Distributed Object-Oriented Architectures

Diplomarbeit von Josef Stepisnik

Abstract:

Distributed computing is playing an increasingly important role in many areas of industry, the sciences, in business processes and in the development of new and emerging technologies. It facilitates inter-process communication across heterogeneous networks, hardware platforms and operating systems.

We compare four distributed and object-oriented architectures: Sockets in Java 2, Sockets in Berkeley Unix, Remote Method Invocation in Java - RMI - and the Common Object Request Broker Architecture - CORBA - of the Object Management Group consortium. We provide a survey of each of the distributed architectures including its constituting components. To present the architectures in a practical context, we amend each survey with a corresponding application framework.

We conclude with a comparative study of the Socket APIs in Java 2 and in Berkeley UNIX and the distributed object models of Java RMI and CORBA. Although the distributed object model as defined by CORBA represents an adopted industry standard, Java RMI has features unattainable by CORBA.

The first part of the discussion offers a comprehensive overview of the Socket architecture in Java 2 and Berkeley UNIX and the distributed object model of Java Remote Method Invocation and the Common Object Request Broker Architecture.

The second part concludes the discussion with a comparative study of selected features with emphasis on the Common Object Request Broker Architecture and Java Remote Method Invocation.

Chapter 1 - The TCP/IP Protocol Suite: We provide an introductory overview of the TCP/IP protocol suite and its architecture including layers and protocols. The TCP/IP architecture is based on three concepts: processes, layers and protocols. There is no official protocol model as compared to the OSI proposal. We can however devise a logical structure of the TCP/IP protocol suit based on the associated protocols and their relationships. The chapter concludes with a brief discussion of Internet-related organizations and standards.

Chapter 2 - Sockets in Berkeley Unix: We present the Berkeley UNIX socket architecture in relation to the Internet communication domain and illustrate connection-oriented and connectionless models of communication. The socket architecture forms the basis for the development of distributed applications. A socket represents an endpoint of communication for connectionless or connection-oriented protocols. A socket address data structure constitutes a platform for the communication of host address information between the operating system and the network application. A connection-oriented model of communication uses the Transmission Control Protocol. Transmission of data is in the form of segments. A connectionless model of communication is based on the User Datagram Protocol and the transmission of data is in the form of datagrams.

Chapter 3 - Sockets in Java 2: We describe the Java 2 socket architecture, outline selected socket operations, introduce related packages and classes and conclude with a framework for a connection-oriented and connectionless model of communication. The Java 2 socket architecture was developed by Sun Microsystems, Inc. and is proprietary and language constrained. It is made available in the form of a class library referred to as package java.net and is functionally similar to the sockets API introduced by Berkeley UNIX. The class library provides for hostname resolution and reverse host name resolution. In addition, classes are available for the transmission of data across connection-oriented or connection-less channels of communication.

Chapter 4 - Remote Method Invocation in Java: We present a distributed object model in Java RMI, provide an overview of related interfaces, classes and packages and discuss security related issues. We conclude with the development of a framework for a distributed object application. Remote Method Invocation was developed by Sun Microsystems, Inc. It is a proprietary and language-constrained technology. Invocations of remote and local methods are syntactically identical. They differ only in the types of parameters and return values. Remote objects can be uniquely identified within a Java distributed object model based on their remote object references. Java RMI uses an enhanced model of security in its recent releases. Every RMI client application and object server requires the presents of an RMI security manager and the specification of a security policy file.

Chapter 5 - Common Object Request Broker Architecture: We introduce a distributed object model for the Common Object Request Broker Architecture and outline design concepts including the Interface Definition Language and the Interoperable Naming Service. We conclude with the development of a framework for a distributed object application. CORBA was developed in the form of a set of specifications by members of the Object Management Group consortium and is vendor-independent. It has developed into an industry standard and provides transparency and interoperability across different hardware platforms and operating systems.

Chapter 6 - Comparative Study of Distributed Architectures: We present a comparative study of socket architectures and distributed object models introduced in part one of this document. It provides an overview of the technical capabilities, similarities and areas of application in industry of each of these architectures.

Chapter 7 – Summary: We conclude the document with a brief summary and detailed annotated bibliographic references of papers, technical articles, standards, and specifications.

This document is based on specifications, guidelines, Internet drafts and Internet standards issued by the Object Management Group consortium, Sun Microsystems, Inc. and the Internet Engineering Task Force – IETF.

In particular, the material for the TCP/IP protocol suit was obtained from IETF. Its responsibility includes the dissemination of Internet Drafts, referred to as Ready for Comments – RFCs. The chapters on the Java 2 Socket API and the Java Remote Method Invocation architecture are based on specifications provided by Sun Microsystems, Inc. The Sockets API in Berkeley UNIX is outlined in the corresponding documentation of Release 4.2. The Common Object Request Broker Architecture is documented in great detail in the form of specifications and introductory material issued by the Object Management Group consortium.

Zusammenfassung:

Die Anwendung verteilter Systeme erhält zunehmende Bedeutung in vielen industriellen Bereichen, in den Wissenschaften, in Geschäftsprozessen und in der Entwicklung neuer und entstehender Technologien. Sie ermöglichen eine Interprozesskommunikation über heterogene Netzwerke, Hardware-Plattformen und Betriebssysteme.

Wir vergleichen vier verteilte und objektorientierte Architekturen: Sockets in Java 2 und in Berkeley UNIX, Remote Method Invokation in Java – RMI – und die Common Object Request Broker Architecture – CORBA – des Object Management Group Konsortiums.

Wir erstellen einen detaillierten Überblick zu den einzelnen verteilten Architekturen einschließlich ihrer konstituierenden Komponenten. Um die Architekturen in einem praktischen Kontext darzustellen, erweitern wir jede unserer Betrachtungen um eine dazugehörige illustrierende Applikation. Wir beenden unsere Betrachtungen mit einer vergleichenden Studie über die Socket APIs in Java 2 und Berkeley UNIX und die verteilten Objektmodelle von Java RMI und CORBA.

Obwohl das verteilte Objektmodell von CORBA einen anerkannten Industriestandard repräsentiert, bietet Java RMI dennoch Eigenschaften, welche für CORBA unerreichbar sind.

Table of Contents:

	Abstract	iv
	Kurzfassung	v
	Introduction	vi
1.	The TCP/IP Protocol Suite	1
	Overview	1
1.1	Architecture	1
	Constituent Components	2
	Protocol Data Unit	4
1.2	Layers and Protocols	5
	Data Link Layer	5
	Network Layer	5
	Transport Layer	8
1.3	Organizations and Standards	8
	Internet-Related Organizations	8
	Internet Standards	9
2.	Sockets in Berkeley Unix	11
	Overview	11
2.1	Architecture	11
	Network Address Data Structure	11
	Network Address Byte Order and Conversion	12
	Host Information	14
	Socket Operations	14
	Additional Socket Information	18
2.2	Connection-Orient Model of Communication	18
	Network Logic	10
	Concurrent Application Framework	20
	Distribution of Incoming TCP Segments in Concurrent Applications	21
2.3	Connectionless Model of Communication	21
	Network Logic	22
	Concurrent Application Framework	23
	Distribution of Incoming UDP Datagrams in Concurrent Applications	24
3.	Sockets in Java 2	25
	Overview	25
3.1	Architecture	25
	Host Information	26
	Identifying and Accessing Resources on the Internet	27
	Connection-Oriented Socket Operations	29
	Connectionless Socket Operations	31
3.2	Connection-Orient Model of Communication	33
	Network Logic	33
	Concurrent Application Framework	33
	Distribution of Incoming TCP Segments in Concurrent Applications	35
3.3	Connectionless Model of Communication	35
	Network Logic	36
	Concurrent Application Framework	36
	Distribution of Incoming UDP Datagrams in Concurrent Applications	38
4.	Java Remote Method Invocation - RMI	39
	Overview	39
4.1	Distributed Object Model in Java RMI	39
	Design Concepts: Remote Interface, Remote Object and Remote Method Invocation	40
	Remote Objects and Non-Remote Objects	40
4.2	Architecture	41
	Overview of Interfaces, Classes and Packages	41
	Communication with Remote Objects	43
	Dynamic Class Downloading in Java RMI	45
4.3	Security Considerations in Java RMI	45
	Security Manager	45
	Security Policy File	46
4.4	Application Development Process in Java RMI	47
	Object Server and Client Application	47
	Security Policy Files and Runtime Options	49
5.	Common Object Request Broker Architecture	50
	Overview	50
5.1	Distributed Object Model in CORBA	50
	Design Concepts	51
	Object Management Group	52
5.2	Interface Definition Language	52
	OMG Interface Definition Language	52
	IDL to Java Language Mapping	52
5.3	Interoperable Naming Service	53
	Overview	53
	Design Objectives	54
	Common Object Services - Naming Service	54
5.4	Portable Object Adapter	55
	Architecture	55
	Overview and Design Objectives	55
5.5	Application Development Process in CORBA	56
	Server Model and Client Model	56
	Naming Service	57
	Interface Definition File	57
	Object Server and Client Application	57
6.	Comparative Study of Distributed Architectures	61
	Overview	61
6.1	Sockets in Java 2 and Berkeley UNIX	61
	Two-Tier Model of Communication	61
	Socket Architectures	63
	Available Communication Domains	64
	Transport Layer Protocols	65
	Scalability of Application Development	66
	Portability and Interoperability	68
	Areas of Application	69
	Application Development Tools and Libraries	70
	Educational Effort	70
6.2	Java RMI and CORBA	70
	Distributed Object Model in Java RMI	70
	Distributed Object Model in CORBA	72
	Expressiveness of OMG IDL in Relation to Java 2	73
	Scalability of Application Development	79
	Portability and Interoperability	79
	Areas of Application	80
	Application Development Tools, Libraries and Policy Files	81
	Educational Effort	81
6.3	Conclusions and Comparative Tables	82
	Sockets in Java 2 and Berkeley UNIX	82
	Java RMI and CORBA	83
	Expressiveness of OMG IDL in Relation to Java 2	84
7.	Summary	85
	Annotated Bibliography	86
	Appendix (Poster)	91