Introduction to Distributed Systems and CORBA Slides for CSCI 3171 Lectures E. W. Grundke

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Introduction to Distributed Systemsand CORBA
Slides for CSCI 3171 Lectures E. W. Grundke

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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/aut
  hor_tanenbaum/custom/dist_sys_1e/
• CDK http//www.cdk3.net/ig/beida/index.html

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Distributed Systems
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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
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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
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Transparency in a Distributed System
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.

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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)
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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 Keaheys Tutorial on CORBA
• http//www.cs.indiana.edu/hyplan/kksiazek/tuto.htm
  l
• 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 objects
  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 Javas 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/ORBd
  iagram.gif