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Introduction

This text provides an introduction to distributed systems, including definitions, scalability problems, and techniques, and an overview of middleware. It covers topics such as transparency, hardware and software concepts, operating systems, and client-server architectures.

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Introduction

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  1. Introduction Chapter 1

  2. Definition of a Distributed System (1) • A distributed system is: • A collection of independent computers that appears to its users as a single coherent system.

  3. Definition of a Distributed System (2) 1.1 A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.

  4. Goals • Connection of users and resources • Transparency • Openness: syntax and semantics • Scalability: asynchronous comm, distribution, replications, etc.

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

  6. Scalability Problems Examples of scalability limitations.

  7. Scaling Techniques (1) 1.4 • The difference between letting: • a server or • a client check forms as they are being filled

  8. Scaling Techniques (2) 1.5 An example of dividing the DNS name space into zones.

  9. Hardware Concepts 1.6 Different basic organizations and memories in distributed computer systems

  10. Multiprocessors (1) 1.7 • A bus-based multiprocessor.

  11. Multiprocessors (2) 1.8 • A crossbar switch • An omega switching network

  12. Homogeneous Multicomputer Systems 1-9 • Grid • Hypercube

  13. Software Concepts

  14. Uniprocessor Operating Systems 1.11 • Separating applications from operating system code through a microkernel.

  15. Multiprocessor Operating Systems (1):MONITOR monitor Counter { private: int count = 0; public: int value() { return count;} void incr () { count = count + 1;} void decr() { count = count – 1;} } • A simple monitor, allowing one access at time: one processes finishes what it is doing, before the other can start • Monitor protects integer count against concurrent access. • A monitor is implemented using semaphores

  16. Multiprocessor Operating Systems (2):MONITOR monitor Counter { private: int count = 0; int blocked_procs = 0; condition unblocked; public: int value () { return count;} void incr () { if (blocked_procs == 0) count = count + 1; else signal (unblocked); } void decr() { if (count ==0) { blocked_procs = blocked_procs + 1; wait (unblocked); blocked_procs = blocked_procs – 1; } else count = count – 1; } } • A evolved monitor to protect an integer against concurrent access • a process is blocked using condition variables

  17. Multicomputer Operating Systems (1) 1.14 • General structure of a multicomputer operating system

  18. Multicomputer Operating Systems (2) 1.15 • Alternatives for blocking and buffering in message passing.

  19. Multicomputer Operating Systems (3) • Relation between blocking, buffering, and reliable communications.

  20. Distributed Shared Memory Systems (1)remote RAM is used as backup storage • Pages of address space distributed among four machines • Situation after CPU 1 references page 10 • Situation if page 10 is read only and replication is used

  21. Distributed Shared Memory Systems (2) 1.18 • False sharing of a page between two independent processes.

  22. Network Operating System (1) 1-19 • General structure of a network operating system. • Services: rlogin, rcp, file servers,

  23. Network Operating System (2) 1-20 • Two clients and a server in a network operating system.

  24. Network Operating System (3) 1.21 • Different clients may mount the servers in different places.

  25. Middleware • Models • Distributed file systems • Remote Procedure Calls • Distributed objects • Distributed Documents • Services • Communication • Naming • Persistence • Distributed Transactions • Security

  26. Positioning Middleware 1-22 • General structure of a distributed system as middleware:communication, naming, data persistence, distributed tx, security.

  27. Middleware and Openness: for interoperability 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.

  28. Comparison between Systems • A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems.

  29. Clients and Server Models • Client Server model has been agreed on as one of the universal DS model, by many researchers

  30. Clients and Servers 1.25 • General interaction between a client and a server.

  31. An Example Client and Server (1) • The header.h file used by the client and server.

  32. An Example Client and Server (2) • A sample server.

  33. An Example Client and Server (3) 1-27 b • A client using the server to copy a file.

  34. Processing Level 1-28 • The general organization of an Internet search engine into three different layers

  35. Client-Server Architectures • Multitiered: vertical • Modern: Horizontal • Peer-to-peer

  36. Multitiered Architectures (1) 1-29 • Alternative client-server organizations (a) – (e).

  37. Multitiered Architectures (2) 1-30 • An example of a server acting as a client.

  38. Modern Architectures 1-31 • An example of horizontal distribution of a Web service.

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