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Understanding Distributed Systems: Key Characteristics and Design Considerations

Distributed systems consist of interconnected autonomous computers designed to function as a cohesive computing entity. Emerging from the advent of LANs in the 1970s, these systems have six defining characteristics: resource sharing, openness, concurrency, scalability, fault tolerance, and transparency. Each feature plays a crucial role in ensuring the system operates efficiently, recovering from failures while allowing for seamless resource access and system extensions. Key design challenges include naming, communication, and workload allocation, influencing how distributed systems are structured and managed.

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Understanding Distributed Systems: Key Characteristics and Design Considerations

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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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