Lec15 - Consistency and Replication Today: Consistency...

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Computer Science Lecture 15, page CS677: Distributed OS Consistency and Replication Today: Consistency models Data-centric consistency models Client-centric consistency models 1 Computer Science Lecture 15, page CS677: Distributed OS Why replicate? Data replication versus compute replication Data replication: common technique in distributed systems Reliability If one replica is unavailable or crashes, use another Protect against corrupted data Performance Scale with size of the distributed system (replicated web servers) Scale in geographically distributed systems (web proxies) 2
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Computer Science Lecture 15, page Replication Issues When to replicate? How many replicas to create? Where should the replicas located? Will return to these issues later (WWW discussion) Today: how to maintain consistency ? Key issue: need to maintain consistency of replicated data If one copy is modified, others become inconsistent CS677: Distributed OS 3 Computer Science Lecture 15, page CS677: Distributed OS Object Replication Approach 1: application is responsible for replication Application needs to handle consistency issues Approach 2: system (middleware) handles replication Consistency issues are handled by the middleware Simplifies application development but makes object-specific solutions harder 4
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Computer Science Lecture 15, page CS677: Distributed OS Replication and Scaling Replication and caching used for system scalability Multiple copies: Improves performance by reducing access latency But higher network overheads of maintaining consistency Example: object is replicated N times Read frequency R, write frequency W If R<<W , high consistency overhead and wasted messages Consistency maintenance is itself an issue What semantics to provide? Tight consistency requires globally synchronized clocks! Solution: loosen consistency requirements Variety of consistency semantics possible 5 Computer Science Lecture 15, page CS677: Distributed OS Data-Centric Consistency Models Consistency model (aka consistency semantics ) Contract between processes and the data store If processes obey certain rules, data store will work correctly All models attempt to return the results of the last write for a read operation Differ in how “last” write is determined/defined 6
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Computer Science Lecture 15, page CS677: Distributed OS Strict Consistency Any read always returns the result of the most recent
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This note was uploaded on 11/22/2011 for the course COMPSCI 677 taught by Professor Shenoy during the Spring '08 term at UMass (Amherst).

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Lec15 - Consistency and Replication Today: Consistency...

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