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Ch4: Distributed Systems Architectures

Ch4: Distributed Systems Architectures. Typically, system with several interconnected computers that do not share clock or memory. Motivation: tie together multiple independent computers over the network to

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Ch4: Distributed Systems Architectures

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  1. Ch4: Distributed Systems Architectures

  2. Typically, system with several interconnected computers that do not share clock or memory. • Motivation: tie together multiple independent computers over the network to • share resources, enhance performance, improve reliability/availability, provide expandability. • One possible classification of architecture • several “minicomputers” (machines with CPU/user < 1) • several hundred / thousand “workstations” (CPU/user ~ 1) • Processor pool (CPU/user > 1) • user typically has a dedicated CPU as well.

  3. What is an OS • An interface between hardware and user processes that provides an abstraction of the machine and manages its resources. • What is a Distributed OS • the same, except for distributed systems • aims at transparency of distribution and presents a virtual uniprocessor to the user. • according to some, the holy grail is to create a “metacomputer” and a “Problem Solving Environment” • The user sees a single machine that automagically provides enough resources to do the task. The task can range from simple hello world programs to complex calculations.

  4. Issues in Distributed OS • Global “state” is not known • due to lack of shared memory and clock, unreliable message transmission. • Need decentralized controls • temporal order of events ? • Naming • how should an object be identified ? • Transparent ? Translucent ? Explicit ? • URIs, URNs, URLs • what if the object is replicated • Scalability • system should continue to work efficiently as resources are added • consider a system that resolves IP addresses using broadcast

  5. Compatibility / Interoperability • binary level, execution level or protocol level • homogeneous vs heterogeneous systems • Process Synchronization • mutual exclusion problem w/o shared memory • Resource Management • how do you get data to the location of the computation • distributed filesystem ? distributed shared memory ? • how do you migrate a computation (remote evaluation) • RPC ? Client-Server ? • how do you migrate running processes (code on demand, mobile objects/agents) • Security • authentication and authorization in a distributed system

  6. Structure of Distributed OS • Monolithic kernel • a (large) single entity that provides all the services of the distributed OS • may not be a good idea since “computer configurations” will vary • do you need to load disk drivers on a diskless client ? • Collective kernel • base OS functionality is in a relatively small microkernel. All other OS services are processes that run on top. • Microkernel will run on all machines • OO approach • same as collective kernel, but OS services are implemented as objects • can provide a more structured approach than collective kernel

  7. Communication • Review Section 4.6 yourself. • Message Passing Model • send and receive type primitives • can be blocking(reliable, unreliable) or non-blocking • use of buffers • synchronous vs asynchronous

  8. Millennium Project • Work at Microsoft Research(paper by Bolosky et al.) • Features • seamless distribution, worldwide scalability, fault tolerance, self-tuning, self configuring, secure, resource control • Principles • aggressive abstraction, storage irrelevance, location irrelevance, JIT binding, introspection.

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