This presentation is based on WS-Membership: Failure Management in Web Services WorldFigure 12.1 A network partitionAgreement in Pepperland (p.51)Consensus in the presence of failure (p.54)Consensus in Space Mission LaunchesShuttle StoryIntroduction to WS-MembershipObduro ProjectFailure ManagementWS-MembershipComponent servicesThe Membership Framework: Five Roles ModeledFile roles (contd.)Activation & RegistrationSee Fig.1 for activation & registration sequenceThe Epidemic MembershipEpidemic membership Service (EMS)Features of EMSTypes of information thru’ gossipOperational DetailsFault model?01/14/19Page 1This presentation is based onWS-Membership: Failure Management in Web Services WorldB. Ramamurthy Based on Paper by Werner Vogels and Chris ReInstructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 12.1A network partitionCrashedrouterAgreement in Pepperland (p.51)•Consider an army in Pepperland: Apples and Oranges: two groups are located on the hills•Blue Meanies are invader. Now located in the valley.•Apple and Orange have to decide when to attack.•They exchange messages on their strength: number of attack items (personal, machinery etc).•They reach a consensus on who will attack first based on strength.•Then attack message is sent from stronger team to weaker team. The message delay {min… max}•Apple (say) send the attack message, waits for min minutes; then starts attack; Other team is supposed to wait for 1 min after it receives attack msg;•Ideal guarantee: Orange will start attack no more than {max-min+1} minutes.Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005Consensus in the presence of failure (p.54)•How do you detect failure? What if the messenger from Apple to Orange is captured?•How does Orange know if Apple has been defeated?•Impossibility in reaching agreement in the presence of failures: to surrender or to attack?•If the messenger is captured there is way to achieve agreement.Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005Consensus in Space Mission Launches•Mission critical applications•Set of divisions each in charge of a module of the (shuttle) mission•They have to agree or come to a consensus to launch (attack in Pepperland) or abort (surrender in Pepperland).•In case of failure of message, consensus is reached to abort the mission.Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005Shuttle Story•“I had a friend who was one of the programmers who worked on the Shuttle guidance system, which used three computers in parallel, and operated under a consensus model.E If two computers agreed on a decision, the third would remain quiescent.E If they disagreed, however, the third computer would jump in to cast a tie-breaking vote.E I asked what would happen if the three computers came up with three different answers,E and he shook his head.EE I asked what would happen if one of those computers dropped out, and he said the other two would end up dead-locked.”EInstructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 200501/14/19Page 7Introduction to WS-Membership•An important factor in the successful deployment of federated web-services-based business activities will be the ability to guarantee reliable distributed operation and execution. •Failure management is essential for systems constructed out web services on the network.•ws-membership–a coordination service–a generic web-service interface for tracking registered web-services and –for providing membership monitoring information. •A prototype membership service –based on epidemic protocol techniques has been implemented •Context: Obduro project which focuses on global scalable distributed systems based on web-service technologies.Obduro Project•Development of advanced distributed services in the context of WS Coordination framework•Development of high performance server technology for web services routing•Integration of reliability and other distributed services into coordination and choreography engines.•Development of a framework for global event management01/14/19Page 8Failure Management•Failure management is essential for building reliable distributed systems•Tracking which services are participating in an activity and what their status is drives the progress of the activity.•Ws-membership is developed in the context of ws-coordination standard of w3c.•Membership can be realized by simple heartbeat (as in Hadoop).•Failure detection can also be used as the building block to simplify the implementation of consensus.•Consensus is used when a set of processes have to agree upon the outcome of an operation.01/14/19Page 901/14/19Page 10WS-Membership•Membership services is about service availability•Coordination protocol•Tracks registered members•Presents membership updates to monitors•Two componentsWS-MembershipFailure detection Membership dissemination01/14/19Page 11Component services•Epidemic communication•State management•Development of advanced distributed services in the context of the web-services Coordination framework.E –These services will include a failure management service, a consensus service and a lightweight distributed state-sharing engine.01/14/19Page 12The Membership Framework: Five Roles Modeled•Coordination service–Receives activation and membership requests and routes them to membership service•Membership Service– Provides failure detection of registered web-services and disseminates membership information01/14/19Page 13File roles (contd.)•Member Service– A software component that has registered itself for failure detection, either directly with a Membership Service •Membership Proxy– A software component that is interposed between a member service and the Membership Service for reasons of efficiency or accuracy •Membership Monitor– This service registers itself with the Membership Service to receive changes to the membership state01/14/19Page 14Activation & Registration•Coordination service provides 2-step access to membership