Introduction to
Distributed Systems
and CORBA
Slides for CSCI 3171 Lectures
E. W. Grundke
References
A. Tanenbaum and M. van Steen (TvS)
Distributed Systems: Principles and Paradigms
Prentice-Hall 2002
G. Coulouris, J. Dollimore and T. Kindberg (CDK)
Distributed System: Concepts and Design
Addison-Wesley 2001
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Acknowledgement
Some slides from:
TvS: http://www.prenhall.com/divisions/esm/app/author_tanenbaum/custom/dist_sys_1e/
CDK: http://www.cdk3.net/ig/beida/index.html
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Distributed Systems
What is a Distributed System?
A collection of independent computers that appears to
its users as a single coherent system.
Examples:
Distributed object-based systems (CORBA, DCOM)
Distributed file systems (NFS)
etc.
TvS 1.2
5
Heterogeneity
Applies to all of the following:
– networks
• Internet protocols mask the differences between networks
– computer hardware
• e.g. data types such as integers can be represented differently
– operating systems
• e.g. the API to IP differs from one OS to another
– programming languages
• data structures (arrays, records) can be represented differently
– implementations by different developers
• they need agreed standards so as to be able to interwork
CDK Ch. 1.4
6
Transparency in a Distributed System
Transparency
Description
Access
Hide differences in data representation and how a
resource is accessed
Location
Hide where a resource is located
Migration
Hide that a resource may move to another location
Relocation
Hide that a resource may be moved to another
location while in use
Replication
Hide that a resource is replicated
Concurrency
Hide that a resource may be shared by several
competitive users
Failure
Hide the failure and recovery of a resource
Persistence
Hide whether a (software) resource is in memory or
on disk
Different forms of transparency in a distributed system.
TvS 1.4
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Layered Protocols: IP
Layers, interfaces, and protocols in the Internet model.
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Layered Protocols: OSI
Layers, interfaces, and protocols in the OSI model.
2-1
TvS 2.2
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Middleware Protocols
2-5
An adapted reference model for networked communication.
TvS 2.6
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Middleware
A software layer that
– masks the heterogeneity of systems
– provides a convenient programming abstraction
– provides protocols for providing general-purpose services
to more specific applications, eg.
• high-level communication protocols
– remote procedure calls (RPC)
– remote method invocation (RMI)
• authentication protocols
• authorization protocols
• distributed commit protocols
• distributed locking protocols
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Middleware
General structure of a distributed system as middleware.
1-22
TvS 1.24
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Middleware and Openness
1.23
In an open middleware-based distributed system, the protocols used by
each middleware layer should be the same, as well as the interfaces
they offer to applications.
TvS 1.25
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Middleware Programming Models
Remote Calls
– remote Procedure Calls (RPC)
– distributed objects and Remote Method Invocation (RMI)
• eg. Java RMI
Common Object Request Broker Architecture (CORBA)
– cross-language RMI
Other programming models
– remote event notification
– remote SQL access
– distributed transaction processing
CDK Ch 1
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CORBA
(Common Object Request Broker
Architecture)
References
A. Tanenbaum and M. van Steen (TvS)
Distributed Systems: Principles and Paradigms
Prentice-Hall 2002
G. Coulouris, J. Dollimore and T. Kindberg (CDK)
Distributed System: Concepts and Design
Addison-Wesley 2001
Kate Keahey’s Tutorial on CORBA
http://www.cs.indiana.edu/hyplan/kksiazek/tuto.html
OMG (Object Management Group) Documentation
http://www.omg.org/
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CORBA
• Common Object Request Broker Architecture
• Language/platform-independent RMI and more
• Specification of the OMG (Object Management Group)
– non-profit consortium
– formed in 1989
– now ~800 members (but not Microsoft)
– CORBA 1: 1990
– CORBA 2: 1996
• (These slides deal with a small subset only.)
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CORBA RMI
• ORB (Object Request Broker)
– communication infrastructure
• IDL (Interface Definition Language):
– language for describing a remote object’s properties
– platform-independent, language-independent
– looks like C
– declarative only; no implementation
(no executable code)
...
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CORBA RMI (cont.)
• General Inter-ORB Protocol (GIOP)
– request/reply protocol
– incl. an xdr named CDR
– remote object references
– TCP/IP version is Internet Inter-ORB Protocol
(IIOP) (port 900)
• CORBA services
– Naming Service (like Java’s RMI Registry)
– Event Service
– etc.
...
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CORBA RMI (cont.)
• CORBA Objects
– exist in the server
– accessible by remote object references
– implement IDL interfaces
– have methods that are callable remotely
– can be implemented in non-OO languages
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CORBA IDL
• Entities
– modules, interfaces, types, attributes, method signatures
• Resembles C/C++
– but new keywords:
interface, in, out, inout, raises etc.
• Supports C++ preprocessing
• Supports inheritance
– interfaces can extend other interfaces
• Compiles to stubs/skeletons using language-specific tools
(for java: idlj)
– mappings into various languages
(not just OO-languages!)
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CORBA Block Diagram
From http://www.omg.org/gettingstarted/Images/ORBdiagram.gif:
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