Middleware combine

M1 Introduction and Evolution

  • This module introduces middleware, its evolution, and its various forms.

View of Middleware

  • Middleware sits as a layer between the operating system and distributed applications.

  • It hides the complexity and heterogeneity of distributed systems, bridging the gap between low-level OS communications and programming language abstractions.

  • Middleware provides a common programming abstraction and infrastructure for distributed applications.

  • More information can be found at http://www.middleware.org.

Definition of Middleware

  • Middleware connects software components or enterprise applications in a distributed system.

  • Examples include enterprise application integration software, telecommunications software, transaction monitors, and messaging-and-queueing software.

Middleware Support

  • Middleware provides support for:

    • Naming, Location, Service discovery, Replication

    • Protocol handling, Communication faults, QoS

    • Synchronization, Concurrency, Transactions, Storage

    • Access control, Authentication

Middleware Dimensions:

  • Request/Reply vs. Asynchronous Messaging

  • Language-specific vs. Language-independent

  • Proprietary vs. Standards-based

  • Small-scale vs. Large-scale

Common Forms of Middleware

  • Sockets

  • Remote Procedure Calls (RPC)

  • Distributed Object Oriented Components (e.g., ORB)

  • Message Oriented Middleware (Message Queues/ Enterprise Message Bus etc.)

  • Service Oriented Architectures

  • Web services (Arbitrary / RESTful)

  • SQL-oriented data access

  • Embedded middleware

  • Cloud Computing

Sockets

  • A socket is an internal endpoint for sending/receiving data within a node on a network.

  • Berkeley/POSIX sockets defined an API for Inter-Process Communication (IPC) within the same host (BSD 4.2 – circa 1983).

  • An early form of Middleware (limited to same host systems).

  • Windows variant (WinSock) based on BSD Sockets.

  • Sockets are treated similar to files in BSD/POSIX and maintained in the File Descriptor table.

  • Supported protocols include TCP/IP (IPv4, IPv6) and UDP.

Socket API

  • socket(): creates a descriptor for use in network communications.

  • connect(): connect to a remote peer (client).

  • write(): send outgoing data across a connection.

  • read(): acquire incoming data from a connection.

  • close(): terminate communication and deallocate a descriptor.

  • bind(): bind a local IP address and protocol port to a socket.

  • listen(): set the socket listening on the given address and port for connections from the client and set the number of incoming connections from a client (backlog) that will be allowed in the listen queue at any one time.

  • accept(): accept the next incoming connection (server).

  • recv(): receive the next incoming datagram.

  • recvmsg(): receive the next incoming datagram (variation of recv).

  • recvfrom(): receive the next incoming datagram and record its source endpoint address.

  • send(): send an outgoing datagram.

  • sendmsg(): send an outgoing datagram (variation of send).

  • sendto(): send an outgoing datagram, usually to a prerecorded endpoint address.

  • shutdown(): terminate a TCP connection in one or both directions.

  • getpeername(): after a connection arrives, obtain the remote machine’s endpoint address from a socket.

  • getsockopt(): obtain the current options for a socket.

  • setsockopt(): change the options for a socket.

Socket Life Cycle

  • TCP Client:

    • socket()

    • connect()

    • TCP connection establishment

    • write(): data (request)

    • read(): data (reply)

    • close(): EOF notification

  • TCP Server:

    • socket()

    • bind()

    • listen()

    • accept(): blocks until connection from client

    • read(): do something

    • write()

    • read()

    • close()

Remote Procedure Call (RPC)

  • Masks remote function calls as being local.

  • Uses a Client/server model.

  • Implemented with message passing in RPC service.

  • Marshalling of function parameters and return value is performed.

Properties of RPC

  • Language-level pattern of function call: easy to understand for programmer

  • Synchronous request/reply interaction:

    • Natural from a programming language point-of-view

    • Matches replies to requests

    • Built in synchronisation of requests and replies

  • Distribution transparency (in the no-failure case):

    • Hides the complexity of a distributed system

  • Various reliability guarantees:

    • Deals with some distributed systems aspects of failure

Failure Modes of RPC

  • Invocation semantics supported by RPC in the light of: network and/or server congestion, client, network and/or server failure

  • Note DS independent failure modes

  • RPC systems differ, many examples, local was Mayflower May beoratmostonce( RPCsystemtries once) Error return – programmer may retry Exactly once (RPC system retries a few times)

  • Hard error return – some failure most likelynote that “exactly once” cannot be guaranteed

Disadvantages of RPC

  • Synchronous request/reply interaction

    • Tight coupling between client and server

    • Client may block for a long time if server loaded leads to multi-threaded programming at client

    • Slow/failed clients may delay servers when replying multi-threading essential at servers

  • Distribution Transparency

    • Not possible to mask all problems

  • RPC paradigm is not object-oriented

    • invoke functions on servers as opposed to methods on objects fork(...) remote call join(...)

Object-Oriented Middleware (OOM)

  • Objects can be local or remote.

  • Object references can be local or remote.

  • Remote objects have visible remote interfaces.

  • Masks remote objects as being local using proxy objects.

  • Remote method invocation.

Properties of OOM

  • Support for object-oriented programming model

    • Objects, methods, interfaces, encapsulation…

    • Exceptions (were also in some RPC systems e.g. Mayflower)

  • Synchronous request/reply interaction –same as RPC

  • Location Transparency

    • System (ORB) maps object references to locations

  • Services comprising multiple servers are easier to build with OOM

    • RPC programming is in terms of server-interface (operation)

    • RPC system looks up server address in a location service

Java Remote Method Invocation (RMI)

  • Distributed objects in Java

  • RMI compiler creates proxies and skeletons

  • RMI registry used for interface lookup

  • Entire system written in Java (single-language system)

Example: ```java
public interface PrintService extends Remote {
int print(Vector printJob) throws RemoteException;
}

# CORBA

*   Common Object Request Broker Architecture
*   Open standard by the OMG (Version 3.0)
*   Language- and platform independent Object Request Broker (ORB)
*   General Inter-ORB Protocol (GIOP) for communication
*   Interoperable Object References (IOR) contain object location
*   CORBA Interface Definition Language (IDL)
*   Stubs (proxies) and skeletons created by IDL compiler
*   Dynamic remote method invocation
*   Interface Repository: Querying existing remote interfaces
*   Implementation Repository: Activating remote objects on demand

# CORBA IDL

*   Definition of language-independent remote interfaces
    *   Language mappings to C++, Java, Smalltalk, …
    *   Translation by IDL compiler
*   Type system
    *   basic types: long (32 bit), long long (64 bit), short, float, char, boolean, octet, any, …
    *   constructed types: struct, union, sequence, array, enum
    *   objects (common super type Object)
*   Parameter passing
    *   in, out, inout
    *   basic & constructed types passed by value
    *   objects passed by reference

*Example:* ```IDL
typedef sequence<string> Files;
interface PrintService : Server {
 void print(in Files printJob);
};

CORBA Services

  • Naming Service: Names remote object references.

  • Trading Service: Attributes (properties) remote object references.

  • Persistent Object Service: Implementation of persistent CORBA objects.

  • Transaction Service: Making object invocation part of transactions.

  • Event Service and Notification Service

    • In response to applications‘ need for asynchronous communication

    • built above synchronous communication with push or pull options. not an integrated programming model with general IDL messages

Disadvantages of OOM

  • Synchronous request/reply interaction only

    • So CORBA oneway semantics added and -

    • Asynchronous Method Invocation (AMI)

    • But implementations may not be loosely coupled

  • Distributed garbage collection

    • Releasing memory for unused remote objects

  • OOM rather static and heavy-weight

    • Bad for ubiquitous systems and embedded devices

Message-Oriented Middleware (MOM)

  • Communication using messages

  • Messages stored in message queues

  • Message servers decouple client and server

  • Various assumptions about message content

Properties of MOM

  • Asynchronous interaction

    • Client and server are only loosely coupled

    • Messages are queued

    • Good for application integration

  • Support for reliable delivery service

    • Keep queues in persistent storage

  • Processing of messages by intermediate message server(s)

    • May do filtering, transforming, logging, …

    • Networks of message servers

  • Natural for database integration

IBM MQSeries

  • One-to-one reliable message passing using queues

    • Persistent and non-persistent messages

    • Message priorities, message notification

  • Queue Managers

    • Responsible for queues

    • Transfer messages from input to output queues

    • Keep routing tables

  • Message Channels

    • Reliable connections between queue managers

  • Messaging API:

    • MQopen: Open a queue

    • MQclose: Close a queue

    • MQput: Put message into opened queue

    • MQget: Get message from local queue

Java Message Service (JMS)

  • API specification to access MOM implementations

  • Two modes of operation specified:

    • Point-to-point

      • one-to-one communication using queues

    • Publish/Subscribe

      • cf. Event-Based Middleware

  • JMS Server implements JMS API

  • JMS Clients connect to JMS servers

  • Java objects can be serialised to JMS messages

  • A JMS interface has been provided for MQ

Disadvantages of MOM

  • Poor programming abstraction (but has evolved)

    • Rather low-level (cf. Packets)

    • Request/reply more difficult to achieve, but can be done

  • Message formats originally unknown to middleware

    • No type checking (JMS addresses this – implementation?)

  • Queue abstraction only gives one-to-one communication

    • Limits scalability (JMS pub/sub – implementation?)

Web Services

  • Use well-known web standards for distributed computing

  • Communication

    • Message content expressed in XML

    • Simple Object Access Protocol (SOAP)

      • Lightweight protocol for sync/async communication

  • Service Description

    • Web Services Description Language (WSDL)

      • Interface description for web services

  • Service Discovery

    • Universal Description Discovery and Integration (UDDI)

      • Directory with web service description in WSDL

Properties of Web Services

  • Language-independent and open standard

  • SOAP offers OOM and MOM-style communication:

    • Synchronous request/reply like OOM

    • Asynchronous messaging like MOM

    • Supports internet transports (http, smtp, …)

    • Uses XML Schema for marshalling types to/from programming language types

  • WSDL says how to use a web service http://api.google.com/Google Search.wsdl

  • UDDI helps to find the right web service

    • Exports SOAP API for access

Disadvantages of Web Services

  • Low-level abstraction

    • Leaves a lot to be implemented

  • Interaction patterns have to be built

    • one-to-one and request-reply provided

    • one-to-many?

    • still synchronous service invocation, rather than notification

    • No nested/grouped invocations, transactions, …

  • No location transparency

What We Lack

  • General interaction patterns

    • we have one-to-one and request-reply

    • one-to-many? many to many?

    • notification?

    • dynamic joining and leaving?

  • Location transparency

    • anonymity of communicating entities

  • Support for pervasive computing

    • data values from sensors

    • lightweight software

Event-Based Middleware

  • a.k.a. Publish/Subscribe

  • Publishers (advertise and) publish events (messages)

  • Subscribers express interest in events with subscriptions

  • Event Service notifies interested subscribers of published events

  • Events can have arbitrary content (typed) or name/value pairs

Topic-Based

  • Event Service matches events against subscriptions

    • What do subscriptions look like?

  • Topic-Based Publish/Subscribe

    • Publishers publish events belonging to a topic or subject

    • Subscribers subscribe to a topic subscribe(PrintJobFinishedTopic, …)

  • (Topic and) Content-Based Publish/Subscribe

    • Publishers publish events belonging to topics and

    • Subscribers provide a filter based on content of events subscribe(type=printjobfinished, printer=‘aspen’, …)

Properties of Publish/Subscribe

  • Asynchronous communication

    • Publishers and subscribers are loosely coupled

  • Many-to-many interaction between pubs. and subs.

    • Scalable scheme for large-scale systems

    • Publishers do not need to know subscribers, and vice-versa

    • Dynamic join and leave of pubs, subs

  • (Topic and) Content-based pub/sub very expressive

    • Filtered information delivered only to interested parties

    • Efficient content-based routing through a broker network

Summary

  • Middleware is an important abstraction for building distributed systems

  • Synchronous vs. asynchronous communication

  • Scalability, many-to-many communication

  • Language integration

  • Ubiquitous systems, mobile systems

    1. Remote Procedure Call

    2. Object-Oriented Middleware

    3. Message-Oriented Middleware

    4. Event-Based Middleware

CORBA

  • Common Object Request Broker Architecture

  • Open standard by the OMG (Version 3.0)

  • Language- and platform independent Object Request Broker (ORB)

  • General Inter-ORB Protocol (GIOP) for communication

  • Interoperable Object References (IOR) contain object location

  • CORBA Interface Definition Language (IDL)

  • Stubs (proxies) and skeletons created by IDL compiler

  • Dynamic remote method invocation

  • Interface Repository – Querying existing remote interfaces

  • Implementation Repository – Activating remote objects on demand

CORBA IDL

  • Definition of language-independent remote interfaces

    • Language mappings to C++, Java, Smalltalk, …

    • Translation by IDL compiler

  • Type system

    • basic types: long (32 bit), long long (64 bit), short, float, char, boolean, octet, any, …

    • constructed types: struct, union, sequence, array, enum

    • objects (common super type Object)

  • Parameter passing

    • in, out, inout

    • basic & constructed types passed by value

    • objects passed by reference
      *Example:

typedef sequence<string> Files;
interface PrintService : Server {
 void print(in Files printJob);
};

CORBA Services

  • Naming Service Names -> remote object references

  • Trading Service Attributes (properties) -> remote object references

  • Persistent Object Service Implementation of persistent CORBA objects

  • Transaction Service Making object invocation part of transactions

  • Event Service and Notification Service

    • In response to applications‘ need for asynchronous communication

    • built above synchronous communication with push or pull options

    • not an integrated programming model with general IDL messages

CORBA

  • The Common Object Request Broker Architecture (CORBA) is a standard architecture for a distributed objects system.

  • CORBA is designed to allow distributed objects to interoperate in a heterogeneous environment, where objects can be implemented in different programming language and/or deployed on different platforms

CORBA vs. Java RMI

  • CORBA differs from the architecture of Java RMI in one significant aspect:

    • RMI is a proprietary facility developed by Sun MicroSystems, Inc., and supports objects written in the Java programming language only.

    • CORBA is an architecture that was developed by the Object Management Group (OMG), an industrial consortium.

CORBA

  • CORBA is not inself a distributed objects facility; instead, it is a set of protocols.

  • A distributed object facility which adhere to these protocols is said to be CORBA-compliant, and the distributed objects that the facility support can interoperate with objects supported by other CORBA-compliant facilities.

  • CORBA is a very rich set of protocols. We will instead focus on the key concepts of CORBA related to the distributed objects paradigm. We will also study a facility based on CORBA: the Java IDL.

Architecture

  • The diagram shows the architecture of CORBA, including the client, object reference, IDL compiler, implementation repository, object (servant), IDL skeleton, DSI, DII, IDL stubs, ORB interface, object adapter, GIOP/IIOP, standard interface, ORB-specific interface, ORB core, standard language mapping, and standard protocol.

Basic Workflow

  • The diagram illustrates the basic workflow of CORBA, including naming lookup, naming service, object client, stub, object implementation, skeleton, ORB, network, operating system, logical data flow, physical data flow.

CORBA vs RMI

  • The diagram compares RMI and CORBA, highlighting the differences in client code, stub generation, network communication, skeleton implementation, and server implementation. Specifically, it notes RMI's use of rmic to generate stubs and skeletons (pre JDK 1.2) and CORBA's use of an IDL compiler.

CORBA vs RMI (Object Lookup)

  • RMI - Client code looks up server instance by name using RMI registry, the server implementation starts instance and bind to a name.

  • CORBA - Client code looks up server instance via naming context, COS Naming Service is used, the server implementation starts instance and bind to a naming context.

CORBA Object Interface

  • A distributed object is defined using a software file similar to the remote interface file in Java RMI.

  • Since CORBA is language independent, the interface is defined using a universal language with a distinct syntax, known as the CORBA Interface Definition Language (IDL).

  • The syntax of CORBA IDL is similar to Java and C++. However, object defined in a CORBA IDL file can be implemented in a large number of diverse programming languages, including C, C++, Java, COBOL, Smalltalk, Ada, Lisp, Python, and IDLScript.

  • For each of these languages, OMG has a standardized mapping from CORBA IDL to the programming language, so that a compiler can be used to process a CORBA interface to generate the proxy files needed to interface with an object implementation or an object client written in any of the CORBA- compatible languages.

Cross-language CORBA application

  • This diagram illustrates a cross-language CORBA application where an object client written in Java communicates with an object implementation written in C++ using respective ORBs and stubs/skeletons generated from the CORBA object interface.

Inter-ORB Protocols

  • To allow ORBs to be interoperable, the OMG specified a protocol known as the General Inter-ORB Protocol (GIOP), a specification which “provides a general framework for protocols to be built on top of specific transport layers.”

  • A special case of the protocol is the Inter-ORB Protocol (IIOP), which is the GIOP applied to the TCP/IP transport layer.

Inter-ORB Protocols

The IIOP specification includes the following elements:

  1. Transport management requirements: specifies the connection and disconnection requirements, and the roles for the object client and object server in making and unmaking connections.

  2. Definition of common data representation: a coding scheme for marshalling and unmarshalling data of each IDL data type.

  3. Message formats: different types of message format are defined. The messages allow clients to send requests to object servers and receive replies. A client uses a Request message to invoke a method declared in a CORBA interface for an object and receives a reply message from the server.

Object Bus

  • An ORB which adheres to the specifications of the IIOP may interoperate with any other IIOP-compliant ORBs over the Internet.

  • This gives rise to the term “object bus”, where the Internet is seen as a bus that interconnects CORBA objects

ORB products

  • There are a large number of proprietary as well as experimental ORBs available:

    • (See CORBA Product Profiles, http://www.puder.org/corba/matrix/)

    • Orbix IONA

    • Borland Visibroker

    • PrismTech’s OpenFusion

    • Web Logic Enterprise from BEA

    • Ada Broker from ENST

    • Free ORBs

Object Servers and Object Clients

  • As in Java RMI, a CORBA distributed object is exported by an object server, similar to the object server in RMI.

  • An object client retrieves a reference to a distributed object from a naming or directory service, to be described, and invokes the methods of the distributed object.

CORBA Object References

  • As in Java RMI, a CORBA distributed object is located using an object reference. Since CORBA is language- independent, a CORBA object reference is an abstract entity mapped to a language-specific object reference by an ORB, in a representation chosen by the developer of the ORB.

  • For interoperability, OMG specifies a protocol for the abstract CORBA object reference object, known as the Interoperable Object Reference (IOR) protocol.

Interoperable Object Reference (IOR)

  • For interoperability, OMG specifies a protocol for the abstract CORBA object reference object, known as the Interoperable Object Reference (IOR) protocol.

  • An ORB compatible with the IOR protocol will allow an object reference to be registered with and retrieved from any IOR-compliant directory service. CORBA object references represented in this protocol are called Interoperable Object References (IORs).

Interoperable Object Reference (IOR)

  • An IOR is a string that contains encoding for the following information:

    • The type of the object.

    • The host where the object can be found.

    • The port number of the server for that object.

    • An object key, a string of bytes identifying the object. The object key is used by an object server to locate the object.

Interoperable Object Reference (IOR)

  • The following is an example of the string representation of an IOR [5]:

IOR:000000000000000d49444c3a677269643a312e3000000
00000000001000000000000004c0001000000000015756c4
72612e6475626c696e2e696f6e612e6965000009630000002
83a5c756c7472612e6475626c696e2e696f6e612e69653a67
7269643a303a3a49523a67726964003a

  • The representation consists of the character prefix “IOR:” followed by a series of hexadecimal numeric characters, each character representing 4 bits of binary data in the IOR.

CORBA Naming Service

  • CORBA specifies a generic directory service. The Naming Service serves as a directory for CORBA objects, and, as such, is platform independent and programming language independent.

  • The Naming Service permits ORB-based clients to obtain references to objects they wish to use. It allows names to be associated with object references.

  • Clients may query a naming service using a predetermined name to obtain the associated object reference.

CORBA Naming Service

  • To export a distributed object, a CORBA object server contacts a Naming Service to bind a symbolic name to the object The Naming Service maintains a database of names and the objects associated with them.

  • To obtain a reference to the object, an object client requests the Naming Service to look up the object associated with the name (This is known as resolving the object name.)

  • The API for the Naming Service is specified in interfaces defined in IDL, and includes methods that allow servers to bind names to objects and clients to resolve those names.

CORBA Naming Service

  • To be as general as possible, the CORBA object naming scheme is necessary complex. Since the name space is universal, a standard naming hierarchy is defined in a manner similar to the naming hierarchy in a file directory

A Naming Context

  • A naming context corresponds to a folder or directory in a file hierarchy, while object names corresponds to a file.

  • The full name of an object, including all the associated naming contexts, is known as a compound name. The first component of a compound name gives the name of a naming context, in which the second component is accessed. This process continues until the last component of the compound name has been reached.

  • Naming contexts and name bindings are created using methods provided in the Naming Service interface.

A CORBA object name

  • The syntax for an object name is as follows:

<naming context > …<naming context><object name>

  • Where the sequence of naming contexts leads to the object name.

Example of a naming hierarchy

  • As shown, an object representing the men’s clothing department is named store.clothing.men, where store and clothing are naming contexts, and men is an object name.

Interoperable Naming Service

  • The Interoperable Naming Service (INS) is a URL-based naming system based on the CORBA Naming Service, it allows applications to share a common initial naming context and provide a URL to access a CORBA object.

CORBA Object Services

CORBA specify services commonly needed in distributed applications, some of which are:

  • Naming Service

  • Concurrency Service

  • Event Service: for event synchronization

  • Logging Service: for event logging

  • Scheduling Service: for event scheduling

  • Security Service: for security management

  • Trading Service: for locating a service by the type (instead of by name)

  • Time Service: a service for time-related events

  • Notification Service: for events notification

  • Object Transaction Service: for transactional processing

  • Each service is defined in a standard IDL that can be implemented by a developer of the service object, and whose methods can be invoked by a CORBA client.

Object Adapters

In the basic architecture of CORBA, the implementation of a distributed object interfaces with the skeleton to interact with the stub on the object client side. As the architecture evolved, a software component in addition to the skeleton was needed on the server side: an object adapter.

Object Adapter

  • An object adapter simplifies the responsibilities of an ORB by assisting an ORB in delivering a client request to an object implementation.

  • When an ORB receives a client’s request, it locates the object adapter associated with the object and forwards the request to the adapter.

  • The adapter interacts with the object implementation’s skeleton, which performs data marshalling and invoke the appropriate method in the object.

The Portable Object Adapter

  • There are different types of CORBA object adapters.

  • The Portable Object Adapter, or POA, is a particular type of object adapter that is defined by the CORBA specification. An object adapter that is a POA allows an object implementation to function with different ORBs, hence the word portable.

Java IDL – Java’s CORBA Facility

  • IDL is part of the Java 2 Platform, Standard Edition (J2SE).

  • The Java IDL facility includes a CORBA Object Request Broker (ORB), an IDL-to-Java compiler, and a subset of CORBA standard services.

  • In addition to the Java IDL, Java provides a number of CORBA-compliant facilities, including RMI over IIOP, which allows a CORBA application to be written using the RMI syntax and semantics.

Key Java IDL Packages

  • org.omg.CORBA - contains interfaces and classes which provides the mapping of the OMG CORBA APIs to the Java programming language

  • org.omg.CosNaming - contains interfaces and classes which provides the naming service for Java IDL

  • org.omg.CORBA.ORB - contains interfaces and classes which provides APIs for the Object Request Broker

Java IDL Tools

  • Java IDL provides a set of tools needed for developing a CORBA application:

    • idlj - the IDL-to-Java compiler (called idl2java in Java 1.2 and before)

    • orbd - a server process which provides Naming Service and other services

    • servertool – provides a command-line interface for application programmers to register/unregister an object, and startup/shutdown a server.

    • tnameserv – an olderTransient Java IDL Naming Service whose use is now discouraged.

A Java IDL application example

Example: Shows the steps required to build and execute a distributed application using Java IDL

The CORBA Interface file

module HelloApp
{
 interface Hello
 {
  string sayHello();
  oneway void shutdown();
 };
};

Compiling the IDL file (using Java)

  • The IDL file should be placed in a directory dedicated to the application. The file is compiled using the compiler idlj using a command as follows:

idlj -fall Hello.idl

  • The -fall command option is necessary for the compiler to generate all the files needed. In general, the files can be found in a subdirectory named <some name>App when an interface file named <some name>.idl is compiled. If the compilation is successful, the following files can be found in a HelloApp subdirectory:

  • HelloOperations.java

  • Hello.java

  • HelloHelper.java

  • HelloHolder.java

  • _HelloStub.java

  • HelloPOA.java

  • These files require no modifications.

The *Operations.java file

  • There is a file HelloOperations.java found in HelloApp/ after you compiled using idlj

  • It is known as a Java operations interface in general

  • It is a Java interface file that is equivalent to the CORBA IDL interface file (Hello.idl)

  • You should look at this file to make sure that the method signatures correspond to what you expect.

HelloApp/HelloOperations.java

  • The file contains the methods specified in the original IDL file: in this case the methods sayHello() and shutdown().

package HelloApp;

/**
 * HelloApp/HelloOperations.java
 * Generated by the IDL-to-Java compiler (portable),
 * version "3.1" from Hello.idl
 */

public interface HelloOperations
{
 String sayHello ();
 void shutdown ();
} // interface HelloOperations

HelloApp/Hello.java

  • The signature interface file combines the characteristics of the Java operations interface (HelloOperations.java) with the characteristics of the CORBA classes that it extends.

```java
package HelloApp;

/**

  • HelloApp/Hello.java

  • Generated by the IDL-to-Java compiler (portable),

  • version "3.1" from Hello.idl
    */

public interface Hello extends HelloOperations,
org.omg.CORBA.Object,
org.omg.CORBA.portable.