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

Introduction to Distributed Systems 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

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

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  1. Introduction to Distributed Systems 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 • Acknowledgment: 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

  3. 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

  4. 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

  5. Transparency in a Distributed System Different forms of transparency in a distributed system. TvS 1.4

  6. Layered Protocols: IP • Layers, interfaces, and protocols in the OSI model.

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

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

  9. 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. • authentication protocols • authorization protocols • distributed commit protocols • distributed locking protocols • high-level communication protocols • remote procedure calls (RPC) • remote method invocation (RMI)

  10. Middleware • General structure of a distributed system as middleware. 1-22 TvS 1.24

  11. 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

  12. 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

  13. External Data Representation See Coulouris, Dollimore and Kindberg (CDK), Sec. 4.3

  14. Motivation • Data in running programs:Not just primitives, but arrays, pointers, lists, trees, etc. • In general: complex graphs of interconnected structures or objects • Data being transmitted: • Sequential! Pointers make no sense. Structures must be flattened. • All the heterogeneities must be masked! (endian, binary formats, etc.) CDK 4.3

  15. Motivation • Data in running programs:Not just primitives, but arrays, pointers, lists, trees, etc. • In general: complex graphs of interconnected structures or objects • Data being transmitted: • Sequential! Pointers make no sense. Structures must be flattened. • All the heterogeneities must be masked! (endian, binary formats, etc.) CDK 4.3

  16. What is an External Data Representation? • “An agreed standard for the representation of data structures and primitive values.” • Internal to external: “marshalling” • External to internal: “unmarshalling” • Examples: • CORBA’s Common Data Representation (CDR)Java Object SerializationSun XDR (RFC 1832)

  17. CORBA CDR • Defined in CORBA 2.0 in 1998 • Primitive types: • Standard data types, both big/little endian, conversion by the receiver. • Constructed types: • sequence, string, array, struct, enumerated, union • (not objects) • Data types are not specified in the external format: receiver is assumed to have access to the definition (via IDL). • (unlike Java Object Serialization!) CDK 4.3

  18. CORBA CDR • only defined in CORBA 2.0 in 1998, before that, each implementation of CORBA had an external data representation, but they could not generally work with one another. That is: • the heterogeneity of hardware was masked • but not the heterogeneity due to different programmers (until CORBA 2) • CORBA CDR represents simple and constructed data types (sequence, string, array, struct, enum and union) • note that it does not deal with objects • it requires an IDL specification of data to be serialised CDK 4.3

  19. CORBA CDR Example Index in sequence 4 bytes wide Notes of bytes • The flattened form represents a Person struct with value:{”Smith”, ”London”, 1934} CDK 4.3

  20. Remote Procedure Calls (RPC)

  21. What is RPC? • Call a procedure (function, subroutine, method, …) • in a program • running on a remote machine, • while hiding communication details from the programmer. • Note: Think C, not java! We deal with objects later!

  22. Conventional Procedure Call • Parameter passing in a local procedure call: the stack before the call to read • The stack while the called procedure is active TvS 2.7

  23. Parameter Passing Techniques • Call-by-value • Call-by-reference • Call-by-copy/restore

  24. Client and Server Stubs • Principle of RPC between a client and server program. TvS 2.8

  25. Steps of a Remote Procedure Call • Client procedure calls client stub in normal way • Client stub builds message, calls local OS • Client's OS sends message to remote OS • Remote OS gives message to server stub • Server stub unpacks parameters, calls server • Server does work, returns result to the stub • Server stub packs it in message, calls local OS • Server's OS sends message to client's OS • Client's OS gives message to client stub • Stub unpacks result, returns to client TvS 2.9

  26. Passing Value Parameters • Steps involved in doing remote computation through RPC 2-8 TvS 2.10

  27. Passing Value Parameters:Data Representation Issues • Original message on the Pentium • The message after receipt on the SPARC • The message after being inverted. The little numbers in boxes indicate the address of each byteBUT: This is not usually a problem with strings! (E.W.G.) TvS 2.11

  28. Parameter Specification and Stub Generation • A procedure • The corresponding message. TvS 2.12

  29. Passing Reference Parameters • Reference variables (pointers):pointers to arrayspointers to structures (objects without methods) • What if the structure contains other pointers? • The server may need a whole “graph” of structures! • “Parameter marshalling” • Interface Definition Language (IDL): • Specifies types, constants, procedures and parameter data types,compiled into client and server stubs.

  30. Asynchronous RPC • The interconnection between client and server in a traditional RPC • The interaction using asynchronous RPC 2-12 TvS 2.14

  31. Asynchronous RPC:Deferred Synchronous RPC • A client and server interacting through two asynchronous RPCs TvS 2.15

  32. Distributed Computing Environment (DCE) • A middleware system • Developed by The Open Group (previously OSF) • Includes • distributed file servicedirectory servicesecurity servicedistributed time service • Adopted by Microsoft for distributed computing

  33. DCE: Binding a Client to a Server 2-15 TvS 2.17

  34. Remote Method Invocation (RMI)

  35. What is RMI? • RPC to a method in an object on another machine. • Note: Now think Java!

  36. Object Orientation:Remote Method Invocation (RMI) • An object encapsulates • State (fields or instance variables) • Methods (often described by an interface) • Distributed object: • An interface known locally may describe an object on another machine.

  37. Distributed Objects • Common organization of a remote object with client-side proxy. 2-16 TvS 2.18

  38. Binding a Client to an Object • An example with implicit binding using only global references • An example with explicit binding using global and local references Distr_object* obj_ref; //Declare a systemwide object referenceobj_ref = …; // Initialize the reference to a distributed objectobj_ref-> do_something(); // Implicitly bind and invoke a method (a) Distr_object objPref; //Declare a systemwide object referenceLocal_object* obj_ptr; //Declare a pointer to local objectsobj_ref = …; //Initialize the reference to a distributed objectobj_ptr = bind(obj_ref); //Explicitly bind and obtain a pointer to the local proxyobj_ptr -> do_something(); //Invoke a method on the local proxy (b) TvS 2.19

  39. Parameter Passing • The situation when passing an object by reference or by value. 2-18 TvS 2.20

  40. The DCE Distributed-Object Model • Distributed dynamic objects in DCE. • Distributed named objects TvS 2.21

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