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Distributed Computing Umar Kalim Dept. of Communication Systems Engineering [email protected] http://www.niit.edu.pk/~umarkalim 19/09/2007 Fall 2007 cs425 1 Agenda Course details Introduction Fall 2007 cs425 2 Reading material Text book: – Andrew S. Tanenbaum, Maarten van Steen “Distributed systems: principles and paradigms”. Reference books: – Jean Dollimore, Tim Kindberg, George Coulouris “Distributed Systems: Concepts and Design” – Joe Wigglesworth and Paula McMillan “Java Programming: Advanced Topics”, 3rd edition – Bruce Eckel “Thinking in Java” 3rd edition Availabe online from Bruce Eckel's web site http://www.mindviewinc.com/ Reference material – Selected publications available online http://www.niit.edu.pk/~umarkalim/dc Fall 2007 cs425 3 Course details Lectures & Handouts – Will be available online Office hours: – Usually one (1) hour after the lecture Fall 2007 cs425 4 Grading policy Assignments 5% Quizzes 10% Survey report/Project 15% OHT 30% End-term 40% Fall 2007 Assignments – Individual – No late submission Quizzes – Mostly unannounced – Occasionally announced cs425 5 Objective of this course? Provide an understanding of the technical issues involved in the design of modern distributed systems Present some of the major current paradigms Outcomes – Appreciation of the main principles underlying distributed systems: processes, communication, naming, synchronization, consistency, fault tolerance, and security. – Familiarity with some of the main paradigms in distributed systems: object-based systems, file systems, and coordination-based systems Fall 2007 cs425 6 Lets begin! Fall 2007 cs425 Ref: csc.liv.ac.uk 7 Definition of a Distributed System A distributed system is: A collection of independent computers that appears to its users as a single coherent system. Fall 2007 cs425 8 Examples of Distributed Systems Computer world: – University computer network – SETI@home Search for ExtraTerrestrial Intelligence – GRID (distributed computing facilities) Ordinary life: – WWW, P2P systems (such as Emule, Azureus etc) – Banks (Cash machines) – Ticket reservation Fall 2007 cs425 9 Goals of a Distributed Systems User-side goals (why distributed system) – Easily connect users/resources – Exhibit transparency Technical goals (how to achieve) – Be open – Be scalable Looking at these goals helps us answer the question: “is building a distributed system worth the effort?” Fall 2007 cs425 10 Connecting Users and Resources Typical resources – Printers, computers, computing power, data Why sharing – Economics – Collaboration, Information Exchange (groupware) Problems with sharing – Security – Unwanted collaboration Fall 2007 cs425 11 Transparency in distributed systems Transparent distributed system: – Appears to its users as if it were only a single computer system Fall 2007 cs425 12 Forms of transparency Transparency Description Access Hide differences in data representation and how a resource is accessed Location Hide where a resource is located Migration Hide that a resource may move to another location Relocation Hide that a resource may be moved to another location while in use Replication Hide that a resource may be shared by several competitive users Concurrency Hide that a resource may be shared by several competitive users Failure Hide the failure and recovery of a resource Persistence Hide whether a (software) resource is in memory or on disk Fall 2007 cs425 13 Degree of transparency Transparency is – Not always desirable Users located in different continents (context-aware) – Not always possible Hiding failures (you can distinguish a slow computer from a failing one) Trade-off between a high degree of transparency and the performance of the system Fall 2007 cs425 14 Openness Open systems – Offer services according to standard rules that describe the syntax and semantics of these services – Enjoy neutral and complete specifications Network protocols Advantages of being open: – Interoperability Open systems can work together – Portability Ability to transform an application from one software or hardware platform to another Open ≠ Free! Fall 2007 cs425 15 Scalability Along three different dimensions – Size (the number of users and/or processes) – Geographical (maximum distance between participants) – Administrative (number of administrative domains) Fall 2007 cs425 16 Scalability problems Concept Example Centralized services A single server for all users Centralized data A single on-line telephone book Centralized algorithms Doing routing based on complete information Fall 2007 cs425 17 Scaling Techniques Three techniques: – Hiding communication latencies Try to avoid waiting for responses to remote service requests as much as possible – Distribution – Replication Fall 2007 cs425 18 Scaling Techniques (1) 1.4 The difference between letting: a) a server or b) a client check forms as they are being filled Fall 2007 cs425 19 Scaling Techniques (2) 1.5 An example of dividing the DNS name space into zones. Fall 2007 cs425 20 Scaling Techniques (3) Replication – – – – Increases availability Balances the load Reduces communication latency But causes consistency problems Caching (client-driven) Fall 2007 cs425 21 Modelling distributed systems When building distributed applications, system builders have often looked to the nondistributed systems world for models to follow (… inspiration?) Consequently, distributed systems tend to exhibit certain characteristics that are already familiar to us This applies equally to hardware concepts as it does to software concepts Fall 2007 cs425 22 Classification of Multiple CPU Computer Systems Two groups: Multiprocessors – Shared memory – Several processors Multicomputers – Each machine has its own memory – Network communication Fall 2007 cs425 23 Bus based multiprocessor systems Simplified representation of a shared-memory supercomputer Initially for supercomputers only, but – AMD and Intel provide 2-, 4-, and 8-processor workstations – Intel Hyper-Thread technology Fall 2007 cs425 24 Multicomputer Systems a) Grid b) Hypercube How to run multiprocessor software on multicomputer systems? – Distributed operating systems Fall 2007 cs425 25 Multicomputer Operating Systems General structure of a (DOS) multicomputer operating system Fall 2007 cs425 26 Modeling on Multicomputers Excessively tightly connected – Such systems tend to be homogeneous (same hardware and software in all nodes) – Compromises openness and scalability Fall 2007 cs425 27 Network Operating System General structure of a network operating system – all the systems are of different types: heterogeneous Fall 2007 cs425 28 Network Operating System Two clients and a server in a network operating system – relatively primitive set of services provided Fall 2007 cs425 29 Distributed Systems on top of Network Operating Systems Too primitive, low level communication – Compromises transparency What to do??? Fall 2007 cs425 30 Best of both worlds “Middleware” – best possible compromise? Middleware = NOS + additional software layer Fall 2007 cs425 31 Distributed Systems as Middleware A distributed system organized as middleware. Note that the middleware layer extends over multiple machines. Fall 2007 cs425 32 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. This is a much higher level of abstraction than (for instance) the NOS Socket API. Fall 2007 cs425 33 Modelling Software Concepts System Description Main Goal DOS Tightly-coupled operating system for multiprocessors and homogeneous multicomputers Hide and manage hardware resources NOS Loosely-coupled operating system for heterogeneous multicomputers (LAN and WAN) Offer local services to remote clients Middleware Additional layer atop of NOS implementing general-purpose services Provide distribution transparency Fall 2007 cs425 34 Comparing DOS/NOS/Middleware Item Distributed OS Network OS Middlewarebased OS Multiproc. Multicomp. Degree of transparency Very High High Low High Same OS on all nodes Yes Yes No No Number of copies of OS 1 N N N Basis for communication Shared memory Messages Files Model specific Resource management Global, central Global, distributed Per node Per node Scalability No Moderately Yes Varies Openness Closed Mostly Closed Open Open A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems. Fall 2007 cs425 35 Summary Distributed Systems … autonomous computers working together to give the appearance of a single, coherent system. They are transparent, scalable and open. Unfortunately, they also tend to be complex. Fall 2007 cs425 36 Questions? That’s all for today! Fall 2007 cs425 37