CS514: Intermediate Course in Operating SystemsConsistencySlide 3Basic “design points”Are actions asynchronous?Mixing modelsReal systems have varied needsDigressionJoint Battlespace InfospherePowerPoint PresentationSlide 11Shop Floor Process Control ExampleThe List Goes OnChallenge faced by developersOne sizes fits all?“Understanding” CATOCSFrench air traffic controlAir traffic controlSlide 19ATC system componentsPossible uses of groupsSlide 22Slide 23Slide 24Air traffic control exampleObservations that can helpA multi-tier APISample but typical issueSeparate concernsWAN architectureConsistency approach in partitionable networkWithin a serverWorst case for a replicated server?Observations?Example of a blurry goalExample of a blurry goal (cont)Much of the theory is misunderstood!When do we need FLP style consistency?Why should this bother us?... but non-uniform protocols are too weak!Optimal TransactionsLong-term prospects?Theory sideOther directions?Self-stabilizationCurious problems?Consistency is ubiquitousMoving on…CS514: Intermediate Course in Operating SystemsProfessor Ken BirmanBen Atkin: TALecture 13: Oct. 5Consistency•How can we relate models of consistency to cost and availability?•Is it possible to reconcile transactional replication with virtual synchrony replication?Consistency•Various models–Multiple copies of some object but behavior mimics a single non-faulty object–ACID: 1-copy SR plus durability–FLP style of consensus –Dynamic uniformity versus static modelBasic “design points”•Does the model guarantee anything relative to “last words” of a process that fails?–Yes for transactions: ACID–No, in virtual synchrony•Can do better using “flush” primitive•And can mimic transactional replication if we require that primary partition is also a quorum of some statically specified set of processesAre actions asynchronous?•No in case of transactions–We can do things locally–But at commit time, we need to synchronize–And most transactional replication schemes are heavily synchronous•Yes for virtual synchrony–But only with cbcast or fbcastMixing models•Virtual synchrony is like “weak transactional serializability”–In fact, connection can be made precise–We use a model called linearizability by Wing and Herlihy•Much recent work on database replication mixes models…Real systems have varied needs•Must match choice of properties to needs of the application•Find that multiple models are hard to avoid–We want the stronger models for database applications–But where data won’t persist, the cheaper models suffice…Digression•Need to strengthen our intuition•Can we find examples of real systems that might need group communication or data replication?–Ideally, systems that can’t be built in any other way–Use this to think about properties required for application correctnessJoint Battlespace InfosphereROBUST INFRASTRUCTUREROBUST INFRASTRUCTUREAvailability for historical data Load balancing and consistentmessage delivery for price distributionParallel execution for analyticsDistributed Trading SystemPricing DB’sHistorical DataAnalyticsCurrent PricingMarketDataFeedsLong-Haul WAN SpoolerTokyo, London, Zurich, ...Trader ClientsDistributed Service NodeOne phone number per personTelephone Trunk LinesCalls,Changes,Adds,DeletesTelephony Data/Digitized Voice PathEthernet= Isis 1.DumbSwitchRISC/UNIX RISC/UNIXReplicated files for digitized voice storeRedundancy for database availabilityLoad balancing for call handling & routingx86/UNIX2. 3.Shop Floor Process Control ExampleStationControllerStationControllerDataCollectionServerRecipeManagementServerOperatorClientOperatorClientOperatorClientOperatorClientFactory equipmentWorkStreamServerVAX HPPC PCHPHP HPHPHPEnetThe List Goes On•Air traffic control system•Medical decision support in a hospital•Providing real-time data in support of banking or major risk-management strategies in finance•Real-time system for balancing power production and consumption in the power grid•Telephone system for providing services in setting with mobile users and complex requirementsChallenge faced by developers•We have multiple notions of consistency now:–Transactional, with persistent data–Process groups with dynamic uniformity–Process groups without dynamic uniformity–Primary partition notion of progress–Non-primary partitions with merge•How can we make the right choices for a given situation?One sizes fits all?•One possibility is that we’ll simply need multiple options–User would somehow specify their requirements–Given this information, system would configure protocols appropriately•Alternative is to decide to standardize on one scheme–Likely to be a strong, more costly option“Understanding” CATOCS•Paper by Cheriton, Skeen in 1993•They argue that end-to-end approach dictates–Simplicity in the GCS–Properties enforced near end-points•Paper is full of mistakes but the point is well taken–People don’t want to pay for properties they don’t actually require!French air traffic control•They wanted to use replication and group communication in a system for high availability controller consoles•Issues they faced–How strong is the consistency need?–Where should we use groups?–Where should we use transactionsAir traffic control•Much use of computer technologies–Flight management system (controls airplane)–Flaps, engine controls (critical subsystems)–Navigational systems–TCAS (collision avoidance system)–Air traffic control system on ground•In-flight, approach, international “hand-off”–Airport ground system (runways, gates, etc)Air traffic control•Much use of computer technologies–Flight management system (controls airplane)–Flaps, engine controls (critical subsystems)–Navigational systems–TCAS (collision avoidance system)–Air traffic control system on ground•In-flight, approach, international “hand-off”–Airport ground system (runways, gates, etc)ATC system componentsControllersAir Traffic Database(flight plans, etc)X.500 DirectoryRadarOnboardPossible uses of groups•To replicate data in console clusters•For administration of console clusters•For administration of the “whole system”•For radar communication from radar to the consoles•To inform consoles when flight plan database is updated•To replicate the database itselfATC system
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