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Distributed systems

Distributed systems. A collection of autonomous computers linked by a network, with software designed to produce an integrated computing facility A well designed distributed system should be perceived as a single, integrated computing facility

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Distributed systems

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  1. Distributed systems • A collection of autonomous computers linked by a network, with software designed to produce an integrated computing facility • A well designed distributed system should be perceived as a single, integrated computing facility • The introduction of LANs at the beginning of the 1970s triggered the development of distributed systems

  2. Key Characteristics • There are 6 key characteristics of a distributed system • Support for resource sharing • Openness • Concurrency • Scalability • Fault Tolerance • Transparency

  3. Resource Sharing • Resource • The range of things that can be shared usefully in a distributed system • Includes things like • Printers, disks, cdroms, etc. • Data • A resource manager is a software module that manages a set of resources of a particular type

  4. Openness • The characteristic that a system can be extended in various ways • Hardware extensions • Software extensions • Historically, computer systems were largely closed • UNIX broke that mold

  5. Concurrency • Concurrency in a distributed system does not necessarily mean concurrency within a single program • Many users invoke similar commands • Many different server processes may be running • Synchronization, of course, is a problem

  6. Scalability • Distributed systems operate at many different scales • Two workstations and a file server • The CS computers… • Often the more important question is not can you scale, but can you scale well • Consider the Internet

  7. Fault Tolerance • Fault tolerance is the ability of a system to recover when failures occur • Required in some systems • Nuisance value in others • Fault tolerant design is based on two approaches • Hardware redundancy • Software recovery

  8. Hardware Redundancy • Two computers are employed for a single application, one acting as a standby • Very costly, but often very effective, solution • Redundancy can be planned at a finer grain • Individual servers can be replicated • Redundant hardware can be used for non-critical activities when no faults are present

  9. Software Redundancy • Software must be designed so that the state of permanent data can be recovered or “rolled back” when a fault is detected • Transaction processing

  10. Transparency • The concealment of the separation of components in a distributed system • Access transparency • Location transparency • Concurrency transparency • Replication transparency • Failure transparency • Migration transparency • Performance transparency • Scaling transparency

  11. Transparency • The two most important forms of transparency are • Access • Location • They are sometimes referred to as network transparency • rlogin command • ptt@cs.rit.edu

  12. Basic Design Issues • Design issues that arise specifically from the distributed nature of systems include • Naming • How do you assign names to resources • Communication • Software structure • Openness • Workload allocation

  13. How is this different? • Resource sharing – yes • Openness – yes • Concurrency – absolutely • Scalability – would love it • Fault tolerance – maybe • Transparency – definitely not

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