L23a.sp11 - Computer Science 425 Computer Science 425...

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Lecture 19-1 Lecture 19-1 Computer Science 425 Distributed Systems CS 425 / CSE 424 / ECE 428 Fall 2010 Computer Science 425 Distributed Systems CS 425 / CSE 424 / ECE 428 Fall 2010 Indranil Gupta (Indy) October 26, 2010 Lecture 19 Replication Control I Reading: Chapter 15 (relevant parts) 010, I . Gupta, K. Nahrtstedt, S. Mitra, N. Vaidya, M. T. Harandi, J. Hou
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Lecture 19-2 Lecture 19-2 Replication Replication Enhances a service by replicating data Increased Availability Of service. When servers fail or when the network is partitioned. Fault Tolerance Under the fail-stop model, if up to f of f+1 servers crash, at least one is alive. Load Balancing One approach: Multiple server IPs can be assigned to the same name in DNS, which returns answers round-robin. P: probability that one server fails= 1 – P= availability of service. e.g. P = 5% => service is available 95% of the time. P n : probability that n servers fail= 1 – P n = availability of service. e.g. P = 5%, n = 3 => service available 99.875% of the time
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Lecture 19-3 Lecture 19-3 Goals of Replication Goals of Replication Replication Transparency User/client need not know that multiple physical copies of data exist. Replication Consistency Data is consistent on all of the replicas (or is converging towards becoming consistent) Client Front End RM RM RM Client Front End Client Front End Service server server server Replica Manager
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Lecture 19-4 Lecture 19-4 Replication Management Replication Management Request Communication Requests can be made to a single RM or to multiple RMs Coordination: The RMs decide whether the request is to be applied the order of requests FIFO ordering: If a FE issues r then r’ , then any correct RM handles r and then r’ . Causal ordering : If the issue of r “happened before” the issue of r’ , then any correct RM handles r and then r’ . Total ordering : If a correct RM handles r and then r’ , then any correct RM handles r and then r’ . Execution: The RMs execute the request (often they do this tentatively – why?).
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Lecture 19-5 Lecture 19-5 Replication Management Replication Management Agreement : The RMs attempt to reach consensus on the effect of the request. E.g., Two phase commit through a coordinator If this succeeds, effect of request is made permanent Response One or more RMs responds to the front end. The first response to arrive is good enough because all the RMs will return the same answer. Thus each RM is a replicated state machine “Multiple copies of the same State Machine begun in the Start state, and receiving the same Inputs in the same order will arrive at the same State having generated the same Outputs.” [Wikipedia, Schneider 90]
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Lecture 19-6 Group Communication: A bulding block Group Communication: A bulding block “Member”= process (e.g., an RM) Static Groups : group membership is pre-defined Dynamic Groups
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This note was uploaded on 02/08/2012 for the course ECE 428 taught by Professor Hu during the Spring '08 term at University of Illinois, Urbana Champaign.

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L23a.sp11 - Computer Science 425 Computer Science 425...

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