJB uses RMI interface, RMI uses IIOP u CORBA 3. 0 promises object compatibility with EJB u Not quite sure what that means u Some u All EJB Servers contain an ORB EJB Objects are also CORBA objects Copyright © 1998 Purple Technology, Inc

CORBA and EJB (Cont. ) u All EJB Servers use CORBA Transactions (via JTS) u That means that any client can make a distributed transaction that includes both CORBA and EJB Objects u Not an either-or decision u You can have both EJB and CORBA working together in a single system Copyright © 1998 Purple Technology, Inc

Java Transactions u Java Transaction Service (JTS) u. A standard Java mapping of the OMG Object Transaction Service (OTS) u packages org. omg. Cos. Transaction and org. omg. Cos. TSPortability u Java Transaction API (JTA) u High-level transaction management specification u package javax. transaction Copyright © 1998 Purple Technology, Inc

Object Transaction Service (OTS) u Distributed u Part transaction specification of CORBA, but also standalone u Transactions can span u multiple queries u multiple objects u multiple servers u multiple databases u multiple vendors’ products Copyright © 1998 Purple Technology, Inc

OTS Features u Distributed u Flat transactions or nested transactions u Two-phase u Vendor

OTS Object Model u Transactional u Can Object participate in a transaction u Recoverable Object u Actually performs commit or rollback on its own data u Resource u. A system or server that can commit or rollback data stored inside it u E. g. a database server Copyright © 1998 Purple Technology, Inc

OTS Object Model (Cont. ) u Current u Reference/wrapper u Control to currently active transaction / Coordinator / Terminator u Registers objects u Performs distributed commit/rollback u Synchronization u An object that gets informed of the progress of the ongoing transaction u Implements callback methods Copyright © 1998 Purple Technology, Inc

Part V: CORBA/Java Case Studies Copyright © 1998 Purple Technology, Inc.

CORBA Case Study u Kaiser Permanente Northern California u KAHUNA a/k/a Web. Chart u

Problem: Specific u Medical records, patient histories, clinical schedules, lab results, and medical images all stored in disparate databases u Many patients per doctor, many doctors per patient u Need immediate access to all information Copyright © 1998 Purple Technology, Inc

Problem: General u getting data u from a multitude of sources u in a multitude of media u putting u into data a single system u distributing data u to a wide variety of desktops u throughout a very large company Copyright © 1998 Purple Technology, Inc

Solution: Java/CORBA u “Not only did Java/CORBA solve all these problems, but Java/CORBA development

Web. Chart Architecture Copyright © 1998 Purple Technology, Inc

This is your brain on Java. Any questions? Copyright © 1998 Purple Technology, Inc

Conclusion Copyright © 1998 Purple Technology, Inc.

Where to Get More Information u Orfali & Harkey, Client/Server Programming with Java and

More Info: Links u This u http: //www. purpletech. com/ u JDK u Presentation 1. 2 CORBA documentation http: //java. sun. com/products/jdk/1. 2/docs/guide/idl/ u Glerum, Web. Chart serves up Kaiser patients, Java. World u http: //www. javaworld. com/jw-11 -1998/jw-11 kaiser. html? 101698 txt Copyright © 1998 Purple Technology, Inc

More Info: Links u Object Management Group - http: //www. omg. org u Java Transactions http: //java. sun. com/products/jta/ u http: //java. sun. com/products/jts/ u ftp: //www. omg. org/pub/docs/formal/97 -12 -17. pdf u Copyright © 1998 Purple Technology, Inc