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

2 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

3 Acknowledgement Some slides from : TvS: CDK:

Distributed Systems

5 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

6 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

7 Transparency in a Distributed System Different forms of transparency in a distributed system. TransparencyDescription Access Hide differences in data representation and how a resource is accessed LocationHide where a resource is located MigrationHide that a resource may move to another location Relocation Hide that a resource may be moved to another location while in use ReplicationHide that a resource is replicated Concurrency Hide that a resource may be shared by several competitive users FailureHide the failure and recovery of a resource Persistence Hide whether a (software) resource is in memory or on disk TvS 1.4

8 Layered Protocols: IP Layers, interfaces, and protocols in the Internet model.

9 Layered Protocols: OSI Layers, interfaces, and protocols in the OSI model. 2-1 TvS 2.2

10 Middleware Protocols An adapted reference model for networked communication. 2-5 TvS 2.6

11 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

12 Middleware General structure of a distributed system as middleware TvS 1.24

13 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 TvS 1.25

14 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

CORBA (Common Object Request Broker Architecture)

16 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 OMG (Object Management Group) Documentation

17 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.)

18 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)...

19 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....

20 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

21 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!)

22 CORBA Block Diagram From From