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3 Acknowledgement Some slides from : TvS: CDK:
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Introduction to Distributed Systems
and CORBA Slides for CSCI 3171 Lectures
E. W. Grundke
2
ReferencesA. Tanenbaum and M. van Steen (TvS)Distributed Systems: Principles and ParadigmsPrentice-Hall 2002
G. Coulouris, J. Dollimore and T. Kindberg (CDK)Distributed System: Concepts and DesignAddison-Wesley 2001
3
AcknowledgementSome 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
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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HeterogeneityApplies 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.
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
TvS 1.4
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Layered Protocols: IPLayers, interfaces, and protocols in the Internet model.
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Layered Protocols: OSILayers, interfaces, and protocols in the OSI model.
2-1
TvS 2.2
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Middleware Protocols
An adapted reference model for networked communication.
2-5
TvS 2.6
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MiddlewareA 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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MiddlewareGeneral structure of a distributed system as middleware.
1-22
TvS 1.24
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Middleware and Openness
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.
1.23
TvS 1.25
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Middleware Programming ModelsRemote 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
CORBA
(Common Object Request Broker Architecture)
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ReferencesA. Tanenbaum and M. van Steen (TvS)Distributed Systems: Principles and ParadigmsPrentice-Hall 2002
G. Coulouris, J. Dollimore and T. Kindberg (CDK)Distributed System: Concepts and DesignAddison-Wesley 2001
Kate Keahey’s Tutorial on CORBAhttp://www.cs.indiana.edu/hyplan/kksiazek/tuto.html
OMG (Object Management Group) Documentationhttp://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 DiagramFrom http://www.omg.org/gettingstarted/Images/ORBdiagram.gif:
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