Software Engineering of Distributed SystemsPowerPoint PresentationCourse LogisticsOutlineDefinition of a Distributed SystemAlternative definition of a distributed systemImplied characteristics?Examples?Why is this hard?Software is hardSoftware is hard - 2Software is hard - 3Necessary DevelopmentsPurposes -- what problem is solved?Access and sharing remotelyTransparency TypesDegree of TransparencyOpennessGoals of OpennessFlexibility -- Policy and MechanismScability Challenges -- SizeScalability Challenges -- GeographicScalability Challenges -- AdministrativeScaling TechniquesDividing DNS name space into zonesSlide 26Hardware ConceptsShared Memory & Private MemoryBus-based & Switch-basedPerformance Impacts--bus, shared memoryPerformance impacts --Performance impacts - heterogeneous multicomputer systemsSoftware ConceptsWhen is a distributed system not a distributed system?Various middleware models (paradigms)Sample ParadigmsEach paradigm must address these issues:Software Engineering of Distributed SystemsSoftware Engineering of Distributed SystemsUniversity of ColoradoBoulderECEN5053bring up www.cdk4.net before starting Tegrity in classAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro2August 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro3Course LogisticsIntroductions http://webct.colorado.eduFormatCalendar – this year, Fall Break is merged into Thanksgiving week – no classes 11/20-11/24/2006Final Exam, Monday December 18, 1:30-4 p.m.Homework – see web siteContact information – [email protected] 303-492-8369 (CU office), ECEE 1B67Text web site: www.cdk4.net -- see key pts.August 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro4OutlineDefinitionsSoftware Engineering issuesPurposesDemands/challengesHardware conceptsSoftware conceptsAn example modelAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro5Definition of a Distributed SystemA distributed system is a collection of independent computers that appears to its users as a single coherent system. Andrew TanenbaumA distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messages. Coulouris et al (your text)concurrency of components’ executionlack of a global clockindependent failures of componentsAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro6Alternative definition of a distributed system“You know you have one when the crash of a computer you’ve never heard of stops you from getting any work done.” Leslie LamportAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro7Implied characteristics?“appears to its users as a single coherent system” – what must be true for that to be true?August 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro8Examples?internetand what else?August 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro9Why is this hard?Hardware tends to smaller, faster, more reliable, cheaper, more predictable to develop and innovateConcurrent, networked, distributed software has grown larger, slower, more error-prone, very expensive and time-consuming to develop, validate, maintain, enhanceSome low-level sw optimizations are no longer needed thanks to new hardwareLifecycle cost continues to riseAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro10Software is hardInherent complexitiesfundamental domain challenges partial failure, distributed deadlock, end-to-end QoSAccidental complexitiesLimitations of software toolsLimitations of development techniquesnon-portable APIs, poor distributed debuggersDeliberate choices of developers who favor low-level languages and tools that don’t scale wellAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro11Software is hard - 2Inadequate methods and techniquesPopular software analysis methods and design techniques have focused largely on constructing single-process, single-threaded applicationsDevelopment of high-quality concurrent, networked, distributed systems with stringent QoS requirements are left to intuition and expertise of skilled sw architects and engineers.Hard to learn – trial & error, platform-specificAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro12Software is hard - 3Continuous re-invention and re-discovery“The software industry has a long history of recreating in compatible solutions to problems that are already solved.”Dozens of non-standard general-purpose and real-time o.s.’s managing the same hw resourcesDozens of incompatible o.s. encapsulation libraries providing slightly different APIs to implement essentially the same features and servicesStarting to shift to recognizing value of patterns but there are fewer available for concurrent, networked, & distributed systemsAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro13Necessary DevelopmentsTake an historical view1945 - 1985 Computers are large & expensiveMost organizations had only a fewlacked a way to connect themoperated independently from one anotherBy mid-80’s ... powerful microprocessors with power of a then-contemporary mainframeHigh speed networks!Result: Easy to combine large numbers of computers via a high-speed network.August 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro14Purposes -- what problem is solved?Easily connect users to remote resourcesShare resources with remote users in a controlled wayHide the fact that the resources are physically distributed over a network -- transparencyShould be an open systemOffers services by standard rules that describe the syntax and semantics of those servicesShould be scalablesize, geography, and administrationAugust 28, 2006 University of Colorado ECEN5053 SW Eng of Distributed Systems Week 1 Intro15Access and sharing remotelyWhy share? economyease of collaboration -- virtual organizationsease of info exchangecommerceConnectivity and sharing lead to security